UNIT II-microwave sources (1).pptx

microwave

1. Sathyabama Institute of science and Technology,Chennai-119 Unit:2 Microwave Sources SECA1701-Microwave and Optical communication

2. Microwave Sources Microwave sources include artificial devices such as circuits, transmission towers,  radar , masers, and  microwave ovens , as well as natural sources such as the Sun and the Cosmic Microwave Background. Microwaves can also be produced by atoms and molecules. The name  Microwave  is derived from the energy used to cook the food,  microwaves , which pass through the cells and molecules of the food, the frequency of the waves causes the water molecules to vibrate, this movement generates heat. These  microwaves  are produced by a device  called  a magnetron within the  microwave  oven. SECA1701-Microwave and Optical communication

3. Microwave tubes Microwave tubes  are electron guns for generating linear beam  tubes . A  microwave tube  generates and amplifies higher frequencies in the  microwave  range of frequency spectrum. When a  microwave tube  is energized, the electrons are emitted from the cathode and are focused on the control grid. Klystron ,  Magnetron  and  Travelling Wave tube  are used in microwave oven SECA1701-Microwave and Optical communication

4. Microwave tubes A  microwave tube  works on the  principle  of velocity modulation. A velocity modulation  principle  generally avoids the problem of frequency limitation that often occurs in  microwave tubes . ... An electron beam generated by a  microwave  source works well in the  operating  temperatures ranging from 910 to 1200 °C. Klystron ,  Magnetron  and  Travelling Wave tube  are used in microwave oven. SECA1701-Microwave and Optical communication

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6. Microwave Tubes SECA1701-Microwave and Optical communication

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9. Cavity Klystron For the generation and amplification of Microwaves, there is a need of some special tubes called as  Microwave tubes . Of them all,  Klystron  is an important one. The essential elements of Klystron are electron beams and cavity resonators. Electron beams are produced from a source and the cavity klystrons are employed to amplify the signals. A collector is present at the end to collect the electrons. The whole set up is as shown in the above figure. SECA1701-Microwave and Optical communication

10. Cavity Klystron The electrons emitted by the cathode are accelerated towards the first resonator. The collector at the end is at the same potential as the resonator. Hence, usually the electrons have a constant speed in the gap between the cavity resonators. Initially, the first cavity resonator is supplied with a weak high frequency signal, which has to be amplified. The signal will initiate an electromagnetic field inside the cavity. This signal is passed through a coaxial cable as shown in the given figure. Due to this field, the electrons that pass through the cavity resonator are modulated. On arriving at the second resonator, the electrons are induced with another EMF at the same frequency. This field is strong enough to extract a large signal from the second cavity. SECA1701-Microwave and Optical communication

11. Cavity Resonator First let us try to understand the constructional details and the working of a cavity resonator. The following figure indicates the cavity resonator. A simple resonant circuit which consists of a capacitor and an inductive loop can be compared with this cavity resonator. A conductor has free electrons. If a charge is applied to the capacitor to get it charged to a voltage of this polarity, many electrons are removed from the upper plate and introduced into the lower plate. The plate that has more electron deposition will be the cathode and the plate which has lesser number of electrons becomes the anode. The following figure shows the charge deposition on the capacitor SECA1701-Microwave and Optical communication

12. Cavity Resonator F igure shows the charge deposition on the capacitor The electric field lines are directed from the positive charge towards the negative. If the capacitor is charged with reverse polarity, then the direction of the field is also reversed. The displacement of electrons in the tube, constitutes an alternating current. This alternating current gives rise to alternating magnetic field, which is out of phase with the electric field of the capacitor. When the magnetic field is at its maximum strength, the electric field is zero and after a while, the electric field becomes maximum while the magnetic field is at zero. This exchange of strength happens for a cycle. SECA1701-Microwave and Optical communication

13. Closed Resonator The smaller the value of the capacitor and the inductivity of the loop, the higher will be the oscillation or the resonant frequency. As the inductance of the loop is very small, high frequency can be obtained. To produce higher frequency signal, the inductance can be further reduced by placing more inductive loops in parallel as shown in the figure. This results in the formation of a closed resonator having very high frequencies. In a closed resonator, the electric and magnetic fields are confined to the interior of the cavity. The first resonator of the cavity is excited by the external signal to be amplified. This signal must have a frequency at which the cavity can resonate. The current in this coaxial cable sets up a magnetic field, by which an electric field originates. SECA1701-Microwave and Optical communication

14. Working of Klystron To understand the modulation of the electron beam, entering the first cavity, let's consider the electric field. The electric field on the resonator keeps on changing its direction of the induced field. Depending on this, the electrons coming out of the electron gun, get their pace controlled. As the electrons are negatively charged, they are accelerated if moved opposite to the direction of the electric field. Also, if the electrons move in the same direction of the electric field, they get decelerated. This electric field keeps on changing, therefore the electrons are accelerated and decelerated depending upon the change of the field. The figure indicates the electron flow when the field is in the opposite direction. SECA1701-Microwave and Optical communication

