PN junction diode brief explanation and pictures and slide shows type

PN JUNCTION

1. Semiconductors Dr. Gurpreet Kaur Teaching Assistant Department of Electrical Engineering and Information Technology

2. Semiconductors The Semiconductors , such as Germanium, Silicon, Carbon, Selenium, etc. are the materials which are neither conductors nor insulators. Semiconductors have some useful properties and are being extensively used for the preparation of solid state devices like the diode, transistor, etc. Although the valence band of these substances is almost filled and the conduction band is almost empty as in case of insulators. Valence band : Electrons move out of valence band. Conduction band: Electrons reaches the conduction band.

3. Semiconductors Therefore, an electric field, smaller than insulators and greater than conductors is required to push the valence electrons to the conduction band

4. Intrinsic Semiconductor and Extrinsic Semiconductor   The pure form of the semiconductor is known as the intrinsic semiconductor and the semiconductor in which intentionally impurities is added for making it conductive is known as the extrinsic semiconductor. Doping: The process by which an impurity is added to a semiconductor is known as Doping. Intrinsic: Silicon (Si), Atomic no. 14 (2,8,4) Germanium (Ge) Atomic no 32 (2,8,18,4) Valency 4 for both Si and Ge

5. Intrinsic Semiconductor and Extrinsic Semiconductor When impurity is added to the semiconductor to increase its conductivity is known as extrinsic semiconductor. Trivalent or Pentavalent impurities are added in the pure semiconductor. Trivalent is for p type and valency added is 3. Ex. Aluminium Pentavalent is for n type and valency added is 5. Ex. Phosphorous (2,8,5)

6. PN Junction The n-type semiconductor is formed by adding pentavalent impurities to the intrinsic semiconductor while p-type semiconductor is formed by adding trivalent impurities to the intrinsic semiconductor . P-N junction is also a fundamental building block of many other semiconductor electronic devices such as transistors, solar cells, light emitting diodes, and integrated circuits.

7. n-type semiconductors electrons are the majority carriers while holes are the minority carriers.  p-type semiconductors holes are the majority carriers while electrons are the minority carriers PN Junction

8. PN Junction

9. Applications computers, control systems, ATM (Automated Teller Machine), mobile phones.

10. Volt-Ampere Characteristic of a PN Junction Diode The  volt-ampere characteristic  of the PN-junction diode is a curve between the  voltage  over the junction and the circuit current.

11. VI Characteristics

12. Forward Biasing Forward Biasing The PN-junction diode is connected in forward biased by keeping the key K close and keeping the double throw switch in position one. the p-type semiconductor material connects to the positive end of the power supply and the n-type semiconductor material is connected to the negative terminal of the supply. When the voltage is increased by varying the resistor value Rh, the circuit curve increases very slowly and the curve becomes non-linear. The current slowly rises in the forward biasing because the applied external voltage is used to cross the potential barrier of the PN-junction diode. when the potential barrier is completely eliminated, and the external voltage applies to the junction increases, the PN-junction behaves like an ordinary diode and circuit current increases sharply

13. Reverse Biasing the p-type material connects to the negative terminal of the supply, and the n-type material is connected to the positive terminal of the supply . Under reverse bias condition, the resistance of the diode becomes very high, and practically no current flows through the diode. But in actual practice current in milliamperes flow through the diode. This current is known as the reverse current. The reverse current is because of the minority charge carrier presents in the semiconductor material at normal room temperature. When the reverse supply increases the reverse current also increases. At this state the resistance of the barrier increases due to which breakdown occurs at the junction. Therefore the reverse biasing current increases and damage the junction permanently.

14. Important Terms Maximum forward current: The maximum value of forward current that a PN junction can carry without damaging itself. Peak Inverse voltage (PIV): The maximum value of reverse voltage that a PN junction can withstand without damaging itself. Maximum power rating: The maximum power that a PN junction can dissipate wit hout damaging itself.

15. PN junction summary No current flows through the diode when the external voltage becomes zero. In forward bias, the current increase slightly till the depletion region is completely wiped off. The forward biased increases suddenly after the knee voltage. The forward current is limited by the series resistance R and the forward resistance Rf. The forward current increases beyond the rated value, destroy the diode. At reverse bias the reverse current increase slightly with the increase of a minority charge carrier. With the increase of the reverse voltage the reverse current increase abruptly to a large value. It is because of this voltage the junction of the transistor breaks and resistance fall suddenly. The double pole double throw switch has two outputs (on and off) for each input.

16. PN Diode as a Circuit Element Diodes are referred to as non-linear circuit elements because of the above characteristic curve. deal diode has having two regions: a conduction region of zero resistance and an infinite resistance non-conduction region. a) Schematic symbol for a diode b ) current versus voltage for an ideal diode

17. A diode can more accurately be described using the equivalent circuit model shown in figure. If a diode is forward biased with a high voltage it acts like a resistor ( Rf ) in series with a voltage source ( V PN ). For reverse biasing it acts simply as a resistor ( Rr ). These approximations are referred to as the linear element model of a diode. Fig: Equivalent circuit model of a junction diode. PN Diode as a Circuit Element

18. Half Wave Rectifier In Half Wave Rectifier, when AC supply is applied at the input, positive half cycle appears across the load, whereas the negative half cycle is suppressed.

19. Operation of Half Wave Rectifier When AC supply is switched ON the alternating voltage (Vin) shown in the figure below appears across the terminal AB at the secondary winding . During the positive half cycle, the terminal A is positive with respect to B and the diode is forward biased. Therefore, it conducts and current flows through the load resistor R L . positive half cycle of the output voltage ( V out  = i RL ) appears across the load resistor R L Peak Inverse Voltage-?

