BEEE unit-3-CO113

Unit-III

Semiconductor Diodes

• Definitions

  • Semiconductor Diode

  • Zener Diode

  • Half-Wave Rectifier

  • Full-Wave Rectifier

  • Clippers and Clampers (no bias/reference voltage)

Bipolar Junction Transistor

• Transistor Operation

• Transistor Configurations

  • Common Base Configuration (CB)

  • Common Emitter Configuration (CE)

  • Common Collector Configuration (CC)

Page 2: Introduction to Atomic Theory

Atom

• Definition: Smallest particle of an element

• Composition:

  • Positively charged Protons

  • Negatively charged Electrons

  • Neutral Neutrons

• Nucleus: Central part consisting of protons and neutrons

• Electrons revolve in various orbits

• Maximum number of electrons in each orbit = 2n²

Types of Electrons

• Valence Electrons: Electrons in the last orbit

• Free Electrons: Loosely connected valence electrons that can move

• Bound Electrons: Tightly attached electrons in inner orbits

Atomic Structure of Silicon and Germanium

• Silicon:

  • Atomic Number: 14

  • Protons: 14, Electrons: 14

  • Electron Distribution: 2 in 1st orbit, 8 in 2nd, 4 in 3rd (Valence: 4)

• Germanium:

  • Atomic Number: 32

  • Protons: 32, Electrons: 32

  • Electron Distribution: 2 (1st), 8 (2nd), 18 (3rd), 4 (4th) (Valence: 4)

• Both are semiconductors

Page 3: Energy Band Diagrams

Energy Bands

• Energy Band: Range of energies in the same orbit

• Valence Band: Highest occupied energy band; range of energies for valence electrons

• Conduction Band: Range of energies for free electrons that conduct electric current

• Forbidden Energy Gap: Energy gap between valence band and conduction band; indicates energy needed to push electrons from valence to conduction band

Materials Classification

• Conductors: Materials allowing easy current flow (e.g., Silver, Copper, Aluminum)

  • No forbidden band; Very small energy required for current flow

• Insulators: Materials where current does not flow (e.g., paper, plastic)

  • Large energy gap; valence band full, conduction band empty

• Semiconductors: Conductivity between conductors and insulators; small forbidden energy gap (approx. 1 eV)

Page 4: Bonding and E-H Pair Generation in Semiconductors

Bonding in Semiconductors

• Elements with 4 valence electrons form covalent bonds for atomic stability.

• Electron-Hole Pair Generation:

  • Created by breaking covalent bonds using external energy.

  • Leads to both free electrons and holes at room temperature.

Page 5: Classification of Semiconductors

Types of Semiconductors

1. Intrinsic Semiconductors:

   • Pure form; weak conductivity; zero signal current causes conduction by electron-hole pairs.

2. Extrinsic Semiconductors:

   • Doped with impurity to enhance conductivity; can be N-type or P-type.

N-Type Semiconductor:

• Doping with pentavalent impurities increases free electrons.

• Majority carriers: electrons, Minority carriers: holes.

P-Type Semiconductor:

• Doping with trivalent impurities creates holes.

• Majority carriers: holes, Minority carriers: electrons.

Page 6: Drift and Diffusion Currents

Current Types

• Drift Current: Flow of current due to charge carriers moving under an electric field.

• Diffusion Current: Movement of charge carriers from high to low concentration (due to repulsion).

Page 7: P-N Junction

Formation and Biasing

• P-N junction formed by doping semiconductor with p-type and n-type materials.

Bias Types

1. Open Circuit:

   • Imbalance of charges creates a depletion region and a potential barrier.

2. Forward Bias:

   • Positive terminal to p-type, negative terminal to n-type, reduces depletion region, current flows.

3. Reverse Bias:

   • Negative terminal to p-type, positive to n-type, widens depletion region, minimal current flows due to minority carriers.

Page 8: V-I Characteristics of Diodes

Forward Bias Characteristics

• Non-linear current increase until barrier potential is overcome (Cut-in Voltage: 0.7V for Si).

Reverse Bias Characteristics

• Minimal current until breakdown (reverse saturation current). Breakdown voltage indicates potential destruction of the junction.

Page 9: Applications of Diodes

• Applications:

  1. Rectifiers

  2. Clippers

  3. Clampers

  4. Logic circuits

  5. Modulation and demodulation circuits

  6. Switches

Diode Specifications

1. Forward Voltage Drop (Vf): Voltage across diode when ON.

2. Peak Inverse Voltage (PIV): Maximum reverse voltage permissible.

3. Junction Capacitance: Capacitance due to depletion region.

4. Package Type: Various mounting options based on application.

Page 10: Zener Diode Characteristics

Construction and Operation

• Highly doped p-n junction; operates in breakdown region as a voltage regulator.

Characterization

• Breakdown occurs at low voltage due to strong electric fields causing covalent bond rupture.

