ELECTRONIC DEVICES & CIRCUITS (PH 114)

Structure and working of BJT

Dr. Rajanikanta Parida

Department of Physics, ITER
Siksha O Anusandhan Deemed to be University

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 Structure and working of BJT

Introduction to Bipolar Junction Transistors (BJTs)

The transistor was invented by a team of three men at Bell Laboratories in 1947. Although
this first transistor was not a bipolar junction device, it was the beginning of a technological
revolution that is still continuing.
All of the complex electronic devices and systems today are an outgrowth of early
developments in semiconductor transistors.
Two basic types of transistors are the bipolar junction transistor (BJT) and the field-effect
transistor (FET).
The BJT is used in two broad areas- as a linear amplifier, electronic switch and oscillator.
Structure of BJT
The BJT (bipolar junction transistor) is
constructed with three doped semiconductor
regions separated by two p-n junctions. The
three regions are called emitter (E), base (B),
and collector (C).
Physical representations of the two types of
BJTs are shown in Figure. One type consists
of two n regions separated by a p region
(npn), and the other type consists of two p
regions separated by n region (pnp). The
term bipolar refers to the use of both holes
and electrons as current carriers in the
transistor structure.

The pn junction joining the base region and the

emitter region is called the base-emitter junction.
The pn junction joining the base region and the collector region is called the base-collector
junction, as indicated in Figure (b).
A wire lead connects to each of the three regions, as shown. These leads are labeled E, B, and
C for emitter, base, and collector, respectively. The base region is lightly doped and very thin
compared to the heavily doped emitter and the moderately doped collector regions. Figure 2
shows the schematic symbols for the npn and pnp bipolar junction transistors.

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 Structure and working of BJT

Basic Transistor Operation

Figure shows the proper bias arrangement for both npn and pnp transistors for active
operation as amplifier. In both cases, the base- emitter (BE) junction is forwarded-biased and
the base-collector (BC) junction is reverse-biased.

Transistor Currents

The directions and schematic symbol of the currents in an npn transistor and those for a pnp
transistor are shown in the figure. The arrow on the emitter of the transistor symbols points is
the direction of convention current. These diagrams show that the emitter current (IE) is the
sum of the collector current (IC) and the base current (IB), expressed as
IE = IC + IB
Transistor Characteristics and Parameters
When the transistor is connected to dc bias voltage, as shown in the Figure (a) for npn and
Figure (b) for pnp types, VBB forward-biases the base-emitter junction, and VCC reverse-
biases the base-collector junction.
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 Structure and working of BJT

DC Beta (βdc) and DC Alpha (αdc)

The ratio of the dc collector current (IC) to the dc base current (IB) is the dc beta (βdc), also αT
is called the Base transport factor. It takes into account any recombination of excess minority
carrier electron in the base.
𝐼𝐼𝐶𝐶
𝛽𝛽𝑑𝑑𝑑𝑑 =
𝐼𝐼𝐵𝐵
The ratio of the dc collector (IC) to the dc emitter current (IE) is the dc alpha (αdc), also called
common base current gain. The alpha is a less-used parameter than beta in transistor circuits
𝐼𝐼𝐶𝐶
𝛂𝛂𝑑𝑑𝑑𝑑 =
𝐼𝐼𝐸𝐸
𝐼𝐼𝐶𝐶
=> 𝛂𝛂𝑑𝑑𝑑𝑑 =
𝐼𝐼𝐶𝐶 + 𝐼𝐼𝐵𝐵
𝐼𝐼𝐶𝐶 /𝐼𝐼𝐵𝐵
=> 𝛂𝛂𝑑𝑑𝑑𝑑 =
𝐼𝐼𝐶𝐶 /𝐼𝐼𝐵𝐵 + 1
𝛽𝛽𝑑𝑑𝑑𝑑
=> 𝛂𝛂𝑑𝑑𝑑𝑑 =
𝛽𝛽𝑑𝑑𝑑𝑑 + 1
Current and Voltage Analysis
Consider the basic transistor bias circuit configuration shown in the Figure. Three transistor
dc currents and three dc voltages can be identified.
• IB: dc base current
• IE: dc emitter current
• IC: dc collector current
• VBE: dc voltage at base with respect to emitter
• VCB: dc voltage at collector with respect to base
• VCE: dc voltage at collector with respect to emitter
VBB forward-biases the base-emitter junction and VCC reverse-biases the base-collector
junction. When the base-emitter junction is forward-biased, it is like a forward-biased diode
and has a nominal forward voltage drop of VBE ≈ 0.7 V. Since the emitter is at ground (0 V),
by Kirchhoff’s voltage law, the voltage across RB is
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 Structure and working of BJT

