Bipolar Junction Transistors [BJT]:
A bipolar junction transistor is a specially constructed, with three terminal semiconductor device containing two PN junctions. It can be formed from a bar of material that has been doped in such a way that it changes from N to P and back to N, or from P to N and back to P. In either case, a junction is created at each of the two boundaries where the material changes from one type to the other. Fig. (1) shows the two ways that it is possible to alternate material types and thereby obtain two junctions.
Figure 1-a
Figure 1-b
Figure 1: Transistor Construction.
When a transistor is formed by sandwiching a single P region between two N regions, as shown in Fig. (1-a), it is called an NPN type. Fig. (1-b) shows the type PNP, containing a single N region between two P regions.
The middle region of each transistor type is called the base of the transistor. Of the remaining two regions, one is called the emitter and the other is called the collector of the transistor, the base region is purposely shown thinner than either the emitter or collector region.
The outer layers of the transistor are heavily doped semiconductor materials having widths much greater than those of the sandwiched P or
N type material depending on the type of the transistor (PNP or NPN).

The ratio of the total width of the transistor to that of the center layer is
150/1. The doping of the sandwiched layer is also considerably less than that of the outer layers (typically 10:1 or less). This lower doping level decreases the conductivity (increases the resistance) of this material by limiting the number of free carriers. These two basic types of transistors along with their circuit symbols are shown in Fig 1.
BJT Operation:
The basic operation of the transistor will now be described using the pnp transistor of Fig. (2). The operation of the npn transistor is exactly the same if the roles played by the electron and hole are interchanged. When the E-B junction is forward-biased, a large number of majority carriers will diffuse across the forward-biased p-n junction into the n-type material (base).
Since the base is very thin and has a low conductivity (lightly doping), a very small number of these carriers will take this path of high resistance to the base terminal. The larger number of these majority carriers will diffuse across the reverse-biased C-B junction into the p-type material (collector). The reason for the relative ease with which the majority carriers can cross the reverse-biased C-B junction is easily understood if we consider that for the reverse-biased diode the injected majority carriers will appear as minority carriers in the n-type base region material. Combining this with the fact that all the minority carriers in the depletion region will cross the reverse-biased junction of a diode accounts for the flow indicated in Fig. (2).
Figure (2)

Applying Kirchhoff's current law to the transistor of Fig. (2), we obtain
IE = IC + IB
The collector current, however, is comprised of two components: the majority and minority carriers. The minority-current component is called the leakage current and is given the symbol I
CO
(I
C
current with emitter terminal Open).
I
C
= I
C
majority + I
CO
minority
Transistor Configurations
A transistor may be connected in any one of three basic configurations: common emitter (CE), common base (CB), and common collector (CC).
The term common is used to denote the element that is common to both input and output circuits. Because the common element is often grounded, these configurations are frequently referred to as grounded emitter, grounded base, and grounded collector.
Common-Emitter (CE) Configuration:
The common-emitter configuration (CE) shown in Fig. (3) is the arrangement most frequently used in practical amplifier circuits, since it provides good voltage, current, and power gain. The common emitter also has a somewhat low input resistance (500 Ω -1500 Ω), because the input is applied to the forward-biased junction, and a moderately high output resistance (30 kΩ -50 kΩ or more), because the output is taken off the reverse-biased junction. Since the input signal is applied to the baseemitter circuit and the output is taken from the collector-emitter circuit, the emitter is the element common to both input and output.
NPN
PNP
Figure (3) Common-Emitter (CE) configuration

