2. Semiconductor Device Physics
Chapter 10
BJT Fundamentals

3. Bipolar Junction Transistors (BJTs)
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. Introduction There are two types of BJT: pnp and npn.
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. Circuit Configurations
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. Modes of Operation Common-Emitter Output Characteristics Mode
Chapter 10
BJT Fundamentals
Modes of Operation
Common-Emitter Output Characteristics
Mode
E-B Junction
C-B Junction
Saturation
forward bias
Active/Forward
reverse bias
Inverted
Cutoff

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

9. BJT Design Important features of a good transistor:
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. Base Current (Active Bias)
Chapter 10
BJT Fundamentals
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
BASE
COLLECTOR 1 4 3 2
p-type
n-type
p-type

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

12. Collector Current (Active Bias)
Chapter 10
BJT Fundamentals
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 2 3
ICB0 :collector current when IE = 0
Common emitter dc current gain:

13. BJT Static Characteristics
Semiconductor Device Physics
Chapter 11
BJT Static Characteristics

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

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

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

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

18. Ideal Transistor Analysis
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 IE IC IB

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

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

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

22. Base Region Solution Since We can write as 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. Simplified Relationships
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. Performance Parameters
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 pnp bipolar 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,