Chapter 10 BJT Fundamentals

2. Semiconductor Device Physics Chapter 10 BJT Fundamentals

3. Chapter 10 BJT Fundamentals Bipolar Junction Transistors (BJTs) • Over the past decades, the higher layout density and low-power advantage of CMOS (Complementary Metal–Oxide–Semiconductor) has eroded away the BJT’s dominance in integrated-circuit products. • Higher circuit density  better system performance • BJTs are still preferred in some digital-circuit and analog-circuit applications because of their high speed and superior gain • Faster circuit speed (+) • Larger power dissipation (–) • Transistor: current flowing between two terminals is controlled by a third terminal

4. Chapter 10 BJT Fundamentals Introduction • There are two types of BJT: pnp and npn. • The convention used in the textbook does not follow IEEE convention, where currents flowing into a terminal is defined as positive. • We will follow the normal convention: . . . . . .

5. Chapter 10 BJT Fundamentals Circuit Configurations Common-Emitter I–V Characteristics Most popular configuration Active Mode Saturation Mode IC < bIB In active mode, bdc is the common emitter dc current gain

6. Chapter 10 BJT Fundamentals Modes of Operation • Common-Emitter Output Characteristics

7. Chapter 10 BJT Fundamentals BJT Electrostatics • Under equilibrium and normal operating conditions, the BJT may be viewed electrostatically as two independent pn junctions. W : quasineutral base width

8. Chapter 10 BJT Fundamentals BJT Electrostatics • Electrostatic potential, V(x) • Electric field, E(x) • Charge density, ρ(x)

9. Chapter 10 BJT Fundamentals BJT Design • Important features of a good transistor: • Injected minority carriers do not recombine in the neutral base region  short base, W << Lp for pnp transistor • Emitter current is comprised almost entirely of carriers injected into the base rather than carriers injected into the emitter  the emitter must be doped heavier than the base pnp BJT, active mode

10. Chapter 10 BJT Fundamentals 2 3 1 4 Base Current (Active Bias) • The base current consists of majority carriers (electrons) supplied for: • Recombination of injected minority carriers in the base • Injection of carriers into the emitter • Reverse saturation current in collector junction • Recombination in the base-emitter depletion region EMITTER COLLECTOR BASE n-type p-type p-type

11. Chapter 10 BJT Fundamentals 2 2 1 1 5 BJT Performance Parameters (pnp) IEn ICn Negligible compared to holes injected from emitter ICp IEp • Emitter Efficiency • Base Transport Factor • Decrease relative to and to increase efficiency • Decrease relative to to increase transport factor Common base dc current gain:

12. Chapter 10 BJT Fundamentals 3 2 Collector Current (Active Bias) • The collector current is comprised of: • Holes injected from emitter, which do not recombine in the base • Reverse saturation current of collector junction ICB0 :collector current when IE = 0 Common emitter dc current gain:

13. Semiconductor Device Physics Chapter 11 BJT Static Characteristics

14. Chapter 11 BJT Static Characteristics Notation (pnp BJT) Minority carrier constants

15. Chapter 11 BJT Static Characteristics Emitter Region • Diffusion equation: • Boundary conditions:

16. Chapter 11 BJT Static Characteristics Base Region • Diffusion equation: • Boundary conditions:

17. Chapter 11 BJT Static Characteristics Collector Region • Diffusion equation: • Boundary conditions:

18. Chapter 11 BJT Static Characteristics Ideal Transistor Analysis • Solve the minority-carrier diffusion equation in each quasi-neutral region to obtain excess minority-carrier profiles • Each region has different set of boundary conditions • Evaluate minority-carrier diffusion currents at edges of depletion regions • Add hole and electron components together  terminal currents is obtained IC IE IB

19. Chapter 11 BJT Static Characteristics Emitter Region Solution • Diffusion equation: • General solution: • Boundary conditions: • Solution

20. Chapter 11 BJT Static Characteristics Collector Region Solution • Diffusion equation: • General solution: • Boundary conditions: • Solution

21. Chapter 11 BJT Static Characteristics Base Region Solution • Diffusion equation: • General solution: • Boundary conditions: • Solution

22. Chapter 11 BJT Static Characteristics Base Region Solution • Since • We can write as

23. Chapter 11 BJT Static Characteristics Base Region Solution • Since

24. Chapter 11 BJT Static Characteristics Terminal Currents • Since • Then

25. Chapter 11 BJT Static Characteristics Simplified Relationships • To achieve high current gain, a typical BJT will be constructed so that W << LB. • Using the limit value Due to VEB • We will have Due to VCB

26. Chapter 11 BJT Static Characteristics Performance Parameters • For specific condition of • “Active Mode”: emitter junction is forward biased and collector junction is reverse biased • W << LB, nE0/pB0 = NB/NE

27. Chapter 11 BJT Static Characteristics Homework 7 • 1. (10.17) • Consider a silicon pnpbipolar transistor at T = 300 K with uniform dopings of NE = 5×1018 cm–3, NB = 1017 cm–3, and NC = 5×1015 cm–3 . Let DB = 10 cm2/s, xB = 0.7 μm, and assume xB << LB. The transistor is operating in saturation with JP = 165 A/cm2 and VEB = 0.75 V. Determine: • (a) VCB, (b) VEC(sat), (c) the number/cm2 of excess minority carrier holes in the base, and (d) the number/cm2 of excess minority carrier electrons in the long collector, take LC = 35 μm. • 2. (10.14) • Problem 10.4, Pierret’s “Semiconductor Device Fundamentals”. • Due: Monday, 25.11.2013.