ECE_Diode____Basic Electronics_2024.pptx

basic electronics

1. Basic Electronics Dr. Prasanta Kumar Guha Autumn Semester(2024) [email protected]

2. References ‘Microelectronic Circuits’ – Sedra Smith (Sixth Edition) ‘Electronic circuits analysis and design’ – Donald A Neamen (Tata Mcgraw Hill) ‘Millman’s Integrated Electronics Analog Digital Circuits and Systems’- J. Millman, C. Halkias and C Parikh (Tata Mcgraw Hill) ‘Electronic Principles’- A. P. Malvino ‘Fundamentals of Microelectronics’ - Razavi IIT Kharagpur, Autumn 2024 2

3. Class Test 1 -7.5 Mid sem - 30 Class Test 2 - 7.5 End sem - 50 Attendance - 5 IIT Kharagpur, Autumn 2024 3

4. Intro to electronics-signal, noise, system (example), RL, RC filter circuit 2) Intrinsic, extrinsic semiconductor, drift & diffusion current, p-n junction, forward bias/ reverse bias, I-V equation (without proof), diode model (ideal, piece wise linear etc), zener diode characteristics 3) half wave, full wave rectifier, bridge rectifier, ripple, zener diode circuit (voltage ref, regulation), filter, clipper, clamper, multi diode circuit 4) BJT Basics, alpha-beta relation, IV equation (no proof) with different regions, DC circuit analysis, common emitter circuit with and without emitter resistor 5) BJT amplifier, load line, Q point, small signal equivalent circuit, common emitter amplifier (gain, input resistance, output resistance). 6) MOSFET basic structure, IV equation (no proof) with different regions, depletion mode, enhancement mode, channel length modulation, DC circuit analysis, common source circuit with and without source resistor 7) OPAMP basic, virtual ground, ideal properties, inverting, non inverting, buffer, differential amplifier, CMRR (all these with ideal and non ideal OPAMP gain), integrator, differentiator. 8) Digital electronics- number system, Digital gates (symbol, truth table), universal gate, sum of product, product of sum, Karnaugh map, RS/D/T Flip Flop. IIT Kharagpur, Autumn 2024 4

5. Introduction Electronics : Motion of charges in a medium (gas, vacuum, semiconductor) Microelectronics : It refers to integrated circuit technology which can produce millions of transistors on a single piece of silicon Nanoelectronics : It refers to the use of nanotechnology on electronic components < 100 nm - very small structures: inter-atomic interactions (e.g. CNT, silicon nano pillars, nanowires) IIT Kharagpur, Autumn 2024 5

6. History Three milestones Vacuum tube (1883, Thomas Alva Edison) - diode, triode...radio, computer...WRI, WRII Transistor (1947, John Bardeen, Walter Brattain, and William Shockley) - functions like the vacuum tube, but it is tiny, weighs less, consumes less power, is much more reliable, and is cheaper to manufacture with its combination of metal contacts and semiconductor materials. Integrated circuits (1958, Jack Kilbi ...Texas Instrument...IC in germanium Robert Noyce ...Fairchild Semiconductor...IC in silicon) IIT Kharagpur, Autumn 2024 6

7. Introduction... Moore’s law – The number of transistors incorporated in a chip will approximately double every 24 months - predicted in 1965 IIT Kharagpur, Autumn 2024 7 Moore's Law - Computer History Museum

8. Electronic Signals Analogue signal: Signal level may have any value (e.g. usually real world signals) -can have infinite amount of signal resolution -noise, distortion Digital signal: Signal level only have discrete values -more immune to noise -signal loss due to quantization error Time V H V L Logic 0 Logic 1 Logic 1 IIT Kharagpur, Autumn 2024 8

9. Electronic Devices Electronic devices are fabricated by semiconductor materials (primarily with Silicon)* - electrical characteristics can controlled very precisely Other key components are insulator (e.g. SiO 2 , silicon nitride) and conductor (e.g. Aluminium, high temperature metal like Tungsten) Semiconductor devices – Diode, Bipolar Junction Transistor (BJT), Metal Oxide Field Effect Transistor (MOSFET) * Gallium arsenide and other III-V compounds can also be used, particularly for high speed applications IIT Kharagpur, Autumn 2024 9

