Bipolar Junction Transistor

Unit VI: Bipolar Junction Transistor (BJT)

Duration: 4 Hours | Credit: ELX 133.3

Introduction to Bipolar Junction Transistors

A Bipolar Junction Transistor (BJT) is a three-terminal semiconductor device that uses two junctions to achieve signal amplification and switching. It is one of the most important electronic components and forms the basis of many integrated circuits.

BJT Structure and Operation

Physical Structure

A BJT consists of three regions of semiconductors:

Emitter (E): Heavily doped region that supplies charge carriers
Base (B): Thin, lightly doped region that controls current flow
Collector (C): Moderately doped region that collects charge carriers

Two Types of BJTs

NPN Transistor: n-type emitter, p-type base, n-type collector
PNP Transistor: p-type emitter, n-type base, p-type collector

BJT Biasing Conditions

Forward-Active Region (Amplification Mode)

Base-Emitter junction is forward-biased (V_BE > 0.6V for silicon)
Base-Collector junction is reverse-biased (V_CB < -0.2V for silicon)
Emitter injects carriers into base
Collector collects carriers from base
Moderate collector current with small base current (amplification occurs)

Saturation Region

Both junctions are forward-biased
Maximum collector current (limited by external resistances)
Transistor acts as a closed switch (very low V_CE, typically < 0.2V)

Cutoff Region

Base-Emitter junction is reverse-biased (V_BE < 0)
Emitter-Base junction is off
Collector current is nearly zero (only leakage current)
Transistor acts as an open switch

BJT Current Relationships

Current Gain (β)

β = I_C / I_B (current gain)

Also called dc current gain or hFE
Typical values: 50-300 for small-signal transistors
Varies with collector current and temperature
More stable at moderate current levels

Emitter Current

I_E = I_C + I_B

I_C = β × I_B

I_E = (1 + β) × I_B = α × I_C / (1 - α)

Transport Factor (α)

α = I_C / I_E (typically 0.98-0.9999)

β = α / (1 - α)

Ebers-Moll Model

Transistor as Two Diodes

The Ebers-Moll model represents a transistor as two diodes with common current source:

Base-Emitter diode (forward-biased in normal operation)
Base-Collector diode (reverse-biased in normal operation)
Controlled current source representing amplification

Hybrid-π Model

Used for small-signal AC analysis with parameters like:

r_e: Emitter resistance
r_b: Base resistance
r_μ: Collector resistance
g_m: Transconductance (mutual conductance)

BJT Configurations

Common Emitter Configuration

Input: Base terminal
Output: Collector terminal
Common: Emitter terminal
Voltage Gain: Moderate to high (50-500)
Current Gain: β (50-300)
Input Impedance: Moderate (1-10 kΩ)
Output Impedance: Moderate to high (10-100 kΩ)
Application: General-purpose amplifier

Common Collector Configuration (Emitter Follower)

Input: Base terminal
Output: Emitter terminal
Common: Collector terminal
Voltage Gain: Less than 1 (unity gain)
Current Gain: β (large, 50-300)
Input Impedance: Very high (100 kΩ - 1 MΩ)
Output Impedance: Very low (10-100 Ω)
Application: Buffer, impedance matching

Common Base Configuration

Input: Emitter terminal
Output: Collector terminal
Common: Base terminal
Voltage Gain: Moderate to high (100-1000)
Current Gain: α (close to 1, 0.98-0.9999)
Input Impedance: Very low (20-100 Ω)
Output Impedance: Very high (100 kΩ - 1 MΩ)
Application: High-frequency amplifier

BJT Biasing and DC Operating Point

Biasing Objectives

Establish a stable Q-point (operating point) independent of β variations
Ensure adequate bias current to maintain linear operation
Maximize available output voltage swing

Fixed-Base Bias

Simple but unstable because I_B depends directly on β which varies with temperature and manufacturing

Voltage Divider Bias (Stiff Bias)

Most practical method:

Uses a voltage divider to establish base voltage
Base voltage is relatively independent of β
More stable with temperature changes

Load Line and Q-Point

DC Load Line Equation

I_C = (V_CC - V_CE) / R_C

Q-Point (Quiescent Point)

The intersection of the dc load line with the transistor characteristic curve determines the operating point (Q-point) with coordinates (V_CE,Q, I_C,Q)

Graphical Solution

Plot the DC load line from (V_CC, 0) to (0, V_CC/R_C)
Plot the I_B line from the specified base current
Find intersection of I_B curve with load line to find Q-point

Temperature Effects and Stability

Temperature Sensitivity

β increases with temperature (0.5%/°C)
V_BE decreases with temperature (2 mV/°C)
I_CO (leakage current) doubles for every 25-50°C

Thermal Runaway

Risk of instability where increased temperature leads to increased current, causing more heat generation

Key Takeaways

BJT has three terminals: Emitter, Base, and Collector
Current gain β = I_C / I_B
Forward-active region provides amplification
Saturation and cutoff regions are used for switching
Common Emitter provides moderate voltage and current gain
Common Collector (Emitter Follower) provides high input impedance
Common Base provides high frequency response
Proper biasing ensures stable operation over temperature range