Types of Breakdowns

1. Zener Breakdown: Heavily doped, low reverse voltage.

2. Avalanche Breakdown: Normally doped, occurs at high reverse voltage.

Page 11: Voltage Regulation with Zener Diodes

Regulation Operation

• Constant output voltage maintained via Zener diode under varying supply/load conditions.

Limitations

1. Fixed output voltage; cannot vary.

2. Power loss through series resistance.

3. Low efficiency.

Applications

• Voltage regulation, peak clippers, fixed reference voltage source.

Page 12: Rectifiers

Definition

• Circuit converting AC to DC.

Types

1. Half-Wave Rectifier: Uses one diode; utilizes only one half of input.

2. Full-Wave Rectifier: Uses two diodes, both half cycles converted.

3. Bridge Rectifier: Uses four diodes.

Ripple Factor

• Ratio of AC to DC component.

Rectification Efficiency

• Ratio of DC output power to AC input power.

Page 13: Full-Wave Rectifier with Centre-Tapped Transformer

Operation

• Converts both half cycles; output through load remains consistent.

Characteristics

• Ripple frequency = 2x supply frequency, Ripple factor: 0.4285, PIV = 2EM.

Page 14: Bridge Rectifier

Operation

• Similar operation as full-wave with both half cycles utilized through 4 diodes.

Characteristics

• Ripple frequency and efficiency characteristics similar to full-wave configuration.

Page 15: Comparison Between Rectifiers

Key Parameters

Page 16: Filters

Capacitor Filter

• Removes ripples; poor voltage regulation.

LC Filter

• Combines capacitor and inductor for good regulation/high current applications.

Page 17: Bipolar Junction Transistor (BJT)

Introduction

• Transistor: Device transforming low resistance flow to high resistance flow.

Construction

• Two PN junctions; one type of semiconductor sandwiched between two other layers.

Configurations

• PNP vs NPN transistors (symbols indicate current flow direction).

Page 18: Transistor Biasing

Definition and Importance

• Application of D.C. voltage across terminals for proper function.

Biasing Details

• Proper voltages across junctions ensure correct operation.

Page 19: Transistor Operation

PNP vs NPN Transistor

• Emitter and Collector Currents

• Emitter current distribution significantly affects collector current.

Current Relationships

• Emitter Current (IE) = Base Current (IB) + Collector Current (IC)

Page 20: Transistor as a Switch

Operating States

1. Cut-off State: No conduction, output voltage equal to supply voltage.

2. Saturation State: Full conduction with zero output voltage.

Advantages

• Increased speed, reliability, and reduced size compared to mechanical switches.

Page 21: Transistor as an Amplifier

Functionality

• Base current control leads to proportionate changes in collector current.

Signal Relationships

• Input voltage variations lead to large output voltage changes across load resistance.

Page 22: Transistor Biasing Methods

Methods

1. Fixed Bias: Simple, poor stability.

2. Collector to Base Bias: Improved stability, can reduce gain.

3. Self-Bias: Widely used for stability and response.

Characteristics

• Criteria for using biasing methods based on desired output characteristics.

Page 23: Collector to Base Bias

Analysis

• Provides improvement over fixed bias method regarding thermal stability.

Page 24: Self-Bias (Voltage Divider Method)

Operation Overview

• Two resistors stabilize operating point; self-adjusts for temperature variations.

Page 25: Stability Factor

Definition

• Ratio indicating thermal stability; minimized for effective biasing.

Page 26: Transistor Configurations

Configurations Overview

• Common Base (CB), Common Emitter (CE), Common Collector (CC) analyzed for performance.

Page 27: Common Base Characteristics

Characteristics Evaluation

• Input and output characteristics are measured; high output resistance.

Page 28: Active, Cut-off, Saturation Regions

Region Functions

1. Active Region: Collector reverse biased; sensitive to IB.

2. Saturation Region: No incremental changes due to IB.

3. Cut-off Region: Both junctions reverse biased.

Page 29: Common Emitter Characteristics

Input/Output Relationships

• Detailed examination between IC and VCE with different IB conditions.

Page 30: Input and Output Characteristics

Measurement Techniques

• Establishing input and output curves based on currents and voltages.

Page 31: Output Resistance in Common Emitter Circuit

Resistance Analysis

• Insight into current relationships and circuit behaviors.

Page 32: Common Collector Configuration

Configuration Relations

• Testing characteristics of circuits and observing dependency on VCE.

Page 33: Current Gain Analysis

Gain Measurement

• Variations in input and output relationships evaluated for performance.

Page 34: Key Amplifier Characteristics

Defined Characteristics

1. Input Impedance (Zi)

2. Output Impedance (Zo)

3. Voltage Gain (AV)

4. Current Gain (Ai)

5. Power Gain (Ap)

Comparison of Configurations

• Summary of characteristics across CB, CE, and CC configurations.