Module VI: Introduction to Transistors

This module covers theory only - numerical problems are not expected.

1. Bipolar Junction Transistor (BJT)

1.1 Structure of BJT

What is a BJT?

A Bipolar Junction Transistor (BJT) is a three-terminal semiconductor device that uses both electrons and holes as charge carriers (hence "bipolar"). It consists of three alternately doped semiconductor regions forming two P-N junctions.

Types of BJT

There are two types of BJT based on the arrangement of doped regions:

NPN Transistor

Two N-type regions sandwiching a thin P-type region
Majority carriers are electrons
More commonly used (electrons have higher mobility)
Arrow on emitter points outward (Not Pointing iN)

PNP Transistor

Two P-type regions sandwiching a thin N-type region
Majority carriers are holes
Less common in modern circuits
Arrow on emitter points inward (Pointing iN)

BJT Structure and Symbols

1.2 Operation of BJT

Operating Principle (NPN Transistor)

For proper operation of an NPN transistor:

Emitter-Base Junction: Forward biased (allows current flow)
Collector-Base Junction: Reverse biased (creates collection field)

Current Flow Mechanism:

When E-B junction is forward biased, electrons from emitter are injected into the thin base region
The base is very thin and lightly doped, so most electrons (95-99%) diffuse through without recombining
These electrons are attracted by the reverse-biased collector and swept into collector region
A small base current (I_B) controls a much larger collector current (I_C)

BJT Current Relationships

I_E = I_C + I_B

I_C = β × I_B = h_FE × I_B

I_C = α × I_E

Where:

β (beta) or h_FE = Current gain (typically 50-300)
α (alpha) = I_C/I_E (typically 0.95-0.99)
α = β/(β+1) and β = α/(1-α)

Key Operating Points

A small change in base current produces a large change in collector current
This current amplification property is the basis of transistor amplifiers
Collector current is almost independent of collector voltage (within limits)
The transistor can be used as a current-controlled current source

1.3 BJT Configurations

A BJT can be connected in three configurations based on which terminal is common to both input and output circuits:

Common Base (CB)
Common Emitter (CE) - Most widely used
Common Collector (CC)

Common Emitter (CE) Configuration

This is the most commonly used configuration because it provides both voltage gain and current gain.

Circuit Arrangement:

Emitter is common to both input and output circuits
Input signal is applied between Base and Emitter
Output is taken between Collector and Emitter

Common Emitter Configuration

Characteristics of CE Configuration

Parameter	Value/Characteristic
Current Gain (β)	High (50-300)
Voltage Gain	High (can be > 100)
Power Gain	Very High (highest of all configurations)
Input Impedance	Medium (1-5 kΩ)
Output Impedance	Medium-High (10-50 kΩ)
Phase Shift	180° (output inverted)

2. Field Effect Transistor (FET)

2.1 Structure and Operation

What is an FET?

A Field Effect Transistor (FET) is a voltage-controlled semiconductor device that uses an electric field to control current flow. Unlike BJT which uses both electrons and holes, FET uses only one type of charge carrier (unipolar device).

Types of FET

JFET (Junction FET)

Uses P-N junction for channel control
N-channel and P-channel types
Depletion mode operation only
Gate must be reverse biased

MOSFET (Metal Oxide Semiconductor FET)

Uses insulated gate (SiO₂ layer)
Enhancement and depletion modes
Higher input impedance than JFET
Most commonly used in digital circuits

N-Channel JFET Structure and Symbol

FET Terminals

Terminal	Function
Gate (G)	Control terminal. Voltage applied here controls current through channel. Very high input impedance.
Drain (D)	Terminal where current exits the channel. Connected to positive supply for N-channel.
Source (S)	Terminal where current enters the channel. Usually connected to ground for N-channel.

