Study Notes on Bipolar Junction Transistor and FET

KALINGA UNIVERSITY – EMERGING DOMAIN IN ELECTRONICS ENGINEERING

Unit II: Bipolar Junction Transistor (BJT)

Introduction

  • A bipolar junction transistor (BJT) is a three-terminal device whose operation relies on the interaction of majority and minority carriers, leading to its classification as 'bipolar'.

  • The BJT is analogous to a vacuum triode and is noted for being smaller in size.

  • Primary functions include:

    • Amplifiers and oscillator circuits.

    • Acting as a switch in digital circuits.

  • Applications include:

    • Computers.

    • Satellites.

    • Modern communication systems.

Construction of BJT and Its Symbols

  • Basic construction includes two PN-junctions generating three terminals:

    • Emitter (E)

    • Base (B)

    • Collector (C)

  • Types of transistors:

    • PNP: Physical arrangement of P-type and N-type semiconductors.

    • NPN: Other configuration of semiconductor arrangement.

  • Transistors function as current-regulating devices by allowing current flow based on the biasing voltage applied to the base terminal.

Functionality

  • Functions of BJTs include:

    1. Switching: In digital electronics, can operate as on-off switches.

    2. Amplification: In analog electronics, increases signal strength.

  • Operating regions of BJTs:

    1. Active Region:

    • Functions as an amplifier.

    • Relation: I_c = eta I_b

    1. Saturation Region:

    • Fully ON state acting as a switch.

    • Saturation current = I(saturation)I_{(saturation)}

    1. Cut-off Region:

    • Fully OFF state, acting as a switch.

    • Collector current Ic=0I_c = 0.

Operating Points and Load Lines

  • When identifying the maximum possible collector current (Y-axis) and collector-emitter voltage (X-axis), the resulting characteristics are:

    • Operating Point: Key point on the load line where active region performance is maintained despite fluctuations in AC signal.

    • Quiescent Point (Q-point): Selected to maintain active region operation regardless of AC signal swings.

  • Load Line Concept:

    • A line connecting these two operating points illustrates the output at the load.

DC Load Line

  • The load line under no input signal is considered a DC condition.

  • Collector-emitter voltage described by: VCE=VCC−ICRCV_{CE} = V_{CC} - I_C R_C

    • This gives a first-degree equation representing a straight line on the output characteristics.

Finding Load Line Intersection Points

  1. Point A (Intersection with Voltage Axis):

    • IA=0I_A = 0

    • VA=VV_A = V (supply voltage, as zero current means no voltage drop across load resistor).

  2. Point B (Intersection with Current Axis):

    • VB=0V_B = 0

    • Apply Ohm's Law: IB=racVRLI_B = rac{V}{R_L}

    • Example with V=10VV = 10V, R=1kΩR = 1kΩ:

      • Point A Coordinates: (10V, 0)

      • Point B Coordinates: (0V, 10mA)

Load Line Equation

  • Derived using:
    V+IRL=VV + I R_L = V

  • Rearranged to depict:
    I=racV−VdropRLI = rac{V - V_{drop}}{R_L}

  • Plot from points A(10V, 0mA) to B(0V, 10mA) on IV characteristic graph.

BJT as an Amplifier

  • Transistors amplify signals by increasing the strength of weak input signals.

  • The emitter-base junction is kept forward-biased by the DC bias voltage, ensuring continuity regardless of signal polarity.

  • Through relationship between input voltage changes and resulting output voltages:

    • Example: A 0.1V change in input induces a 5V change in output across a collector load resistor of 5kΩ.

BJT as a Switch

  • Operation as a switch entails alternating between saturation and cutoff regions.

  • When biased:

    • Cutoff: No current flows, $V_{CE} = V_{CC}$, $I_C = 0A$.

    • Saturation: Currents flow, $V_{CE} = 0$, $I_C = rac{V_{CC}}{R_C}$.

Characteristics of BJT Switches

  • Assumptions for switching behavior:

    1. Base current controlled by voltage across biasing systems.

    2. Assumed ideal conditions neglect current leakage in cutoff and voltage drop in saturation (0.2 to 0.4 V range).

Configuration and Characteristics

Common-Emitter Configuration

  • Configuration: Input applied across base-emitter, output taken between collector-emitter.

    • Input current: IinI_{in}, Output current: IoutI_{out}

Common Collector Configuration

  • Configuration: Input between base-collector, output from emitter-collector.

Common Base Configuration

  • Configuration: Input between emitter-base, output from collector-base.

  • Total collector current described in terms of injected carriers and leakage current.

  • Current amplification factor eta defined. Range: 0.95 to 0.995.

Properties Comparison Table

Property

Common Base

Common Emitter

Common Collector

Input Port

Emitter

Base

Base

Output Port

Collector

Collector

Emitter

Input Resistance

Small

Small

Large

Output Resistance

Large

Large

Small

Current Gain

Low

High

High

Voltage Gain

High

Medium

Low

Power Gain

Medium

High

Medium

Applications

Cascode amplifiers

Cascade amplifiers

Impedance matching

Field Effect Transistor (FET)

Introduction to FET

  • Another semiconductor device designed as an amplifier or switch, primarily voltage-operated.

  • High input resistance, as it requires virtually no input current, leading to lower operational noise.

  • Composition of FET includes:

    • Source: Analogous to BJT's emitter.

    • Drain: Comparable to collector.

    • Gate: Equivalent to base.

  • Unique in that it solely operates from majority carriers, contrasting with BJTs.

Types of FETs

  1. Junction Field Effect Transistor (JFET)

  2. MOSFET (Metal-Oxide-Semiconductor FET)

    • Subdivided into P-channel and N-channel.

N-Channel JFET Construction and Operation

  • Consists of an N-type channel bordered by P-type materials forming a gate.

  • Operation controlled via gate biasing to vary current flow based on channel width.

Characteristics of N-Channel JFET

  1. Drain Characteristics: Relation between drain-to-source voltage and drain current.

  2. Transfer Characteristics: Relating drain current to gate-to-source voltage for differing VDS values (Pinch-off concept).

MOSFET Details

  • N-Channel MOSFET: Involves structures where P-type substrates create zones when biased.

  • Two operational modes:

    • Depletion Mode: Conducts even with $V_{gs} = 0V$.

    • Enhancement Mode: Requires positive $V_{gs}$ to induce channel conductivity.

Applications of MOSFET

  • Essential in digital electronics and computer technologies, particularly in logic families such as CMOS.

BJT vs. FET: Key Differences

  1. Principle of Operation: BJT is current-controlled, FET is voltage-controlled.

  2. Input Impedance: FETs offer higher input impedance than BJTs.

  3. Current Gain: BJTs exhibit high current gain along with operational versatility.

  4. Noise Levels: FETs present lower noise characteristics, making them better for low-noise applications.

  5. Cost Considerations: BJTs generally less expensive compared to FETs.