Transistors: Bipolar Junction Transistors (BJT) General configuration and definitions Notes | EduRev

: Transistors: Bipolar Junction Transistors (BJT) General configuration and definitions Notes | EduRev

 Page 1


Transistors: 
Bipolar Junction Transistors (BJT) 
 
General configuration and definitions 
 
The transistor is the main building block “element” of electronics. It is a semiconductor 
device and it comes in two general types: the Bipolar Junction Transistor (BJT) and the 
Field Effect Transistor (FET). Here we will describe the system characteristics of the BJT 
configuration and explore its use in fundamental signal shaping and amplifier circuits. 
 
The BJT is a three terminal device and it comes in two different types. The npn BJT and 
the pnp BJT. The BJT symbols and their corresponding block diagrams are shown on 
Figure 1. The BJT is fabricated with three separately doped regions. The npn device has 
one p region between two n regions and the pnp device has one n region between two p 
regions. 
The BJT has two junctions (boundaries between the n and the p regions). These junctions 
are similar to the junctions we saw in the diodes and thus they may be forward biased or 
reverse biased. By relating these junctions to a diode model the pnp BJT may be modeled 
as shown on Figure 2. 
 
The three terminals of the BJT are called the Base (B), the Collector (C) and the Emitter 
(E). 
 
C
C
BB
EE
n
n
p
 
(a) npn transistor 
C
C
BB
EE
n
p
p
 
(b) pnp transistor 
 
Figure 1. BJT schematics and structures. (a) npn transistor, (b) pnp transistor 
 
B
E
C
Base-Emitter junction
Base-Collector junction Base-Collector junction
Base-Emitter junction
B
E
C
 
Figure 2 
22.071/6.071 Spring 2006, Chaniotakis and Cory  1 
Page 2


Transistors: 
Bipolar Junction Transistors (BJT) 
 
General configuration and definitions 
 
The transistor is the main building block “element” of electronics. It is a semiconductor 
device and it comes in two general types: the Bipolar Junction Transistor (BJT) and the 
Field Effect Transistor (FET). Here we will describe the system characteristics of the BJT 
configuration and explore its use in fundamental signal shaping and amplifier circuits. 
 
The BJT is a three terminal device and it comes in two different types. The npn BJT and 
the pnp BJT. The BJT symbols and their corresponding block diagrams are shown on 
Figure 1. The BJT is fabricated with three separately doped regions. The npn device has 
one p region between two n regions and the pnp device has one n region between two p 
regions. 
The BJT has two junctions (boundaries between the n and the p regions). These junctions 
are similar to the junctions we saw in the diodes and thus they may be forward biased or 
reverse biased. By relating these junctions to a diode model the pnp BJT may be modeled 
as shown on Figure 2. 
 
The three terminals of the BJT are called the Base (B), the Collector (C) and the Emitter 
(E). 
 
C
C
BB
EE
n
n
p
 
(a) npn transistor 
C
C
BB
EE
n
p
p
 
(b) pnp transistor 
 
Figure 1. BJT schematics and structures. (a) npn transistor, (b) pnp transistor 
 
B
E
C
Base-Emitter junction
Base-Collector junction Base-Collector junction
Base-Emitter junction
B
E
C
 
Figure 2 
22.071/6.071 Spring 2006, Chaniotakis and Cory  1 
Since each junction has two possible states of operation (forward or reverse bias) the BJT 
with its two junctions has four possible states of operation. 
 
For a detailed description of the BJT structure see: Jaeger and Blalock, Microelectronic 
Circuit Design, McGraw Hill. 
Here it is sufficient to say that the structure as shown on Figure 1 is not symmetric. The n 
and p regions are different both geometrically and in terms of the doping concentration of 
the regions. For example, the doping concentrations in the collector, base and emitter may 
be, , and  respectively. Therefore the behavior of the device is not electrically 
symmetric and the two ends cannot be interchanged. 
15
10
17
10
19
10
 
Before proceeding let’s consider the BJT npn structure shown on Figure 3. 
 
Base-Emitter junction Base-Collector junction
p nn
B
C E
R
R
-     V     + -     V     +
I I
I
EC
B
BE CB
E
C
 
 
Figure 3. Biasing voltages of npn transistor 
 
With the voltage and as shown, the Base-Emitter (B-E) junction is forward biased 
and the Base-Collector (B-C) junction is reverse biased. 
BE
V
CB
V
 
The current through the B-E junction is related to the B-E voltage as 
 
 
( )
/
1
BE T
VV
Es
IIe = - (1.1) 
Due to the large differences in the doping concentrations of the emitter and the base regions 
the electrons injected into the base region (from the emitter region) results in the emitter 
current 
E
I . Furthermore the number of electrons injected into the collector region is 
directly related to the electrons injected into the base region from the emitter region.  
 
Therefore, the collector current is related to the emitter current which is in turn a function 
of the B-E voltage. 
 
The voltage between two terminals controls the current through the third terminal. 
 
This is the basic principle of the BJT 
 
22.071/6.071 Spring 2006, Chaniotakis and Cory  2 
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