In the dc mode the levels of IC and IE due to the majority carriers are related by a quantity called alpha and defined by the following equation:
The current relations developed in the common-emitter configuration is:
I
E
= I
C
+ I
B
and I
C
= α I
E
+ I cbo
.
I
E
= α I
E
+ I cbo
+ I
B
The external voltage source V
BB
is used to forward bias the E-B junction and the external voltage source V
CC
is used to reverse bias C-B junction.
The magnitude of V
CC
must be greater than V
BB
to ensure the CB junction remains reverse biased, since, as can be seen in the Fig. (3),
V
CB
= V
CC
- V
BB
When a transistor is connected in a common-emitter configuration, the input signal is injected between the base and emitter, which is a low resistance, low-current circuit. As the input signal swings positive, it also causes the base to swing positive with respect to the emitter. This action decreases forward bias which reduces collector current (IC ) and increases collector voltage (making VC more negative). During the negative alternation of the input signal, the base is driven more negative with respect to the emitter. This increases forward bias and allows more current carriers to be released from the emitter, which results in an increase in collector current and a decrease in collector voltage (making
VC less negative or swing in a positive direction). The collector current that flows through the high resistance reverse-biased junction also flows through a high resistance load (not shown), resulting in a high level of amplification.
Since the input signal to the common emitter goes positive when the output goes negative, the two signals (input and output) are 180 degrees

out of phase. The common-emitter circuit is the only configuration that provides a phase reversal.
The common-emitter is the most popular of the three transistor configurations because it has the best combination of current and voltage gain. The term GAIN is used to describe the amplification capabilities of the amplifier. It is basically a ratio of output versus input. Each transistor configuration gives a different value of gain even though the same transistor is used. The transistor configuration used is a matter of design consideration. However, as a technician you will become interested in this output versus input ratio (gain) to determine whether or not the transistor is working properly in the circuit.
The current gain in the common-emitter circuit is called BETA (β).
Beta is the relationship of collector current (output current) to base current (input current). To calculate beta, use the following formula:
For example, if the input current (IB) in a common emitter changes from
75 A to 100 A and the output current (IC) changes from 1.5 mA to 2.6 mA, the current gain (β) will be 44.
The resistance gain of the common emitter can be found in a method similar to the one used for finding beta:
Once the resistance gain is known, the voltage gain is easy to calculate since it is equal to the current gain (β) multiplied by the resistance gain
(E= βR). And, the power gain is equal to the voltage gain multiplied by the current gain (P = βE).
The input (base) characteristics for the CE configuration are a plot

of the base (input) current ( I
B
) versus the base-to-emitter (input) voltage
(V
BE
) for a range of values of collector-to-emitter (output) voltage (V
CE
) as shown in Fig.(4-a). Note that I
B
increases as V
CE
decreases, for a fixed value of V
BE
.
The output (collector) characteristics for CE configuration are a plot of the collector (output) current ( I
C
) versus collector-to-emitter (output) voltage (V
CE
) for a range of values of base (input) current ( I
B
) as shown in Fig. (4-b).
Example:
(a) Using the characteristics of Fig. 3.14a, determine dc at IB _ 80 _A and
VCE _ 5 V.
Figure (4-a): Common-emitter output characteristics.
Figure (4-b): Common-emitter input characteristics.
(b) Repeat part (a) at IB _ 5 _A and VCE _ 15 V.
(c) Repeat part (a) at IB _ 30 _A and VCE _ 10 V.
(d) Reviewing the results of parts (a) through (c), does the value of dc change from point to point on the characteristics?
Where were the higher values found?
Can you develop any general conclusions about the value of dc on a set of characteristics such as those provided in Fig. 3.14a?

Example:
(a) Using the characteristics of Fig. 4, determine I
C at I
B
_ 30 _A and V
CE
_
10 V.
(b) Using the characteristics of Fig. 4, determine I
C at V
BE
_ 0.7 V and V
CE
_
15 V.
Solution
(a) At the intersection of I
B
= 30 µA and V
CE
= 10 V, I
C
= 3.4 mA.
(b) Using Fig. 4-b, I
B
=20 µA at V
BE
= 0.7 V. From Fig. 4-a we find that
I
C
= 2.5 mA at the intersection of I
B
=20 µA and V
CE
= 15 V.