15. Formation of Electron Bunches While moving, these electrons enter the field free space called as the  drift space  between the resonators with varying speeds, which create electron bunches. These bunches are created due to the variation in the speed of travel. These bunches enter the second resonator, with a frequency corresponding to the frequency at which the first resonator oscillates. As all the cavity resonators are identical, the movement of electrons makes the second resonator to oscillate. The figure shows the formation of electron bunches. SECA1701-Microwave and Optical communication

16. The induced magnetic field in the second resonator induces some current in the coaxial cable, initiating the output signal. The kinetic energy of the electrons in the second cavity is almost equal to the ones in the first cavity and so no energy is taken from the cavity. The electrons while passing through the second cavity, few of them are accelerated while bunches of electrons are decelerated. Hence, all the kinetic energy is converted into electromagnetic energy to produce the output signal. Amplification of such two-cavity Klystron is low and hence multi-cavity Klystrons are used. The figure depicts an example of multi-cavity Klystron amplifier. SECA1701-Microwave and Optical communication

17. M ulti-cavity Klystron amplifier With the signal applied in the first cavity, we get weak bunches in the second cavity. These will set up a field in the third cavity, which produces more concentrated bunches and so on. Hence, the amplification is larger. This microwave generator, is a Klystron that works on reflections and oscillations in a single cavity, which has a variable frequency. Reflex Klystron consists of an electron gun, a cathode filament, an anode cavity, and an electrode at the cathode potential. It provides low power and has low efficiency. SECA1701-Microwave and Optical communication

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20. Construction of Reflex Klystron The electron gun emits the electron beam, which passes through the gap in the anode cavity. These electrons travel towards the Repeller electrode, which is at high negative potential. Due to the high negative field, the electrons repel back to the anode cavity. In their return journey, the electrons give more energy to the gap and these oscillations are sustained It is assumed that oscillations already exist in the tube and they are sustained by its operation. The electrons while passing through the anode cavity, gain some velocity. The constructional details of this reflex klystron is as shown in the above figure. SECA1701-Microwave and Optical communication

21. Reflex Klystron SECA1701-Microwave and Optical communication

22. Operation of Reflex Klystron The operation of Reflex Klystron is understood by few assumptions. The electron beam is accelerated towards the anode cavity. Let us assume that a reference electron  e r  crosses the anode cavity but has no extra velocity and it repels back after reaching the Repeller electrode, with the same velocity. Another electron, let's say  e e  which has started earlier than this reference electron, reaches the Repeller first, but returns slowly, reaching at the same time as the reference electron. We have another electron, the late electron  e l , which starts later than both  e r  and  e e , however, it moves with greater velocity while returning back, reaching at the same time as er and ee . Now, these three electrons, namely  e r ,  e e  and  e l  reach the gap at the same time, forming an  electron bunch . This travel time is called as  transit time , which should have an optimum value. SECA1701-Microwave and Optical communication

23. Operation of Reflex Klystron The anode cavity accelerates the electrons while going and gains their energy by retarding them during the return journey. When the gap voltage is at maximum positive, this lets the maximum negative electrons to retard. The optimum transit time is represented as T=n + …….Where ‘n’ is an integer This transit time depends upon the Repeller and anode voltages.   SECA1701-Microwave and Optical communication

24. Applications of Reflex Klystron Reflex Klystron is used in applications where variable frequency is desirable, such as − Radio receivers Portable microwave links Parametric amplifiers Local oscillators of microwave receivers As a signal source where variable frequency is desirable in microwave generators SECA1701-Microwave and Optical communication

25. Magnetron Unlike the tubes discussed so far, Magnetrons are the cross-field tubes in which the electric and magnetic fields cross, i.e. run perpendicular to each other. In TWT, it was observed that electrons when made to interact with RF, for a longer time, than in Klystron, resulted in higher efficiency. The same technique is followed in Magnetrons. SECA1701-Microwave and Optical communication

26. Types of Magnetrons There are three main types of Magnetrons. Negative Resistance Type The negative resistance between two anode segments, is used. They have low efficiency. They are used at low frequencies  <500MHz Cyclotron Frequency Magnetrons The synchronism between the electric component and oscillating electrons is considered Useful for frequencies higher than 100MHz . Travelling Wave or Cavity Type The interaction between electrons and rotating EM field is taken into account. High peak power oscillations are provided. Useful in radar applications. SECA1701-Microwave and Optical communication

27. Cavity Magnetron The Magnetron is called as Cavity Magnetron because the anode is made into resonant cavities and a permanent magnet is used to produce a strong magnetic field, where the action of both of these make the device work. Construction of Cavity Magnetron A thick cylindrical cathode is present at the center and a cylindrical block of copper, is fixed axially, which acts as an anode. This anode block is made of a number of slots that acts as resonant anode cavities. The space present between the anode and cathode is called as  Interaction space . The electric field is present radially while the magnetic field is present axially in the cavity magnetron. This magnetic field is produced by a permanent magnet, which is placed such that the magnetic lines are parallel to cathode and perpendicular to the electric field present between the anode and the cathode. The figures show the constructional details of a cavity magnetron and the magnetic lines of flux present, axially. SECA1701-Microwave and Optical communication

