Solid State Physics and

Devices
Noor Fatima
Email: noorfatima@ucp.edu.pk
Overview
• Bipolar Junction Transistor
• CE Configuration
• BJT as a Switch
Bipolar Junction Transistor
• A bipolar junction transistor is a three-terminal semiconductor device
that consists of two p-n junctions which are able to amplify or
magnify a signal. It is a current controlled device. The three terminals
of the BJT are the base, the collector, and the emitter.
Transistor Configuration
• There are three possible configurations possible when a transistor is
connected in a circuit:
(a) Common base, (b) Common emitter (c) Common collector.
• We will be focusing on the common emitter configurations.
• The behavior of a transistor can be represented by DC current-
voltage (I-V) curves, called the static characteristic curves of the
device.
Transistor Configuration
• The three important characteristics of a transistor are:
(i) Input characteristics
(ii) Output characteristics
(iii) Transfer Characteristics.
• These characteristics give information about various transistor
parameters, e.g. input and output dynamic resistance, current
amplification factors, etc.
Common Emitter Transistor
Characteristics
• Input Characteristics: The variation of the base current IB with the
base-emitter voltage VBE keeping the collector-emitter voltage VCE
fixed, gives the input characteristic in CE mode.
• Input Dynamic Resistance (ri): This is defined as the ratio of change in
base emitter voltage (∆VBE) to the resulting change in base current
(∆IB) at constant collector-emitter voltage (VCE). This is dynamic and it
can be seen from the input characteristic, its value varies with the
operating current in the transistor:
Common Emitter Transistor
Characteristics
• Output Characteristics: The variation of the collector current IC with
the collector-emitter voltage VCE is called the output characteristic.
The plot of IC versus VCE for different fixed values of IB gives one output
characteristic. Since the collector current changes with the base
current, there will be different output characteristics corresponding to
different values of IB.
• Output Dynamic Resistance (ro): This is defined as the ratio of change
in collector-emitter voltage (∆VCE) to the change in collector current
(∆IC) at a constant base current IB.
Problem
• In a transistor of operating in CE mode a constant base current of 20
µA flows. The collector current changes from 3.0mA to 3.5mA when
the collector emitter voltage changes from 6V to 12V. Find the value
of output resistance and β at 12V.
Solution:
Solution
Common Emitter Transistor
Characteristics
• The output characteristic of common emitter configuration consists of
three regions: Active, Saturation and Cut‐off.
• Active region: In this region base‐emitter junction is forward biased
and base‐collector junction is reversed biased. The curves are
approximately horizontal in this region.
• Saturation region: In this region both the junctions are forward biased.
• Cut‐off: In this region, both the junctions are reverse biased. When the
base current is made equal to zero, the collector current is reverse
leakage current ICEO. The region below IB = 0 is the called the cutoff
region.
Common Emitter Transistor
Characteristics
• Transfer Characteristics: The transfer characteristics are plotted
between the input and output currents (IB versus IC). Both IB and IC
increase proportionately.
• Current amplification factor (β) This is defined as the ratio of the
change in collector current to the change in base current at a constant
collector-emitter voltage (VCE) when the transistor is in active state.
Common Emitter Transistor
Characteristics
• This is also known as small signal current gain and its value is very
large. The ratio of IC and IB we get what is called βdc of the transistor.
Hence,

• Since IC increases with IB almost linearly, the values of both βdc and βac
are nearly equal.
Circuit Diagrams
Problem
• In a transistor connected in CE mode if the base current is changed
from 20 µA to 30 µA at a fixed VCE = 7.5V, the collector current
changes from 2.2mA to 3.2 mA. Find βAC and also βDC at the mean
operating point.
Solution
Given Data:
• VCE = 7.5 V, IB changed from 20 µA to 30 µA, IC changes from 2.2mA
to 3.2 mA.

• βAC = = 100

• βDC = = 108
BJT as a Switch
• Switch is a device for the “on and off” of current in circuit.

Forward Bias Reversed Bias

Input in Emitter side Output in Collector side
Using Kirchhoff’s Voltage Law
For the input circuit: In case of Si transistor VBE is 0.6 to 0.7
• -VBB + IB RB + VBE = 0
• VBB = IB RB + VBE
• Vi = IB RB + VBE
For the output circuit: In case of Si transistor VCE
• -VCC + IC RC + VCE = 0
• VCE = VCC – IC RC
• Vo = VCC – IC RC
Transfer Characteristics Curve
Case 1: Vi < 1.6, then IB = 0. So, IC = 0 then from Vo = VCC
• Now, the transistor will be in cut off state or switch off state.
• Case 2: Vi > 1.6 V, IB exist and IC also exist.
• As IC increase, the value of Vo decrease, with increase in Vi, IC Increase
and Vo decrease till Vi become I V. In this, situation the transistor is in
active state.
• Now the situation comes that IC is maximum when Vo is decrease
towards zero but never become zero. Now the transistor is in
saturation state or switched on state.
Transfer Characteristics Curve
• So low input state, switches the transistor off and in high input state,
switch it on. This indicates that a transistor, act as a switch.
Problem
Solution
Solution
Solution
Problem
Problem
Solution
Solution
Difference between NPN and PNP
Transistor
NPN PNP
• Made of two N-type materials • Made of two P-type materials
separated by a thin layer of P- separated by a thin layer of N-
type material. type material.
• The conventional current flows • The conventional current flows
from the collector to the emitter. from the emitter to the collector.
• Preferred for high-speed • Suitable for low-side switching
switching and digital circuits. applications.