10. Periodic Table The table contains elements with three to five valence electrons, with Si being the most important. IIT Kharagpur, Autumn 2024 10

11. Silicon Si has four valence electrons. Therefore, it can form covalent bonds with four of its neighbors. When temperature goes up, electrons in the covalent bond can become free. That is, electrons can jump from valence band to conduction band. E V E C E g =1.1 eV electron IIT Kharagpur, Autumn 2024 11

12. Silicon: Electron Hole Pair E V E C E g =1.1 eV With free electrons breaking off covalent bonds, holes are generated. Holes can be filled by absorbing other free electrons, so effectively there is a flow of charge carriers. The usual concept is electrons move in conduction band and holes in valence band. But one can also say that electrons move in valence band that will move the holes effectively in the opposite direction of the motion of holes. electron hole IIT Kharagpur, Autumn 2024 12

13. Electron Density with Temperature E g , or bandgap energy determines how much effort is needed to break off an electron from its covalent bond. There exists an exponential relationship between the free-electron density and bandgap energy. B – related to specific semiconductor material ...e.g. Si- 5.23x10 15 (cm -3 K -3/2 ) not enough n ( p ) for appreciable amount of current IIT Kharagpur, Autumn 2024 13

14. Semiconductor Intrinsic semiconductor Extrinsic semiconductor IIT Kharagpur, Autumn 2024 14

15. Intrinsic Semiconductor Single crystal semiconductor with no other impurities (types of atoms)- density of electrons ( n ) and holes ( p ) are same = n i Free carrier electrons/holes- generated due to breaking of covalent bonds With increase in temperature electron/hole pairs increases- hence conductivity of Si increases No of electrons = no of holes, The carriers can randomly move...hence some electron may fill some holes ...... recombination Recombination  no of free electrons and holes  generation In thermal equilibrium recombination rate=generation rate Carrier concentration depends on temperature ( not good news! ) Not enough n ( p ) for appreciable amount of current IIT Kharagpur, Autumn 2024 15

16. Extrinsic Semiconductor Impure semiconductor (achieved through doping) This will substitute semiconductor atoms with the impurity atoms It introduces allowed energy states within the band gap but very close to the energy band that corresponds to the dopant type. For Si, desirable impurities are usually from Group V (Phosphorus, Arsenic).........n-type Group III (Boron, Gallium).................p-type IIT Kharagpur, Autumn 2024 16

17. Doping (N type) Pure Si can be doped with other elements to change its electrical properties. For example, if Si is doped with P (Phosphorous), then it has more electrons, or becomes type N (electron). This is because Phosphorous is in Group V of the periodic table, that is it has 5 electrons at the outer most shell. So when it forms covalent bond with four Si atoms, there will be one additional electron. IIT Kharagpur, Autumn 2024 17

18. Doping (P type) If Si is doped with B (Boron), then it has more holes, or becomes type P. This is because Boron has 3 electrons at outer shell, so collectively Boron and Four neighboring Si will have one electron less (hole) compared to complete octave IIT Kharagpur, Autumn 2024 18

19. Extrinsic semiconductor n-type semiconductor Majority carrier-electron Pentavalent impurities donate excess electrons and therefore named as donor or n-type impurities Allowable energy levels are introduced very small distance below conduction band p-type semiconductor Majority carrier-hole Trivalent impurities accept electrons and thus form holes and therefore named as acceptor or p-type impurities Allowable energy levels are introduced just above the valence band Conduction band Valence band E c E A E v Conduction band Valence band E v E c E D Eg electron hole Small amount of energy required to donate electron (from E D state to E C ) in case of donor and to accept electron (from E V to E A state) hence produce holes in valence band in case of acceptor ...thus they become predominantly n (or p) type. IIT Kharagpur, Autumn 2024 19

20. Charge Density in a Semiconductor Semiconductor is electrically neutral – so magnitude of positive charge density must equal to that of negative charge density - concentration of donor ions, - concentration of acceptor ions (considering practically all ionised) Consider an n -type material having N A =0, and also in n -type n >> p which implies in an n-type material free electron concentration is approximately equal to the concentration of donor atoms... independent of temperature From now on, electron concentration in n-type is n n and hole concentration in n-type is p n ... this depends on temperature Similarly for a p-type semiconductor, IIT Kharagpur, Autumn 2024 20 Laws of mass action

21. Current in Semiconductor Drift Current – movement of carriers caused by electric field Diffusion Current –movement of carriers caused by concentration gradient IIT Kharagpur, Autumn 2024 21 Movement of charge carriers (electrons and holes) generate current.