Substituting for VRB yields

Solving for IB

The voltage VCE can be written as: VCE = VCC - VRC

Since the drop across RC is VRC = ICRC
The voltage at the collector with respect to the emitter can be written as
VCE = VCC - ICRC
Where 𝐼𝐼𝐶𝐶 = 𝛽𝛽𝑑𝑑𝑑𝑑 𝐼𝐼𝐵𝐵
The voltage across the reverse-biased collector-base junction is
VCB = VCE - VBE

Modes of BJT Operation

BJT has four operating modes
1. Cut-off mode B – E junction is reversed biased
B – C is reversed biased.
This mode occurs when Ic=0
2. Saturation mode B – E junction is forward biased
B – C is forward biased.
Saturation occurs when there is no longer change of Ic for a change
of IB
3. Active Mode B – E junction is forward biased
B – C is reversed biased.
This mode occurs if Ic= β IB valid

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 Structure and working of BJT

B – E junction is reversed biased

B – C is forward biased.
Here the role of emitter and collector are reversed. An inverse
4. Inverse active mode
active characteristic is not same as forward active characteristics.
So the transistor is not a symmetrical device.

1. Let B – E voltage is zero or reversed biased i.e

𝑉𝑉𝐵𝐵𝐵𝐵 ≤ 0 (with the base lead open as shown in the
figure). Then majority carrier electron from emitter
will not be injected into the base, resulting in a base
current of zero (IB = 0).
Again B – C junction is also reversed biased
Thus IE = 0 and IC = 0
This condition is called cut off condition.
Under this condition, there is very small of collector leakage current, ICEO, due mainly to
thermally produced carriers. Because, ICEO is extremely small, it will usually be
neglected in circuit analysis so that VCE = VCC.
2. If B – E junction is forward biased then, IE, IB and IC exist.
Now applying Kirchhoff’s voltage law to the collector loop we get
VCC = ICRC + VCB + VBE
 VCC = ICRC + VCE
 VCC = VR + VCE
If VBE is less then IC will be less and VR will be less accordingly. If VCC is large enough
and VR is small enough then, VCB > 0.
Thus B – C junction is reversed biased. This is the condition of active mode of BJT.
3. If VBE increases then IC will increase.
 IC RC (= VR) will increase
 VCE will decrease ( Since VCE = VCC – ICRC).
At some point then IC is large enough that VR + VCE = 0
across B-C junction. Then VCE reaches its saturation value,
VCE(sat) .
Then B – C junction is forward biased (VCB < 0) and IC can
increase no further even with a continued increase in IB. And VCE(sat) is usually only 0.2 –
0.3 V for silicon transistors.
This condition is called saturation. Thus at saturation mode of operation we have both B
– C junctions are forward biased.