28. Cavity Magnetron This Cavity Magnetron has 8 cavities tightly coupled to each other. An N-cavity magnetron has N modes of operations. These operations depend upon the frequency and the phase of oscillations. The total phase shift around the ring of this cavity resonators should be 2n 𝝿.where n is an integer. If ϕ v represents the relative phase change of the AC electric field across adjacent cavities, then ϕ v=2 π n/N Where,n=0,±1,±2,±(N2−1),±N2 Which means that N/2 mode of resonance can exist if  N is an even number If n=N/2 then ϕ v= π This mode of resonance is called as π - mode n=0 then ϕ v=0 This is called as the  Zero mode , because there will be no RF electric field between the anode and the cathode. This is also called as  Fringing Field  and this mode is not used in magnetrons SECA1701-Microwave and Optical communication

29. Operation of Cavity Magnetron When the Cavity Klystron is under operation, we have different cases to consider. Let us go through them in detail. Case 1 If the magnetic field is absent, i.e. B = 0, then the behavior of electrons can be observed in the following figure. Considering an example, where electron  a  directly goes to anode under radial electric force. SECA1701-Microwave and Optical communication

30. Operation of Cavity Magnetron Case: 2 If there is an increase in the magnetic field, a lateral force acts on the electrons. This can be observed in the figure, considering electron  b  which takes a curved path, while both forces are acting on it. Radius of this path is calculated as R = mv/ eB It varies proportionally with the velocity of the electron and it is inversely proportional to the magnetic field strength SECA1701-Microwave and Optical communication

31. Operation of Cavity Magnetron Case 3 If the magnetic field  B  is further increased, the electron follows a path such as the electron  c , just grazing the anode surface and making the anode current zero. This is called as " Critical magnetic field “ ( Bc ) which is the cut-off magnetic field SECA1701-Microwave and Optical communication

32. Operation of Cavity Magnetron Case 4 If the magnetic field is made greater than the critical field, B> Bc Then the electrons follow a path as electron  d , where the electron jumps back to the cathode, without going to the anode. This causes " back heating " of the cathode. This is achieved by cutting off the electric supply once the oscillation begins. If this is continued, the emitting efficiency of the cathode gets affected. SECA1701-Microwave and Optical communication

33. Operation of Cavity Magnetron with Active RF Field We have discussed so far the operation of cavity magnetron where the RF field is absent in the cavities of the magnetron staticase. Let us now discuss its operation when we have an active RF field. As in TWT, let us assume that initial RF oscillations are present, due to some noise transient. The oscillations are sustained by the operation of the device. There are three kinds of electrons emitted in this process, whose actions are understood as electrons  a ,  b  and  c , in three different cases. Case 1 When oscillations are present, an electron  a , slows down transferring energy to oscillate. Such electrons that transfer their energy to the oscillations are called as  favored electrons . These electrons are responsible for  bunching effect . SECA1701-Microwave and Optical communication

34. Case 2 In this case, another electron, say  b , takes energy from the oscillations and increases its velocity. As and when this is done, It bends more sharply. It spends little time in interaction space. It returns to the cathode. These electrons are called as  unfavored electrons . They don't participate in the bunching effect. Also, these electrons are harmful as they cause "back heating". Case 3 In this case, electron  c , which is emitted a little later, moves faster. It tries to catch up with electron  a . The next emitted electron  d , tries to step with  a . As a result, the favored electrons  a ,  c  and  d  form electron bunches or electron clouds. It called as "Phase focusing effect". This whole process is understood better by taking a look at the following figure. SECA1701-Microwave and Optical communication

35. Figure A shows the electron movements in different cases while figure B shows the electron clouds formed. These electron clouds occur while the device is in operation. The charges present on the internal surface of these anode segments, follow the oscillations in the cavities. This creates an electric field rotating clockwise, which can be actually seen while performing a practical experiment. While the electric field is rotating, the magnetic flux lines are formed in parallel to the cathode, under whose combined effect, the electron bunches are formed with four spokes, directed in regular intervals, to the nearest positive anode segment, in spiral trajectories. SECA1701-Microwave and Optical communication

36. Microwave solid state Devices Solid State Devices The classification of solid state Microwave devices can be done − Depending upon their electrical behavior Non-linear resistance type. Example − Varistors  variable resistances Non-Linear reactance type. Example − Varactors  variable reactors Negative resistance type. Example − Tunnel diode, Impatt diode, Gunn diode Controllable impedance type. Example − PIN diode Depending upon their construction Point contact diodes Schottky barrier diodes Metal Oxide Semiconductor devices  MOS Metal insulation devices T he types of diodes which we have mentioned here have many uses such as amplification, detection, power generation, phase shifting, down conversion, up conversion, limiting modulation, switching, etc. SECA1701-Microwave and Optical communication