22. Drift Current Drift Current Due to applying electric field electrons acquire a drift velocity µ n – electron mobility (~1350cm 2 / Vsec )... how well an electron can move in a semiconductor In one sec the electron charge crosses a plane A is Drift current density, Drift current density for holes , So total drift current density where - conductivity,  - resistivity Charge particles will move at a velocity that is proportional to the electric field. Electric current is calculated as the amount of charge in v meters that passes through a cross-section A if the charge travel with a velocity of v m/s. IIT Kharagpur, Autumn 2024 22 Unit - Siemens per meter or Mho per meter Unit - Ohm meter

23. Velocity Saturation At lower electric field drift velocity increases with electric field The carrier drift velocity will eventually saturate to a critical value v sat at high E. IIT Kharagpur, Autumn 2024 23 drift velocity E v sat

24. Diffusion current Charge particles move from a region of higher concentration to a region of lower concentration. Diffusion current is proportional to the gradient of charge ( dn /dx) along the direction of current flow. Its total current density consists of both electrons and holes. Diffusion current density due to motion of electrons Diffusion current density due to motion of holes D n -electron diffusion co-efficient D p -hole diffusion co-efficient Problem! Consider a bar of Si in which hole concentration profile was described by p=p e(-x/L). Find the value of hole current at x=0, given L=1um and cross sectional area=100um 2 ,Dp=12 cm 2 /sec,p =10 16 /cm 3 Diffusion Current IIT Kharagpur, Autumn 2024 24

25. Einstein's Relation This is Einstein Relation Problem! Find out the value of D p at room temperature, given µ p =480 cm 2 /Vsec 1.38 × 10 −23 J⋅K −1 8.62 × 10 −5 eV⋅K −1 1.38 × 10 −16 erg⋅K −1 Boltzmann constant k IIT Kharagpur, Autumn 2024 25

26. PN Junction (Diode) When N-type and P-type dopants are introduced in a semiconductor, a PN junction or a diode is formed. P side is known as anode, and N side as cathode. Diode Symbol IIT Kharagpur, Autumn 2024 26 Boron doped Phosphorus doped p side n side

27. PN Junction (Diode) In order to understand the operation of a diode, it is necessary to study its three operation regions: - equilibrium - reverse bias - forward bias IIT Kharagpur, Autumn 2024 27

28. Equilibrium – no bias voltage Each side of the junction contains an excess of holes or electrons compared to the other side, hence there exists a large concentration gradient. Therefore, a diffusion current flows across the junction from each side. As free electrons and holes diffuse across the junction, a region of fixed ions is left behind. That is, p side will have –ve immobile ions and n side will have +ve immobile ions. This region is known as the “depletion region.” This region is free (or deplete) of mobile carriers. IIT Kharagpur, Autumn 2024 28 Positive Negative donor ion acceptor ion Free Free electrons holes Depletion region

29. Equilibrium – no bias voltage The fixed ions in depletion region create an electric field that results in a drift current. At equilibrium, the drift current flowing in one direction cancels out the diffusion current flowing in the opposite direction, creating a net zero current. IIT Kharagpur, Autumn 2024 29

30. Built in Potential at Equilibrium The electric field in the space charge region gives rise to potential barrier...this is known as built in potential. V T – thermal voltage at room temperature =25.8 mV=  26 mV From Einstein equation we can write x=0 Doping concentration V bi Potential x2 x1 IIT Kharagpur, Autumn 2024 30

