India-Singapore working on semiconductor ecosystem

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 Who Invented the Transistor?
• Developed by John Bardeen, Walter Brattain, William Shockley at Bell Telephone
Labs in Murray Hill, New Jersey, 1947.
 PNP and NPN transistor
structure
The transistor regions are:

P N – Emitter (E) – send the

N P
carriers into the base
BJT is bipolar because both holes P N
region and then on to the
(+) and electrons (-) will take part
collector
in the current flow through the
– Base (B) – acts as control
device Ic(mA) IC(mA)
IB(µA) IB(µA)
region. It can allow
carriers to flow
IE(mA) IE(mA)– Collector (C) – collects

the carriers
Arrow shows the current flow direction
 Transistor Construction
A transistor has three doped regions.
For both types, the base is a narrow region sandwiched between the larger
collector and emitter regions.

▪ The emitter region is heavily doped

and its job is to emit carriers into the N C
base.

▪ The base region is very thin and

lightly doped.
▪ Most of the current carriers
P B
injected into the base pass on to the
collector.

▪ The collector region is moderately N E

doped and is the largest of all three
regions.
 Poll
The doped region in a transistor are ________
a) Emitter and Collector
b) Emitter and Base
c) Collector and Base
d) Emitter, Collector and Base

Ans (d)
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 Transistor Configuration
Transistor configuration : 3 types of configuration:

Common Base Common Collector Common Emitter

 Common-Emitter Configuration
Emitter terminal is common for input and output circuit Input – BE
Output – CE
Mostly applied in practical amplifier circuits, since it provides good
voltage, current and power gain
 Common-Collector Configuration
The input signal is applied to the base terminal and the output
is taken from the emitter terminal.
Collector terminal is common to the input and output of the
circuit Input – BC
Output – EC
 Common-Base Configuration
Base terminal is a common point for input and output. Input – EB
Output – CB
Not applicable as an amplifier because the relation between input
current gain (IE) and output current gain (IC) is approximately 1
 Poll
Which junction is forward biased when transistor is used as an amplifier?
a) Emitter-Base
b) Emitter-Collector
c) Collector-Base
d) No junction is forward biased

Ans (a)
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 Poll
Which of the following is true in construction of a transistor?
a) the collector dissipates lesser power
b) the emitter supplies minority carriers
c) the collector is made physically larger than the emitter region
d) the collector collects minority charge carriers

Ans (c)
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 Common Emitter Configuration
• Emitter is common to both the input and output circuits.
• Input signal is applied to the base of the transistor.
• Output is taken from the collector.
• Dc biasing Vbb and Vcc.
• Rb and RL input and output resistance
• High Voltage Gain
• Output signal is 180 degree out of phase
• From input signal.
 CE configuration : Input characteristic
Input characteristic:
input current (IB) against
input
voltage (VBE) for several
output voltage (VCE)
From the graph

– I =0A
B VBE < 0.7V
– I = value
B (Si) VBE >
0.7V (Si)

The transistor turned on

when VBE = 0.7V
 CE configuration : Output characteristic
Output characteristic: output
current (IC) against output
voltage (VCE) for several input
current (IB)
3 operating regions:

– Saturation region
– Cut-off region
– Active region

EveryCircuit - Basic BJT Topologies

I-V Characteristic for CE configuration : Output
characteristic

• Saturation region – in which both junctions are forward-biased

and IC increase linearly with VCE

• Cut-off region – where both junctions are reverse-biased,

the IB is very small, and essentially no IC flows, IC is
essentially zero with increasing VCE
• Active region – in which the transistor can act as a linear
amplifier, where the BE junction is forward-biased and BC
junction is reverse-biased. IC increases drastically although
only small changes of IB.
• Saturation and cut-off regions – areas where the transistor
can operate as a switch
• Active region – area where transistor operates as an
amplifier
 Transistor as a Switch
1.Off State (Cutoff Region):
1. When the transistor is in the cutoff region, no current flows between the collector and emitter

2. In the case of a BJT, the base-emitter junction is not forward biased, and no base current flows. This means
the transistor is effectively "off" and behaves like an open switch.

2.On State (Saturation Region):

1. When the transistor is in the saturation region (also called the on state), the transistor conducts current
freely between the collector and emitter.

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 Applications
•Amplifiers (audio, RF, and general-purpose)
•Signal processing
•Oscillators
•Switches (in digital circuits)
 Poll
When does the transistor act like an open switch?
a) cut off region
b) inverted region
c) saturated region
d) active region

Ans (a)
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 Poll
In the output characteristics, the resistance is the __________
a) Slope of the curve
b) Trace of the curve
c) Asymptote of the curve
d) Reciprocal of the slope of the curve

Ans (d)
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 Current Relationships
Relations between IC and IE :
• α = IC
IE
Value of α usually 0.9998 to 0.9999, α ≈
• 1
• Relations between IC and IB :
β = IC @ IC = βIB IB
•Value of β usually in range of 50 - 400
•The equation, IE =IC + IB can also
written in β
IC = βIB
IEThe
= βIcurrent
B + IB => gain
IE = (βfactor
+ 1)IB, α and
• β is:
α=β @β= α
 Poll
A bipolar junction transistor has beta=250 and base current=10micro ampere. What is the
collector current?
a) 25 micro ampere
b) 10 micro ampere
c) 2.5 milli ampere
d) 10 milli ampere

Ans (c) 22
 Poll
What is the value of current gain (α) where emitter current(Ic)=1.6 milli ampere; collector
current=1.5 milli ampere and base current=0.1 milli ampere?
a) 3.1
b) 0.937
c) 0.8
d) 0.31

Ans (b) 23
Checking a Transistor with an Ohmmeter

▪ Low resistance across the junctions in both directions:

transistor is shorted.

▪ High resistance on both directions: transistor is open.

▪ In these cases, the transistor is defective and must be

replaced.
 Homework
Which configuration of BJT is called as Emitter Follower? Explain.

Can back-to-back diodes can be used as a transistor? Justify the answer.