37. PIN Diode SECA1701-Microwave and Optical communication

38. PIN Diode A  PIN diode  is a  diode  with a wide, undoped  intrinsic semiconductor  region between a  p-type semiconductor  and an  n-type semiconductor  region. The p-type and n-type regions are typically heavily  doped  because they are used for  ohmic contacts . The wide intrinsic region is in contrast to an ordinary  p–n diode . The wide intrinsic region makes the PIN diode an inferior  rectifier  (one typical function of a diode), but it makes it suitable for attenuators, fast switches, photodetectors, and high-voltage power electronics applications SECA1701-Microwave and Optical communication

39. The  PIN diode  is  used as  a photo detector to convert the light into the current which takes place in the depletion layer of a photo  diode , Ri sing the depletion layer by inserting the intrinsic layer progresses the performance by increasing the volume in, where light change occurs. The  microwave  signal is amplitude-modulated using the  PIN modulator . When limited to a  modulation  frequency, detection can be carried out in very narrow bandwidths. The  PIN modulator  can be used in the linear characteristic range as an analog amplitude  modulator  and in switching mode for digital  modulation . SECA1701-Microwave and Optical communication

40. In a PIN diode the depletion region exists almost completely within the intrinsic region. This depletion region is much larger than in a PN diode and almost constant-size, independent of the reverse bias applied to the diode. This increases the volume where electron-hole pairs can be generated by an incident photon. Some  photodetector devices, such as PIN photodiodes and phototransistors (in which the base-collector junction is a PIN diode), use a PIN junction in their construction. The diode design has some design trade-offs. Increasing the area of the intrinsic region increases its stored charge reducing its RF on-state resistance, while also increasing reverse bias capacitance and increasing the drive current required to remove the charge during a fixed switching time, with no effect on the minimum time required to sweep the charge from the’ I’ region. SECA1701-Microwave and Optical communication

41. Increasing the thickness of the intrinsic region increases the total stored charge, decreases the minimum RF frequency, and decreases the reverse-bias capacitance, but doesn't decrease the forward-bias RF resistance and increases the minimum time required to sweep the drift charge and transition from low to high RF resistance. Diodes are sold commercially in a variety of geometries for specific RF bands A PIN diode operates under what is known as  high-level injection . In other words, the intrinsic " i " region is flooded with charge carriers from the "p" and "n" regions. Its function can be linked to filling up a water bucket with a hole on the side. Once the water reaches the hole's level it will begin to pour out. SECA1701-Microwave and Optical communication

42. Similarly, the diode will conduct current once the flooded electrons and holes reach an equilibrium point, where the number of electrons is equal to the number of holes in the intrinsic region. When the diode is  forward biased the injected carrier concentration is typically several orders of magnitude higher than the intrinsic carrier concentration. Due to this high level injection, which in turn is due to the  depletion process , the electric field extends deeply (almost the entire length) into the region. This electric field helps in speeding up of the transport of charge carriers from the P to the N region, which results in faster operation of the diode, making it a suitable device for high-frequency operation. SECA1701-Microwave and Optical communication

43. Characteristics of PIN diode The  PIN diode  obeys the standard  diode  equation for low-frequency signals. At higher frequencies, the diode looks like an almost perfect (very linear, even for large signals) resistor. The P-I-N diode has a relatively large stored charge adrift in a thick  intrinsic region . At a low-enough frequency, the stored charge can be fully swept and the diode turns off. At higher frequencies, there is not enough time to sweep the charge from the drift region, so the diode never turns off. The time required to sweep the stored charge from a diode junction is its  reverse recovery time , and it is relatively long in a PIN diode. For a given semiconductor material, on-state impedance, and minimum usable RF frequency, the reverse recovery time is fixed. This property can be exploited; one variety of P-I-N diode, the  step recovery diode, exploits the abrupt impedance change at the end of the reverse recovery to create a narrow impulse waveform useful for  frequency multiplication with high multiples. The high-frequency resistance is inversely proportional to the DC bias current through the diode. A PIN diode, suitably biased, therefore acts as a variable resistor. This high-frequency resistance may vary over a wide range (from 0.1 Ω to 10 kΩ  in some cases , the useful range is smaller, though). The wide intrinsic region also means the diode will have a low capacitance when  reverse-biased SECA1701-Microwave and Optical communication

44. Operation A microwave PIN diode is a semiconductor device that operates as a variable resistor at RF and microwave frequencies A PIN diode is a current controlled device in contrast to a varactor diode which is a voltage controlled device When the forward bias control current of the PIN diode is varied continuously ,it can be used for attenuating ,levelling and amplitude modulating an RF signal When the control current is switched on and off, or in discrete steps, the device can be used for switching, pulse modulating, and phase shifting an RF signal . SECA1701-Microwave and Optical communication

