BJT - Operational Principle

1. BJT – Operational
Principle
Yong Heui Cho @ Mokwon University
Some of slides are referred to:
[1] A. S. Sedra & K. C. Smith, Microelectronic Circuits.

2. 2
Electronic Circuits
6. Diode – Basic Applications
7. Diode – Advanced
Applications
8. BJT – Operational Principle
9. BJT – Model

3. 3
BJT Structure
• BJT: Bipolar Junction TR
• TR: Transistor (Trans + Resistor)
• 3 layers of semiconductor
• 4 different operation modes

4. 4
BJT Modes
Mode EBJ CBJ
Cutoff Reverse Reverse
Active Forward Reverse
Rev. Act. Reverse Forward
Saturation Forward Forward
digital (off)
analog (amp.)
X
digital (on)

5. 5
Operation in Active

6. 6
Carrier Injection
• Minority carrier injection (base)

7. 7
Base Current
For short emitter
LP
IB2 = Qn/τb
1
1 1
Qn = AE·q·1/2·np(0)·W = (AE·q·W·ni
2/2NA)evBE/VT
IB = IB1 + IB2 = IC/β
Dp
Dn
NA
ND
WB
Lp
W2
2Dnτb
β = +[ ]
-1
: common emitter current gain
CBE III 

8. 8
Collector Current
CBE III 

9. 9
Emitter Current
: common base current gain
α = αF, β = βF : at forward active
B
CBE
I
III
)1( 



BC II 