31. Diode (Reverse Bias) in equilibrium in reverse bias (p side connected to –ve terminal & n side to +ve terminal of supply voltage) Reverse Bias- +ve terminal of the battery applied to the n region and -ve terminal applied to the p region...this increases the potential barrier at the junction The increased space charge region prevents any carrier to cross the junction...hence there is very low current The depletion layer stops growing when its difference of potentials equals the source voltage Thermal energy continuously creates a limited number of electrons and holes on both sides of the junction...this creates very small amount of current...called saturation current I S I S is independent of applied voltage...only depends on thermal energy (~ doubles for each 10  C rise in temperature ) IIT Kharagpur, Autumn 2024 31

32. Diode (Forward Bias) in forward bias (p side connected to +ve terminal & n side to -ve terminal of supply voltage) The depletion width is shortened and the built-in electric field decreased. This reduces the potential barrier at the junction This disturbs the equilibrium between diffusion and electric field force Majority carrier holes from p region crosses the potential barrier and moves to the n region...thus they constitute an injected minority carrier current in n side Similarly electron from n side form minority carrier current in the p region after crossing the junction. Total forward current = hole minority current + electron minority current Under forward bias, minority carriers in each region increase due to the lowering of built-in field/potential. Therefore, diffusion currents increase to supply these minority carriers. IIT Kharagpur, Autumn 2024 32

33. pn junction IV Relationship Total current I D n side p side electron diffusion current electron current hole diffusion current hole current Total current I D is constant throughout the device...but the proportion due to holes and that due to electrons varies with distance In p side main current is due to majority carrier i.e. hole...holes after crossing junction becomes hole diffusion current...hole diffusion current decreases exponentially Similarly n side main current is due to electron and when electrons cross the junction...they become electron diffusion current in the p side IIT Kharagpur, Autumn 2024 33

34. Diode Characteristics p n I D Diode can be thought of a voltage controlled switch... off for a reverse bias voltage on for a forward bias voltage There exists a cut in / threshold ( V  ) voltage for diode...below which the current is very small...and above which the current rises very rapidly... ( V  ) – (0.2V for germanium and 0.6V for silicon) The current and voltage relationship of a PN junction is exponential in forward bias region, and relatively constant in reverse bias region. n – ideality factor 1  n  2 depends on the fabrication process and semiconductor material IIT Kharagpur, Autumn 2024 34

35. Reverse Breakdown With increase in reverse voltage I S remains very small initially (may be of the order of pA or fA) At large reverse voltage (known as breakdown voltage) the diode can conduct very heavily with very little increase in reverse voltage...hence can be used as constant voltage device Breakdown - (i) Avalanche (ii) Zener IIT Kharagpur, Autumn 2024 35

36. Zener vs. Avalanche Breakdown Avalanche – Carriers crossing the space charge region gain sufficient energy from the high electric field...able to break the covalent bonds...the generated electron hole pair then in turn gets enough energy to knock other covalent bonds...this cumulative process known as Avalanche Multiplication. Zener Breakdown - Tunnelling of carriers across the junction...needs high doping concentration IIT Kharagpur, Autumn 2024 36

37. Zener Diode At certain reverse voltage current increases enormously the diode breakdown voltage is essentially constant over a wide range of currents it can be used as constant voltage reference in a circuit I Z IIT Kharagpur, Autumn 2024 37

38. Diode Model (2) Constant Voltage model Diode operates as an open circuit if V D < V D,on and a constant voltage source of V D,on if V D tends to exceed V D,on. I D V D reverse bias forward bias (1) Ideal diode During reverse bias current through the diode is zero (open circuit) During forward bias voltage across the diode is zero (short circuit) V D , on IIT Kharagpur, Autumn 2024 38

39. Diode Model (3) Piecewise linear diode model Diode voltage greater than (cut in voltage), current increases linearly with voltage - slope is 1/R f Diode voltage less than , current is zero (or very close to zero) parallel to V axis - so diode reverse resistance ( R r ) can be taken as  v  v  v  V I R f IIT Kharagpur, Autumn 2024 39

40. Different types of Diode Solar Cell Photodiode Light-Emitting Diode Schottky Barrier Diode Zener Diode IIT Kharagpur, Autumn 2024 40

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