UNIT-3: Bipolar Junction Transistor (BJT)

Transistor transfers a signal from a low resistance to high resistance circuit.

The prefix ‘trans’ means the signal transfer property of the device while ‘istor’
classifies it as a solid element in the same general family with resistors.
A “transistor” is nothing but active components, which consists of two P– N
junctions formed by sandwiching either P – type or N – type semiconductor
between a pair of opposite type.
Generally, the operation of a transistor depends on the interaction of both
majority and minority carriers. Due to this reason, transistor also known as bipolar
junction transistor.
Construction of BJT
A BJT comprises a thin slice of doped semiconductor material sandwiched
between two other layers of doped semiconductor material, which can be p-type or
n-type, as shown in Fig. The three pins are doped differently and vary in size. The
three doped regions of a transistor are:
 Emitter
 Base
 Collector
Emitter:
From the structure of a transistor, emitter is the left segment of the transistor,
which supplies a large number of majority carriers. The emitter is always
forward biased with respect to base, so that it can supply a large number of
majority carriers to its junction with the base. The emitter is more heavily doped
than the collector.
Base:
The middle section which forms two segments between the emitter and the
collector is known as base. It is extremely thin, lightly doped and its width is very
small, so that it can pass most of the majority carriers to the collector. The base
emitter junction is forward biased, where as base collector junction is
reversed biased.
Collector:
Collector is a segment, which situated at the right side of the structure of the
transistor, which collects the major amount of majority carriers, which emitted by
the emitter. Normally, this segment is moderately doped and its size is larger than
the emitter and base of a transistor. Collector base junction is always reverse
biased.

From the above structure of a transistor, the PN junction joining the base region
and the emitter region is known as base–emitter junction, where as the PN
junction joining the base and collector region is known as base–collector
junction. The arrow on the emitter leg in keeping with the diode symbol arrow
indicates the direction of current flow when the base emitter junction is forward
biased.
In transistors, the following points are important to be considered as follows.
 There are two PN junctions. Therefore, a transistor may be regarded as
a combination to two diodes connected back to back.
 There are three terminals, taken from each type of semiconductor.
 The middle section is a very thin layer. This is the most important factor
in the function of a transistor.
Operating Principle of BJT
 A bipolar junction transistor (BJT) operates by controlling the flow of
current between its emitter and collector terminals using a small current applied to
the base terminal. This control is achieved by forward-biasing the emitter-base
junction and reverse-biasing the collector-base junction. The key to its operation is
the thin, lightly doped base region which allows most of the injected charge
carriers from the emitter to reach the collector, resulting in a larger collector
current than the base current.
Current Flow Mechanism:
 When the emitter-base junction is forward-biased, charge carriers (electrons
in NPN, holes in PNP) are injected from the emitter into the base.
 The base is intentionally made thin and lightly doped. This means there are
fewer majority carriers in the base to recombine with the injected carriers.
 Therefore, most of the injected carriers diffuse across the base and are swept
into the collector region due to the reverse bias of the base-collector
junction.
 This results in a collector current that is significantly larger than the base
current.
Current Gain:
 The ratio of collector current to base current is called the current gain (β).
 A high current gain is a key characteristic of BJTs, allowing them to amplify
signals.
Transistors are three terminal active devices made from different semiconductor
materials that can act as either an insulator or a conductor by the application of a
small signal voltage. The transistor's ability to change between these two states
enables it to have two basic functions: "switching" (digital electronics) or
"amplification" (analogue electronics).
Then bipolar transistors have the ability to operate within three different
regions:
1. Active Region - the transistor operates as an amplifier and Ic =β.Ib
2. Saturation - the transistor is "fully-ON" operating as a switch and Ic
=I(saturation)
3. Cut-off - the transistor is "fully-OFF" operating as a switch and Ic =0
Bipolar Transistors are current regulating devices that control the amount
of current flowing through them in proportion to the amount of biasing voltage
applied to their base terminal acting like a current-controlled switch.

Types of BJTs
Based on structure, generally a transistor is of two types such as: NPN
transistor and PNP transistor. So, NPN transistor is active components which
consist of two N-type semiconductors separated by a thin segment of P-type
semiconductor as shown in the above figure [a], where as PNP transistor is active
components which consist of two P–type materials, separated by a thin segment of
N–type semiconductor as shown in the above figure [b].
Working Principle of PNP Transistor:

The emitter is biased positive with respect to base. This is the case of
forward bias. The external battery opposes the internal potential carrier in this case.
The current through the junction is carried by majority carrier holes from the P–
type emitter. This is because the holes are repelled by the positive potential of the
battery, and by electrons from the N–type base.
The concentration of impurity into the emitter is greater than that of base. So
the holes carrying the emitter base current greatly outnumbered the electrons. So
the current flowing across the emitter into the thin base layer, due to transistor
action. Most of them cross the base by diffusion and on reaching the collector base
junction are swept into the collector region. These are collected by the collector.
Therefore, the collector current is almost same as the emitter current. This collector
current IC is totally independent of the collector voltage. There will be no change
in Ic even if the collector voltages are increased to a larger amount.
Working Principle of NPN Transistor:
The given figure [ b ] shows the polarity and other arrangements of NPN
transistor. The operation of this transistor is similar to that of PNP only the bias
polarity and direction of current flow is reversed. The current is due to the majority
carrier electrons instead of holes. Emitter is negatively biased, because this is the
condition for a forward bias. The emitter region has the greater conductivity;
therefore, the electrons are o u t n u m b e r e d. The base collector junction should
be reverse biased, so in NPN transistors this required the collector to be biased
positively with respect to base. The electrons entering the thin base region from the
emitter cross the base section by diffusion and on reaching the base collector
junction are swept into the collector region to give a collector current, which is also
known as injected current because this current is created due to electrons injected
from emitter region.