20. Full Wave Rectifier Transformer is used in place of a center tapped transformer. The circuit forms a bridge connecting the four diodes D 1 , D 2 , D 3 , and D 4 .

21. Operation of Full Wave Bridge Rectifier When an AC supply is switched ON, the alternating voltage Vin appears across the terminals AB of the secondary winding of the transformer. During the positive half cycle of the secondary voltage, the end A becomes positive, and end B becomes negative

22. The diodes D 1  and D 3  are forward biased and the diodes D 2  and D 4  is reversed biased. Therefore, diode D 1  and D 3  conduct and diode D 2  and D 4  does not conduct . Peak Inverse Voltage-?  

23. Objective questions In a P type semiconductors, the minority carriers are_____ While forming PN junction, the diffusion is restricted by____ Under reverse bias condition, a diode cannot withstand the voltage more than its rated___ A capacitor does not allow to pass ___ component A zener diode is always operated in the region___

24. Clippers

25. Parallel Clipper

26. Parallel Clipper

27. Clampers

28. Mathematical modelling of Clampers Apply KVL in the loop

29. Mathematical modelling of Clampers Apply KVL

30. Clippers Clippers Circuits are used to remove the part of a signal that is above or below some defined reference level. One of the simple example of a clipper is the half-wave rectifier – that circuit basically cut off everything at the reference level of zero and let only the positive-going (or negative-going) portion of the input waveform through. Clipping circuits (also known as limiters, amplitude selectors).

31. Clipper Circuits Biased Clipper Circuit Unbiased Clipper Circuit Biased Positive Clipper Biased Negative Clipper Unbiased Negative Clipper Unbiased Positive Clipper Clippers Classification

32. Unbiased Positive clippers Positive clippers are used to clip positive portions of the input signal and allow the negative portions of the signal to pass through, Figure below shows the input and output signal along with the positive clipper. The positive cycle is completely clipped off by the clipper .

33. Unbiased Negative clippers Negative clippers are used to clip Negative portions of the input signal and allow the positive portions of the signal to pass through Figure shows the input and output signal along with the negative clipper. The negative cycle is completely clipped off by the clipper .

34. Voltage Doubler A multiplier circuit that generates a dc output voltage having amplitude twice the maximum amplitude of the ac input supply voltage is known as Voltage Doubler. Types Half Wave Voltage Doubler Full Wave Voltage Doubler

35. Half Wave Voltage Doubler

36. Full Wave Voltage Doubler

37. Numerical Problems Q1. Determine the current flowing through the silicon diode in the given fig. Assume forward resistance to be zero. Q2. The applied input a.c . power to a half-wave rectifier is 500 watts. The d.c. output power obtained is 100 watts. (i) What is the rectification efficiency ? (ii) What happens to remaining 400 watts ?

38. Capacitor Filter The main function of this filter is to allow the ac components and blocks the dc components of the load. The construction of a filter circuit can be done with the basic electronic components like resistors, inductors, and capacitors. There are different types of filters available namely LPF (low pass filter), BPF ( bandpass filter), HPF (high pass filter), capacitor filter, etc.

39. Half Wave Rectifier with Capacitor Filter The main function of half wave rectifier is to change the AC (Alternating Current) into DC (Direct Current). However, the acquired output DC is not pure and it is an exciting DC. This DC is not constant and varies with time.

40. Half Wave Rectifier with Capacitor Filter Whenever AC voltage is applied to the circuit throughout the positive half cycle, then the diode lets the flow of current through it. The flow of current always chooses to supply through a low resistance lane. So when the flow of current gets the filter, the ac components experience a low-resistance and dc components experience a high-resistance from the capacitor . Throughout the conduction time, the capacitor gets charged to the highest value of the voltage supply. As the voltage among the two plates of the capacitor is equivalent to the voltage supply, then it is said to be completely charged.

41. Half Wave Rectifier with Capacitor Filter When it gets charged then it holds the supply until the supply of i/p AC toward the rectifier achieves the negative half cycle. Once the rectifier reaches to negative half cycle,   the diode acquires reverse biased & stops letting the flow of current through it. Throughout this, the supply voltage is low then the voltage of a capacitor. Thus the capacitor releases all the stored current through the RL. This stops the o/p load voltage from falling to nil.

42. Full Wave Rectifier with Capacitor Filter

43. Transistor The transistor is a semiconductor device   which transfers a weak signal from low resistance circuit to high resistance circuit. The transistor consists two PN diode connected back to back. It has three terminals namely emitter, base and collector.

44. Emitter – The section that supplies the large section of majority charge carrier is called emitter. The emitter is always connected in forward biased with respect to the base so that it supplies the majority charge carrier to the base. The emitter-base junction injects a large amount of majority charge carrier into the base because it is heavily doped and moderate in size . Collector – The section which collects the major portion of the majority charge carrier supplied by the emitter is called a collector. The collector-base junction is always in reverse bias. Its main function is to remove the majority charges from its junction with the base. The collector section of the transistor is moderately doped, but larger in size so that it can collect most of the charge carrier supplied by the emitter. Base – The middle section of the transistor is known as the base. The base forms two circuits, the input circuit with the emitter and the output circuit with the collector. The emitter-base circuit is in forward biased and offered the low resistance to the circuit. The collector-base junction is in reverse bias and offers the higher resistance to the circuit. The base of the transistor is lightly doped and very thin due to which it offers the majority charge carrier to the base.

45. Transistor Symbols

46. Working of Transistor silicon is used for making the transistor because of their high voltage rating, greater current and less temperature sensitivity