Lecture #25
Chapter 5 of Jaeger, Chapter 2 of Spencer
Bipolar Junction Transistors
Outline/Learning Objectives:
• Describe the physical structure of the bipolar junction transistor (BJT).
• Identify the npn and pnp BJT circuit diagram symbols.
• Describe qualitatively and quantitatively the physical operating principles of the BJT.
• Describe qualitatively and quantitatively the i-v characteristics (output and transfer) of
the BJT.
• Determine the regions of operation (cutoff, forward-active, and saturation).
•
Selected problems:
• 5.32, 5.61, 5.65, 5.73, 5.74, 5.82, 5.91, 5.95, 5.98
•
•
Lecture 25
25 - 1
Physical Structure of the BJT
C
iC
n
Emitter-Base
Junction
p
iE
n
E
iB
B
Collector-Base
Junction
Emitter
Base
Collector
Active Base Region
Lecture 25
25 - 2
Simplified Cross-Section of an npn BJT
Currents are for “Normal” Operation
p
+
+
n
Collector
p
+
p
n+
+
n
Emitter
n epitaxy
iC
p
n+
p
i
Base
E
n epitaxy p
p
+
i
+
B
n buried layer
p
+
p
Active Transistor
Region
p
p
Lecture 25
25 - 3
Idealized npn BJT and Circuit Symbol
iC
vBC
+
Collector (C)
n
iC
Collector
iB
iB
p
Base (B)
Base
v
iE
n
BE
Emitter
Emitter (E)
(b)
(a)
Lecture 25
iE
25 - 4
Forward Characteristics
BE voltage --> i E
C
iC
n
i E is composed of i F & i B
iF
Collector
iF
 v BE


i C = i F = I S  exp  --------- – 1 
 VT 


with 10
– 18
iB
B
iF
βF
p
Base
–9
A ≤ I S ≤ 10 A
I S is proportional to the cross-sectional
v
BE
area of the active base region.
iF
IS 
 v BE

--------iB =
exp  --------- – 1  with 20 ≤ β F ≤ 500 .
=

βF
βF 
 VT 

Emitter
iF
βF
n
iE
E
IS  
IS 

 v BE
 v BE


i E = i C + i B =  I S + ------  exp  --------- – 1  = ------  exp  --------- – 1  with 0.95 ≤ α F < 1 .
β F 
αF 

 VT 
 VT 


βF
αF
--------------- .
--------------and β F =
αF =
βF + 1
1 – αF
i C = β F i B , i E = i B + i C = ( β F + 1 )i B and i C = α F i E .
Lecture 25
25 - 5
Reverse Characteristics
BC voltage --> i R & i R ⁄ β R
 v BC


i R = I S  exp  --------- – 1  and i E = – i R
 VT 


iR
IS 
 v BC

i B = ------ = ------  exp  --------- – 1 
βR
βR 
 VT 

0 < β R ≤ 20 and β R is the reverse CE cur-
rent gain.
C
iC
n
v
Collector
BC
iB
B
p
iR
βR
iR
Base
Emitter
n
iE
E
IS  
–IS 

 v BC
 v BC


i C = i E – i B = –  I S + ------  exp  --------- – 1  = -------  exp  --------- – 1  with 0 < α R ≤ 0.95 .
β R 
αR 

 VT 
 VT 


βR
αR
α R = --------------- and β R = ---------------- .
βR + 1
1 – αR
Lecture 25
25 - 6
α F or α R
αF
αR
----------------------------βF =
or β R =
1 – αF
1 – αR
0.1
0.5
0.9
0.99
0.11
1
9
499
Full Transport Model Equations - Arbitrary Bias Conditions
 v BE
 v BC  I S 
 v BC


i C = I S  exp  --------- – exp  ---------  – ------  exp  --------- – 1  .
 VT 
 VT   βR 
 VT 


 v BE
 v BC  I S 
 v BE


i E = I S  exp  --------- – exp  ---------  + ------  exp  --------- – 1  .
 VT 
 VT   βF 
 VT 


IS 
 v BE
 v BC
 IS 

i B = ------  exp  --------- – 1  + ------  exp  --------- – 1  .
βF 
 VT 
 VT 
 βR 

Require 4 parameters - I S , β F , β R and T to characterize an individual
BJT.
Lecture 25
25 - 7
Example
Determine iC, iE and iB if IS = 10
V T = 25mV , β F = 100 and β R = 2 .
V BB = V BE = 0.7V and
– 15
A,
VBC
-
IC
C
B +
V
BB
+
-
V BC = V BB – V CC = – 3.3V
IB
+
VBE
0.75
0.7VV
VCC
+
-
4
5V
E
- IE
 v BE
 v BC  I S 
 v BC


I C = I S  exp  --------- – exp  ---------  – ------  exp  --------- – 1  = 1.45mA
 VT 
 VT   βR 
 VT 


 v BE
 v BC  I S 
 v BE


I E = I S  exp  --------- – exp  ---------  + ------  exp  --------- – 1  = 1.46mA
 VT 
 VT   βF 
 VT 


IS 
 v BE
 v BC
 IS 

I B = ------  exp  --------- – 1  + ------  exp  --------- – 1  = 14.5µA
βF 
 VT 
 VT 
 βR 

Lecture 25
25 - 8
PNP Transistor
iE
C
E
Emitter (E)
v
p
EB
iE
Emitter
iB
iB
n
B
Base
Base (B)
iC
p
v
CB
Collector
(a)
iC
Collector (C)
C
(b)
E
 v EB
 v CB  I S 
 v CB


------------------i C = I S  exp 
 – exp 
  –  exp  --------- – 1  .
 VT 
 VT   βR 
 VT 


 v EB
 v CB  I S 
 v EB


i E = I S  exp  --------- – exp  ---------  + ------  exp  --------- – 1  .
 VT 
 VT   βF 
 VT 


IS 
 v EB
 v CB
 IS 

i B = ------  exp  --------- – 1  + ------  exp  --------- – 1  .
βF 
 VT 
 VT 
 βR 

Lecture 25
25 - 9
E
iE
iF
v
EB
iB
B
p
n
Base
B
B
p
iC
Emitter
i
iF
βF
E
iE
iR
β
R
v
Collector
CB
C
Emitter
n
Base
p
iR
iC
p
Collector
C
Operating Regions of the Bipolar Transistor
Each pn junction may be independently biased.
Have 4 regions of operation.
Both pn junctions reverse biased - cut-off region.
Both pn junctions forward biased - saturation region (closed switch).
Lecture 25
25 - 10
BE junction forward and BC junction reverse biased - forward active region
(normal mode). Have high current, high voltage and high power gains. ECL
logic switch between cut-off and forward active (fastest bipolar logic).
BE junction reverse and BC junction forward biased - reverse active region
(inverse-active mode). Have low common emitter gain and this mode is not
often used. It is used in TTL circuits and for some analog switching applications.
Base-Emitter Junction
Lecture 25
Base-Collector Junction
Forward Bias
Reverse Bias
Forward Bias
Saturation Region
(Closed Switch)
Forward-Active Region
(good amplifier)
Reverse Bias
Reverse-Active Region
(poor amplifier)
Cut-Off Region
(open switch)
25 - 11