Lesson Overview

Unit 9 introduces semiconductors, diodes, rectification, Zener regulation, transistors, FETs, operational amplifiers, and digital logic (logic gates and flip-flops). This unit is essential for understanding modern electronic devices and circuits.

1. Core Concepts (Short Notes)

9.1 Semiconductors & Diodes

• Intrinsic semiconductor: pure Si/Ge.

• Extrinsic semiconductor: doped (p-type or n-type).

• p–n junction forms depletion region & potential barrier (~0.7 V for Si).

• Diode characteristics: forward & reverse bias.

• Diode symbol: triangle and bar.

9.2 Rectification

• Half-wave: uses one diode.

• Full-wave bridge: uses four diodes.

• Smoothing: capacitor filters ripples.

9.3 Zener Diode

• Operates in reverse breakdown region.

• Used for voltage regulation.

9.4 Transistors (BJT)

• npn / pnp bipolar devices.

• Three regions: base (thin), emitter (heavily doped), collector.

• Common emitter configuration most common.

• Transistor regions:

  • Cut-off

  • Active (amplification)

  • Saturation

9.5 Amplifiers

• Current amplification: I_C = βI_B.

• Voltage amplification: output is 180° out of phase.

9.6 FET (Field Effect Transistor)

• JFET: n-channel and p-channel.

• Voltage-controlled device.

9.7 Operational Amplifiers (Op-Amps)

• High-gain differential amplifier.

• Golden Rules:

  1. Input currents ≈ 0.

  2. V⁺ = V⁻ (for linear region).

• Inverting & non-inverting configurations.

• Voltage comparator.

9.8 Logic Gates

• Basic gates: AND, OR, NOT, NAND, NOR, XOR, XNOR.

• Truth tables & Boolean expressions.

• SR Flip-flop for memory.

2. Detailed Notes for Each Section

9.1 Intrinsic & Extrinsic Semiconductors

Intrinsic

• Pure Si/Ge.

• Temperature increases → electron-hole pairs increase.

Extrinsic

• p-type: doped with Group III (acceptors → holes).

• n-type: doped with Group V (donors → electrons).

p–n Junction

• Formation of depletion region.

• Potential barrier prevents further diffusion.

• Typical values:

  • Si: 0.7 V

  • Ge: 0.3 V

9.2 Diode Characteristics

Forward Bias

• Reduces barrier → allows current.

• Current rises sharply after knee voltage.

Reverse Bias

• Depletion widens → small leakage current.

• Breakdown occurs if reverse voltage > PIV.

I-V Characteristics

• Non-linear.

• Important points: knee voltage, reverse breakdown.

9.3 Rectification

Half-Wave Rectifier

• Only positive half allowed.

• Low efficiency.

Full-Wave Bridge

• Four diodes.

• Uses both halves of AC.

Smoothing

• A capacitor reduces ripple.

• Ripple decreases with larger capacitance.

9.4 Zener Diode Regulation

• Operates in reverse breakdown.

• Maintains constant output voltage.

• Requires series resistor for safety.

9.5 Bipolar Junction Transistor (BJT)

Modes of Operation

• Cutoff: I_B ≈ 0; no conduction.

• Active: I_C ≈ βI_B; amplifier region.

• Saturation: Both junctions forward biased.

Common-Emitter Amplifier

• Input: base.

• Output: collector.

• Voltage gain: inverted (180° phase shift).

Characteristics

• Input: I_B vs V_BE.

• Output: I_C vs V_CE.

• Transfer: I_C vs I_B.

9.6 JFET (Junction Field Effect Transistor)

Features

• Unipolar (single carrier type).

• Gate reverse-biased.

• Drain current controlled by gate voltage.

Common-Source Configuration

• Voltage gain without phase inversion (for some cases).

• Characteristic curves: I_D vs V_DS.

9.7 Operational Amplifiers

Symbol & Pins

• non-inverting input

• – inverting input

• Output terminal

• Power supply pins

Golden Rules

1. Input currents ≈ 0.

2. V⁺ = V⁻ in linear mode.

Inverting Amplifier

Gain:

Non-Inverting Amplifier

Gain:

Comparator

• Output saturates to ±V_supply.

9.8 Logic Gates & Boolean Algebra

Basic Gates

• AND, OR, NOT.

• Universal gates: NAND, NOR.

• XOR & XNOR.

Truth Tables

Used to predict output states.

SR Flip-Flop

• Basic memory element.

• Based on NOR or NAND gates.

• Stores 1 bit.

Timing Diagrams

Show how outputs follow inputs over time.

3. Formula Summary (Unit 9)

• Current gain: β = I_C / I_B

• Inverting Op-Amp gain: A_v = – R_f/R_in

• Non-inverting gain: A_v = 1 + R_f/R_in

• Rectifier efficiency: η ≈ 40% (half-wave), 80% (full-wave)

4. Common Mistakes to Avoid

• Confusing p-type with positive charge carriers (they are holes, not protons!).

• Not including series resistor with Zener diode.

• Using transistor without biasing.

• Forgetting phase inversion in CE amplifiers.

• Mixing XOR and OR logic conditions.

5. Exam Tips

• Draw semiconductor diagrams clearly (show depletion region!).

• Label diode I-V curves properly.

• Show transistor regions (cutoff, active, saturation) on graphs.

• Use truth tables to design logic circuits.

• For op-amp questions, always apply Golden Rules.

6. Quick Revision Table