This document summarizes the operational principle of a bipolar junction transistor (BJT). It describes the basic BJT structure as having 3 layers of semiconductor material and lists the 4 main operation modes. It then explains active mode operation in more detail, discussing how minority carrier injection at the base leads to collector and emitter currents that are proportional to the base current, with current gain factors alpha and beta.

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 