1. COURSE NAME: SEMICONDUCTORS Course Code: PHYS 473 Week No. 8

2. Transistor A three lead semiconductor device that acts as: (1)an electrically controlled switch (2)a current amplifier. 2

3. Bipolar Junction Transistor (BJT) A bipolar junction transistor (BJT) is a type of transistor that relies on the contact of two types of semiconductor for its operation. A transistor consisting of two n- and one p-type layers of material called NPN transistor. A transistor consisting of two p- and one n-type layers of material called PNP transistor. 3

4. Basic Models of BJT 4

5. Bipolar Junction Transistor Fundamentals 5

6. Basic principle of the BJT The basic principle of the BJT is: “The voltage between two terminals controls the current through the third terminal”. 6

7. Working of NPN transistor. Figure shows the NPN transistor with forward bias to emitter base junction and reverse bias to collector-base junction. The forward bias causes the electrons in the n-type emitter to flow towards the base. This constitutes the emitter current I E. 7

8. Working of NPN transistor. As these electrons flow through the p-type base, they tend to combine with holes. As the base is lightly doped and very thin, therefore, only a few electrons (less than 5%) combine with holes to constitute base current I B. 8

9. Working of NPN transistor. The remainder (more than 95%) cross over into the collector region to constitute collector current I C. In this way, almost the entire emitter current flows in the collector circuit. It is clear that emitter current is the sum of collector and base currents i.e I E = I B + I C 9

10. Working of PNP transistor. The forward bias causes the holes in the p-type emitter to flow towards the base. This constitutes the emitter current I E. As these holes cross into n-type base, they tend to combine with the electrons. As the base is lightly doped and very thin, therefore, only a few holes (less than 5%) combine with the electrons. 10

11. Working of PNP transistor. The remainder (more than 95%) cross into the collector region to constitute collector current I C. In this way, almost the entire emitter current flows in the collector circuit. 11

12. Conventional currents 12

13. Conventional current flow (NPN) For NPN connection, it is clear that conventional current flows out of the emitter as indicated by the outgoing arrow in Fig. 13

14. Conventional current flow (PNP) For PNP connection, it is clear that conventional current flows into the emitter as indicated by the inward arrow in Fig. 14

15. Types of configurations Common Base Configuration (CBC): Common Emitter Configuration (CEC): Common Collector Configuration (CCC): 15

16. Common Base Configuration (CBC) Current amplification factor (α). The ratio of change in collector current to the change in emitter current at constant collector base voltage VCB is known as current amplification factor i.e. 16

17. Characteristics of Common Base Connection Input characteristic. 1.The emitter current I E increases rapidly with small increase in emitter-base voltage V EB. It means that input resistance is very small. 2.The emitter current is almost independent of collector-base voltage V CB. This leads to the conclusion that emitter current is almost independent of collector voltage. 17

18. Characteristics of Common Base Connection Output characteristic. 1.The collector current IC varies with V CB only at very low voltages ( < 1V). The transistor is never operated in this region. 18

19. Characteristics of Common Base Connection Output characteristic. 2.When the value of V CB is raised above 1 − 2 V, the collector current becomes constant as indicated by straight horizontal curves. It means that now I C is independent of V CB and depends upon I E only. The transistor is always operated in this region. 19

20. Characteristics of Common Base Connection Output characteristic. 3. A very large change in collector- base voltage produces only a tiny change in collector current. This means that output resistance is very high. 20

21. Common Base Configuration (CBC) Q: 1 Q: 2 21

22. Common Base Configuration (CBC) Q. 3 22

23. Common Base Configuration (CBC) Q. 4 23

24. Common Emitter Connection Base current amplification factor (β). The ratio of change in collector current (ΔI C ) to the change in base current (ΔI B ) is known as base current amplification factor i.e. 24

25. Common Emitter Connection Q. 5 Q. 6 25

26. Common Emitter Connection Q. 7 26

27. Common Emitter Connection Q. 8 27

28. Characteristics of Common Emitter Connection Input characteristic. As compared to CB arrangement, I B increases less rapidly with V BE. Therefore, input resistance of a CE circuit is higher than that of CB circuit. 28

29. Characteristics of Common Emitter Connection Output characteristic. 29

30. Characteristics of Common Emitter Connection 30

31. Characteristics of Common Emitter Connection 1. Cutoff region: Base-emitter junction is reverse biased. No current flow. 31

32. 2. Saturation region: Base-emitter junction forward biased. Collector-base junction is forward biased. I C reaches a maximum which is independent of I B and β. V CE < V BE 32 Characteristics of Common Emitter Connection

33. 3. Active region: Base-emitter junction forward biased Collector-base junction is reverse biased V BE < V CE < V CC 33 Characteristics of Common Emitter Connection

34. 4. Breakdown region: I C and V CE exceed specifications Damage to the transistor 34 Characteristics of Common Emitter Connection

35. Common Collector Connection Current amplification factor γ. The ratio of change in emitter current (ΔIE) to the change in base current (ΔIB) is known as current amplification factor in common collector (CC) arrangement i.e. 35

36. Types of configurations Common Base Configuration (CBC): Common Emitter Configuration (CEC): Common Collector Configuration (CCC): 36

37. BJT relationships-Equations I E = I C + I B 37

38. Example No. 1 38 In the given figure, if I B = 50  A, I C = 1mA, then find the followings: 1) I E 2)  3)  Solution: 1) Since: I E = I B + I C I E = 50  A + 1 mA I E = 0.05 mA + 1 mA I E = 1.05 mA 2) Since:  = I C /I B  = 1 mA / 0.05 mA = 20  = 20

39. Example No. 1 continued………… 39 Solution: 3) Since  = I C / I E = 1 mA / 1.05 mA = 0.95238  = 1 mA / 1.05 mA  = 0.95238

40. Example No. 2 An NPN Transistor has a DC current gain, (Beta) value of 200. Calculate the base current I B required to switch a resistive load of 4mA. Solution: 40

41. Example No. 3 If the collector current I C =7.95 mA and the emitter current I E =8mA, then calculate current gain alpha. Solution: 41

42. Modes of Operation Depending on the bias condition on its two junctions, the BJT can operate in one of three possible modes: cut-off (both junctions reverse biased) active (the EBJ forward-biased and CBJ reversed) saturation (both junctions forward biased) 42

43. Advantages of transistors 1. High voltage gain. 2. Lower supply voltage. 3. No heating. 4. Mechanically strong due to solid-state 43

44. Disadvantages of transistors 1. Lower power dissipation 2. Lower input impedance 3. Temperature dependence 4. Inherent variation of parameters 44

45. Summary of Bipolar Junction Transistors BJT is a three layer device constructed form two semiconductor diode junctions joined together, one forward biased and one reverse biased. There are two main types of BJT, the NPN and the PNP transistor. Transistors are "Current Operated Devices" where a much smaller Base current causes a larger Emitter to Collector current, which themselves are nearly equal, to flow. The most common transistor connection is the Common-emitter configuration. 45