Collector Characteristics Curves

The collector characteristic curves explains how IC varies with VCE, for specified values of
IB.
A family of collector characteristics curves are shown in the figure
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 Structure and working of BJT

Assume that VBB is set to produce a certain

value of IB and VCC is zero. For this condition,
both the base-emitter junction and the base-
collector junction are forward-biased because
the base is at approximately 0.7 V (for Si) while
the emitter and the collector are at 0 V.
Here, the base current is through the base-
emitter junction because the low impedance
path to ground, and IC is zero. When both
junctions are forward-biased, the transistor is in
the saturation region of its operation.
As VCC is increased, VCE increases gradually as the collector current increases. When VCE
exceeds VK (0.7 V for Si), the base-collector junction becomes reverse-biased and the
transistor goes into the active or linear region of its operation. Once the base-collector
junction is reverse-biased, IC remains essentially constant for a given value of IB as VCE
continues to increase. For this region of the characteristic curve, the value of IC is determined
only by the relationship expressed as
𝐼𝐼𝐶𝐶 = 𝛽𝛽𝑑𝑑𝑑𝑑 𝐼𝐼𝐵𝐵
When VCE reaches a sufficiency high voltage, the reverse-biased base- collector junction goes
into breakdown; and the collector current increases rapidly. A transistor should never be
operated in this breakdown region.
When IB = 0, the transistor is in the cutoff region although there is a very small collector
leakage current.
Cutoff
When IB = 0, the transistor is in the cutoff region of
its operation. This is shown in Figure with the base
lead open, resulting in a base current of zero. Under
this condition, there is very small of collector leakage
current, ICEO, due mainly to thermally produced
carriers. Because, ICEO is extremely small, it will
usually be neglected in circuit analysis so that VCE =
VCC. Moreover, in cutoff mode, both the base-emitter
and the base-collector junction are reverse-biased.
Saturation
When the base-emitter junction becomes forward-
biased and the base-current is increased, the
collector current also increases and VCE decreases
as a result of more drop across the collector
resistor (VCE = VCC – ICRC).
This is illustrated in Figure. When VCE reaches its
saturation value, VCE(sat) , the base-collector
junction becomes forward-biased and IC can
increase no further even with a continued increase in IB.
And VCE(sat) is usually only 0.2 to 3V for silicon transistors.

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 Structure and working of BJT

DC Load Line
Cutoff and saturation mode can be illustrated in
relation to the collector characteristics curves by
the use of a load line.
DC load line drawn on a family of curves
connecting the cutoff point and the saturation
point is shown in the figure .
The bottom of the load line is at ideal cutoff
where IC = 0 and VCE = VCC.
The top of the load line is at saturation where IC =
IC(sat) and VCE = VCE(sat).
In between cutoff and saturation along the load
line is the active region of the transistor’s operation.
Transistor as a Switch
The basic operation as a switching device is illustrated in Figure below. In part (a), the
transistor is in the cutoff region because the base-emitter junction is not forward-biased. In
this condition, there is, ideally, an open between collector and emitter, as indicated by the
switch equivalent. In part (b), the transistor is in the saturation region because the base-
emitter junction and the base-collector junction are forward-biased and the base current is
made large enough to cause the collector to reach its saturation value.
In this condition, there is, ideally, a short between collector and emitter, as indicated by the
switch equivalent. Actually, a voltage drop of up to a few tenths of a volt normally occurs,
which is the saturation voltage, VCE(sat) .
Conditions in Cutoff:
As mentioned before, a transistor is in the cutoff region when the base-emitter junction it not
forward-biased. Neglecting leakage current, all of the currents are zero, and
VCE = VCC. Or VCE(cutoff) = VCC

Conditions in Saturation:
When the base-emitter junction is forward-biased and there is enough base current to produce
a maximum collector current, the transistor is saturated. The formula for collector saturation
current is

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 Structure and working of BJT

Since VCE(sat) is very small, it is neglected.

The minimum value of base current needed to produce saturation is

IB should be significantly greater than IB(min) to keep the transistor well into saturation.
Application of a Transistor Switch:
The transistor shown in Figure is used as a switch to turn the LED on
and off. For example, a square wave input voltage with a period of 2 s
is applied to the input as indicated. When the square wave is at 0 V, the
transistor is in cutoff and, since there is no collector current, the LED
does not emit light. When the square wave goes to its high level, the
transistor saturates. These forward-biased the LED, and the resulting
collector through the LED causes it to emit light. So, we have a
blinking LED that is on for 1 s and off for 1 s.
Solution:

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