45. Characteristics of PIN diode as an RF switch Under zero or reverse bias, a PIN diode has a low  capacitance . The low  capacitance  will not pass much of an RF signal. Under a forward bias of 1 mA, a typical PIN diode will have an RF resistance of about 1 ohm, making it a good RF conductor. Consequently, the PIN diode makes a good RF switch. The microwave PIN diode's small physical size compared to a wavelength, high switching speed, and low package parasitic reactance, make it an ideal component for use in miniature, broadband RF signal control circuits.  PIN diode has the ability to control large RF signal power while using much smaller levels of control power. In modulator circuit, PIN diode’s minority carrier provide a low level of RF Intermodulation Distortion. (switching speed ) SECA1701-Microwave and Optical communication

46. Application of PIN Diode PIN diode is used primarily in UHF and microwave applications. They are also used as RF switches in many amateur radio systems. The low reverse capacitance and current controlled resistance make it ideal for high frequency communication circuits Most common applications are RF switches M odulators  A ttenuators   photodetectors phase shifters SECA1701-Microwave and Optical communication

47. Tunnel Diode The  tunnel diode  is a type of  microwave  semiconductor  diode  that can be used in oscillators and also amplifiers. Rather than using the standard physics of the ordinary PN junction, the  tunnel diode  uses a quantum mechanical effect called tunnelling – from which it gains its name. The oscillator circuit that is built using a tunnel diode is called as a Tunnel diode oscillator. If the impurity concentration of a normal PN junction is highly increased, this  Tunnel diode  is formed. It is also known as  Esaki diode , after its inventor. SECA1701-Microwave and Optical communication

48. When the impurity concentration in a diode increases, the width of depletion region decreases, extending some extra force to the charge carriers to cross the junction. When this concentration is further increased, due to less width of the depletion region and the increased energy of the charge carriers, they penetrate through the potential barrier, instead of climbing over it. This penetration can be understood as  Tunneling  and hence the name,  Tunnel diode SECA1701-Microwave and Optical communication

49. Tunnel Diode Oscillator The tunnel diode helps in generating a very high frequency signal of nearly 10GHz. A practical tunnel diode circuit may consist of a switch S, a resistor R and a supply source V, connected to a tank circuit through a tunnel diode D. Working The value of resistor selected should be in such a way that it biases the tunnel diode in the midway of the negative resistance region A practical tunnel diode oscillator circuit is shown in figure. SECA1701-Microwave and Optical communication

50. In this circuit, the resistor R 1  sets proper biasing for the diode and the resistor R 2  sets proper current level for the tank circuit. The parallel combination of resistor R p  inductor L and capacitor C form a tank circuit, which resonates at the selected frequency. When the switch S is closed, the circuit current rises immediately towards the constant value, whose value is determined by the value of resistor R and the diode resistance. However, as the voltage drop across the tunnel diode V D  exceeds the peak-point voltage V p , the tunnel diode is driven into negative resistance region. In this region, the current starts decreasing, till the voltage V D  becomes equal to the valley point voltage V v . At this point, a further increase in the voltage V D  drives the diode into positive resistance region. As a result of this, the circuit current tends to increase. This increase in circuit will increase the voltage drop across the resistor R which will reduce the voltage V D . SECA1701-Microwave and Optical communication

51. V-I characteristic curve The curve AB indicates the negative resistance region as the resistance decreases while the voltage increases. It is clear that the Q-point is set at the middle of the curve AB. The Q-point can move between the points A and B during the circuit operation. The point A is called  peak point  and the point B is called  valley point . During the operation, after reaching the point B, the increase in circuit current will increase the voltage drop across the resistor R which will reduce the voltage V D . This brings the diode back into negative resistance region. The decrease in voltage V D  is equal to the voltage V P  and this completes one cycle of operation. The continuation of these cycles produces continuous oscillations which give a sinusoidal output . SECA1701-Microwave and Optical communication

52. Advantages The advantages of a tunnel diode oscillator are as follows − It has high switching speeds. It can handle high frequencies. Disadvantages The disadvantages of a tunnel diode oscillator are as follows − They are low power devices. Tunnel diodes are a bit costly. Applications The applications of a tunnel diode oscillator are as follows − It is used in relaxation oscillators. It is used in microwave oscillators. It is also used as Ultra high speed switching device. It is used as logic memory storage device. SECA1701-Microwave and Optical communication

53. Gunn Diode Gunn Effect Devices J B Gunn discovered periodic fluctuations of current passing through the  n-type GaAs  specimen when the applied voltage exceeded a certain critical value. In these diodes, there are two valleys,  L & U valleys  in conduction band and the electron transfer occurs between them, depending upon the applied electric field. This effect of population inversion from lower L-valley to upper U-valley is called  Transfer Electron Effect. Hence these are called Transfer Electron Devices. The examples of the devices that come under this category are IMPATT, TRAPATT and BARITT diodes. SECA1701-Microwave and Optical communication