FET Operation

For an N-channel JFET:

Current flows from Drain to Source through the N-channel
Gate-Source voltage (V_GS) controls the channel width
More negative V_GS → Wider depletion region → Narrower channel → Less current
At pinch-off voltage (V_P), the channel is completely depleted and current stops
FET is voltage-controlled, unlike BJT which is current-controlled

BJT vs FET Comparison

Parameter	BJT	FET
Type	Bipolar (uses electrons and holes)	Unipolar (uses one carrier type)
Control	Current-controlled	Voltage-controlled
Input Impedance	Low to Medium (1-5 kΩ)	Very High (10¹⁰ - 10¹⁴ Ω)
Noise	Higher	Lower
Temperature Stability	Less stable	More stable
Switching Speed	Moderate	Fast (especially MOSFETs)

3. Applications of BJT and FET

3.1 Amplification

Transistor as an Amplifier

Both BJT and FET can amplify weak signals. The transistor is biased in the active region (BJT) or saturation region (FET).

How Amplification Works:

A small input signal modulates the base/gate current/voltage
This causes a larger change in collector/drain current
The larger current through the load resistor produces an amplified voltage
Voltage gain = Output voltage change / Input voltage change

Amplifier Action

Amplifier Applications

Audio Amplifiers: Amplify sound signals in speakers, headphones, microphones
RF Amplifiers: Boost radio frequency signals in communication systems
Operational Amplifiers: Building blocks of analog circuits
Instrumentation Amplifiers: Amplify sensor signals in measurement systems
Power Amplifiers: Drive speakers, motors, and other loads

3.2 Switching

Transistor as a Switch

Transistors can act as electronic switches that are faster, more reliable, and have no moving parts compared to mechanical switches.

Two States of Operation:

Cut-off State (OFF)

BJT: No base current → No collector current
FET: Gate voltage below threshold → No drain current
Transistor acts as open switch
Very high resistance between output terminals

Saturation State (ON)

BJT: Maximum base current → Maximum collector current
FET: Gate voltage above threshold → Maximum drain current
Transistor acts as closed switch
Very low resistance between output terminals

Transistor Switching Action

Switching Applications

Digital Logic Gates: AND, OR, NOT, NAND, NOR gates in computers
Microprocessors: Billions of transistor switches in CPUs
Memory Circuits: RAM, Flash memory storage
Motor Control: H-bridge circuits for motor direction control
Power Electronics: Switching power supplies, inverters
LED Drivers: Controlling LED on/off states
Relay Drivers: Low-power signals controlling high-power relays

3.3 Oscillators

Transistor in Oscillators

An oscillator is a circuit that generates a continuous AC signal from DC power without any external input signal.

Basic Principle:

An amplifier with positive feedback
Part of the output is fed back to the input in phase
When loop gain ≥ 1, oscillations are sustained
Frequency determined by reactive components (L, C) in feedback network

Amplifier
(Transistor)

→

Output
(AC Signal)

↓

Feedback Network
(LC or RC)

↓

Positive Feedback
(In-phase)

Types of Oscillators

Type	Frequency Range	Application
RC Oscillators (Phase Shift, Wien Bridge)	Audio frequencies (20 Hz - 20 kHz)	Audio signal generators, test equipment
LC Oscillators (Colpitts, Hartley)	RF frequencies (100 kHz - 100 MHz)	Radio transmitters, receivers
Crystal Oscillators	Very stable frequencies (MHz range)	Clocks, computers, watches

Oscillator Applications

Clock Generators: Timing signals for digital circuits and microprocessors
Radio/TV Transmitters: Generate carrier frequencies for broadcasting
Signal Generators: Test and measurement equipment
Musical Instruments: Electronic keyboards, synthesizers
Communication Systems: Modulation, frequency synthesis
Timing Circuits: Watches, clocks, timers

Summary: Transistors

Topic	Key Points
BJT	Three terminals (B, C, E); Current-controlled; NPN/PNP types; I_C = β × I_B
CE Configuration	Most common; High current, voltage, and power gain; 180° phase shift
FET	Three terminals (G, D, S); Voltage-controlled; Very high input impedance; JFET/MOSFET types
Amplification	Small input signal produces large output; Active/saturation region; Audio, RF applications
Switching	ON (saturation) / OFF (cut-off) states; Digital logic; Power control; High-speed operation
Oscillators	Amplifier + Positive feedback; Generate AC from DC; RC, LC, Crystal types