54. Gunn Diode A diode is a two-terminal semiconductor  electronic component  that exhibits nonlinear current-voltage characteristics. It allows current in one direction at which its resistance is very low (almost zero resistance) during forward bias. Similarly, in the other direction, it doesn’t allow the flow of current – as it offers a very-high resistance (infinite resistance acts as open circuit) during reverse bias The  diodes are classified into different types  based on their working principles and characteristics. These include Generic diode, Schotty diode, Shockley diode, Constant-current diode,  Zener diode Light emitting diode, Photodiode, Tunnel diode, Varactor, Vacuum tube, Laser diode, PIN diode, Peltier diode, Gunn diode, and so on. SECA1701-Microwave and Optical communication

55. Gunn Diode A Gunn Diode is considered as a type of diode even though it does not contain any typical PN diode junction like the other diodes, but it consists of two electrodes. This diode is also called as a Transferred Electronic Device. This diode is a negative differential resistance device, which is frequently used as a low-power oscillator to generate  microwaves. It consists of only N-type semiconductor in which electrons are the majority charge carriers. To generate short radio waves such as microwaves, it utilizes the Gunn Effect. T he central region shown in the figure is an active region, which is properly doped N-type GaAs and epitaxial layer with a thickness of around 8 to 10 micrometers. The active region is sandwiched between the two regions having the Ohmic contacts. A heat sink is provided to avoid overheating and premature failure of the diode and to maintain thermal limits. For the construction of these diodes, only N-type material is used, which is due to the transferred electron effect applicable only to N-type materials and is not applicable to the P-type materials. The frequency can be varied by varying the thickness of the active layer while doping SECA1701-Microwave and Optical communication

56. Gunn Effect It was invented by John Battis combe Gunn in 1960s; after his experiments on GaAs (Gallium Arsenide), he observed a noise in his experiments’ results and owed this to the generation of electrical oscillations at microwave frequencies by a steady electric field with a magnitude greater than the threshold value. It was named as Gunn Effect after this had been discovered by John Battiscombe Gunn. The Gunn Effect can be defined as generation of microwave power (power with microwave frequencies of around a few GHz) whenever the voltage applied to a semiconductor device exceeds the critical voltage value or threshold voltage value. SECA1701-Microwave and Optical communication

57. Gunn Diode Oscillator Gunn diodes are used to build oscillators for generating microwaves with frequencies ranging from 10 GHz to THz. It is a Negative Differential Resistance device It is also called as transferred  electron device oscillator   which is a tuned circuit consisting of Gunn diode with DC bias voltage applied to it. And, this is termed as biasing the diode into negative resistance region. Due to this, the total differential resistance of the circuit becomes zero as the negative resistance of the diode cancels with the positive resistance of the circuit resulting in the generation of oscillations . SECA1701-Microwave and Optical communication

58. Gunn Diode’s Working This diode is made of a single piece of  N-type semiconductor  such as Gallium Arsenide and InP (Indium Phosphide). GaAs and some other semiconductor materials have one extra-energy band in their electronic band structure instead of having only two energy bands, viz. valence band and conduction band like normal semiconductor materials. These GaAs and some other semiconductor materials consist of three energy bands, and this extra third band is empty at initial stage. If a voltage is applied to this device, then most of the applied voltage appears across the active region. The electrons from the conduction band having negligible electrical resistivity are transferred into the third band because these electrons are scattered by the applied voltage. The third band of GaAs has mobility which is less than that of the conduction band. Because of this, an increase in the forward voltage increases the field strength (for field strengths where applied voltage is greater than the threshold voltage value), then the number of electrons reaching the state at which the effective mass increases by decreasing their velocity, and thus, the current will decrease. Thus, if the field strength is increased, then the drift velocity will decrease; this creates a negative incremental resistance region in V-I relationship. Thus, increase in the voltage will increase the resistance by creating a slice at the cathode and reaches the anode. But, to maintain a constant voltage, a new slice is created at the cathode. Similarly, if the voltage decreases, then the resistance will decrease by extinguishing any existing slice. SECA1701-Microwave and Optical communication

59. Gunn Diode’s Characteristics The current-voltage relationship characteristics of a Gunn diode are shown in the above graph with its negative resistance region. These characteristics are similar to the characteristics of the tunnel diode. As shown in the above graph, initially the current starts increasing in this diode, but after reaching a certain voltage level (at a specified voltage value called as threshold voltage value), the current decreases before increasing again. The region where the current falls is termed as a negative resistance region, and due to this it oscillates. In this negative resistance region, this diode acts as both oscillator and amplifier, as in this region, the diode is enabled to amplify signals . SECA1701-Microwave and Optical communication

60. Among the Microwave measurement devices, a setup of Microwave bench, which consists of Microwave devices has a prominent place. This whole setup, with few alternations, is able to measure many values like guide wavelength, free space wavelength, cut-off wavelength, impedance, frequency, VSWR, Klystron characteristics, Gunn diode characteristics, power measurements, etc. The output produced by microwaves, in determining power is generally of a little value. They vary with the position in a transmission line. There should be an equipment to measure the Microwave power, which in general will be a Microwave bench setup. SECA1701-Microwave and Optical communication

61. Gunn Diode’s Applications Used as Gunn oscillators to generate frequencies ranging from 100mW 5GHz to 1W 35GHz outputs. These Gunn oscillators are used for  radio communications military and commercial radar sources. Used as sensors for detecting trespassers, to avoid derailment of trains. Used as efficient microwave generators with a frequency range of up to hundreds of GHz. Used for remote vibration detectors and rotational speed measuring  tachometers Used as a microwave current generator (Pulsed Gunn diode generator). Used in microwave transmitters to generate microwave radio waves at very low powers. Used as fast controlling components in microelectronics such as for the modulation of semiconductor injection lasers. Used as sub-millimeter wave applications by multiplying Gunn oscillator frequency with diode frequency. Some other applications include door opening sensors, process control devices, barrier operation, perimeter protection, pedestrian safety systems, linear distance indicators, level sensors, moisture content measurement and intruder alarms . SECA1701-Microwave and Optical communication

62. Applications of Gunn Diodes Gunn diodes are extensively used in the following devices − Radar transmitters Transponders in air traffic control Industrial telemetry systems Power oscillators Logic circuits Broadband linear amplifier The process of having a delay between voltage and current, in avalanche together with transit time, through the material is said to be Negative resistance. The devices that helps to make a diode exhibit this property are called as  Avalanche transit time devices . SECA1701-Microwave and Optical communication

63. IMPATT Diode IMPATT diode  is a very high power semiconductor device that is utilized for microwave applications. It is basically used as oscillator and amplifier at microwave frequencies. The operating range of the  IMPATT diode  lies in the range of 3 to 100 GHz. This is a high-power semiconductor diode, used in high frequency microwave applications. The full form IMPATT is  Impact ionization Avalanche Transit Time diode . SECA1701-Microwave and Optical communication

64. IMPATT Diode A voltage gradient when applied to the IMPATT diode, results in a high current. A normal diode will eventually breakdown by this. However, IMPATT diode is developed to withstand all this. A high potential gradient is applied to back bias the diode and hence minority carriers flow across the junction. Application of a RF AC voltage if superimposed on a high DC voltage, the increased velocity of holes and electrons results in additional holes and electrons by thrashing them out of the crystal structure by Impact ionization. SECA1701-Microwave and Optical communication

65. IMPATT Diode I t consists of 4 regions namely P + -N-I-N + The structure of the IMPATT diode is somewhat similar to the PIN diode. However, it operates on a very high voltage gradient of around  400KV/cm , so as to produce avalanche current. Generally, materials like  GaAs ,  Si ,  Ge  or  InP  are used for its construction. However,  GaAs  is preferred because of its low noise behaviour . Basically, it uses a slightly different structure from a normal diode. Because we know that a normal PN junction diode breaks down under avalanche condition. As the generation of a large amount of current causes the generation of heat inside it. So variation in construction is adopted to produce RF signals at microwave frequencies SECA1701-Microwave and Optical communication

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67. Working of IMPATT Diode T hese diodes operate on the principle of avalanche breakdown and transit time delay. Avalanche Condition(breakdown) . An action that causes an abrupt increase in the junction current in reverse biased condition of pn junction diode leading to junction breakdown is known as avalanche breakdown. W. k .T In reverse biased condition the width of the depletion region becomes extremely thick. Due to which only minority carriers drift across the junction. In the presence of a high electric field, the mobile charge carriers move with greater velocity. During their movement, the high-velocity carriers collide with other atoms in the crystal and generates electron-hole pairs. This causes multiplication of charge carriers inside the crystal structure . SECA1701-Microwave and Optical communication

68. Thus the moving charges generate high current inside the device. This is known as  avalanche condition or impact ionization  and is utilized in IMPATT diodes. It is to be noted here that the overall external field provided to the diode is the summation of RF ac signal and dc voltage. Initially when ac voltage is 0 then due to applied low dc voltage, a very small amount of current flows through the diode. This current is generally known as  pre-breakdown current . But as the applied potential increases then the electric field inside the diode increases. And as we have already discussed that with an increase in the electric field there will be an increase in the number of generated electron-hole pairs due to impact ionization. The above figure clearly shows the avalanche region and drift space in the structure of the diode. The increase in  superimposed ac field and dc potential  causes the electrons in the p+ region to get injected into the I region in order to drift towards n+ region. This is so because with the increase in the applied field the electrons will move towards the anode and holes towards the cathode. SECA1701-Microwave and Optical communication

69. The moving electrons cause charge multiplication in the presence of a high electric field. By this time the ac field now starts approaching 0 but due to secondary charge generation, the concentration of electrons in the avalanche region will be extremely high . This shows a phase shift of 90⁰ now gets generated between the ac input signal and concentration of charge carriers in the avalanche region. Thus while drifting from avalanche region to anode, the electrons generate high current with a phase opposite to that of the applied ac signal. During the negative half of the ac signal, even the dc potential is high, still, the reduction in the overall electric field will cause decay in the concentration of carriers present in the avalanche region. Thus the current flowing through it also gets reduced. SECA1701-Microwave and Optical communication

70. In order to have the desired phase shift between the ac signal and diode current, the thickness of the drift region must be properly selected. The thickness of the drift region must be such that the electron bunch must be collected at the anode till the time ac voltage is approaching 0. Thereby providing a phase shift of 90⁰. This is so because the thickness of the drift region decides the time taken by the carriers to reach the respective electrode. Though all the carriers travel unequal distance while approaching the electrode. But the introduced phase-shift due to drifting generates negative resistance. The figure below represents the negative resistance characteristic of IMPATT diode with respect to transit angle. Hence in this way current through an IMPATT diode is generated. SECA1701-Microwave and Optical communication

71. The efficiency of IMPATT diode is represented as SECA1701-Microwave and Optical communication

72. Advantages & Disadvantages, application A dvantages It provides high operating range. It shows compactness in size. IMPATT diodes are economical. It provides reliable operation at high temperature Disadvantages It is noisy as avalanche is a noisy process The rate of generation of electron-hole pair in the avalanche region causes the generation of high noise. Thus makes the system noisy. It offers a low tuning range. It offers high sensitivity to different operating conditions. Tuning range is not as good as in Gunn diodes Applications Microwave oscillator Microwave generators Modulated output oscillator Receiver local oscillator Negative resistance amplifications Intrusion alarm networks  Police radar  Low power microwave transmitter  FM telecom transmitter  CW Doppler radar transmitter  SECA1701-Microwave and Optical communication

73. Differences between IMPATT &TRAPATT The TRAPATT diode is normally used as a microwave oscillator. It has the advantage of a greater level of  efficiency  when compared to an IMPATT microwave diode. Typically the DC to RF signal conversion  efficiency  may be in the region of 20% to 60% which is high. TRAPATT . A microwave oscillator device with a similar structure to the  IMPATT  diode is the  TRAPATT  diode, which stands for "trapped plasma avalanche triggered transit". This mode of operation produces relatively high power and efficiency, but at lower frequency than a device operated in  IMPATT  mode. SECA1701-Microwave and Optical communication

74. TRAPATT Diode The full form of TRAPATT diode is Trapped Plasma Avalanche Triggered Transit diode. A microwave generator which operates between hundreds of MHz to GHz. These are high peak power diodes usually n+- p-p+ or p+-n-n+ structures with n-type depletion region, width varying from 2.5 to 1.25 µm. The  TRAPAT  belongs to the same basic family as the IMPATT  diode  but it provides a number of advantages in microwave applications. The  TRAPATT diode  is normally used as a microwave oscillator . SECA1701-Microwave and Optical communication

75. TRAPATT construction The electrons and holes trapped in low field region behind the zone, are made to fill the depletion region in the diode. This is done by a high field avalanche region which propagates through the diode. The following figure shows a graph in which AB shows charging, BC shows plasma formation, DE shows plasma extraction, EF shows residual extraction, and FG shows charging. SECA1701-Microwave and Optical communication

76. Let us see what happens at each of the points. A:  The voltage at point A is not sufficient for the avalanche breakdown to occur. At A, charge carriers due to thermal generation results in charging of the diode like a linear capacitance. A-B:  At this point, the magnitude of the electric field increases. When a sufficient number of carriers are generated, the electric field is depressed throughout the depletion region causing the voltage to decrease from B to C. C:  This charge helps the avalanche to continue and a dense plasma of electrons and holes is created. The field is further depressed so as not to let the electrons or holes out of the depletion layer, and traps the remaining plasma. D:  The voltage decreases at point D. A long time is required to clear the plasma as the total plasma charge is large compared to the charge per unit time in the external current. E:  At point E, the plasma is removed. Residual charges of holes and electrons remain each at one end of the deflection layer. E to F:  The voltage increases as the residual charge is removed. F:  At point F, all the charge generated internally is removed. F to G:  The diode charges like a capacitor. G:  At point G, the diode current comes to zero for half a period. The voltage remains constant as shown in the graph above. This state continues until the current comes back on and the cycle repeats. SECA1701-Microwave and Optical communication

77. The avalanche zone velocity Vs is represented as SECA1701-Microwave and Optical communication

78. Advantages &Disadvantages Advantages of TRAPATT diode It offers higher efficiency compare to IMPATT diode. Efficiency of about 15 to 40 % can be achieved. It has very low power dissipation. It is most suitable for pulsed operation. It can operate from 3 to 50 GHz. D isadvantages of TRAPATT diode It is not used for continuous operation mode as it offers high power densities i.e. 10 to 10 2  W/m 2 . It has very high noise figure which is about 60 dB. It supports frequencies below millimeter band. SECA1701-Microwave and Optical communication

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