Lecture 4 Semiconductor Diodes and Applications Part 3

Chapter Overview

  • Title: Semiconductor Diodes and Applications

  • Authors: Satya Sai Srikant, Prakash Kumar Chaturvedi

2.1 The pn Junction Diode

Historical Context

  • Discovery: 1931 - Semiconductor discovered by Wilson.

  • Doping Process: Adding p- and n-type impurities led to practical applications.

  • Key Development: Shockley's work in 1948 on the pn junction mechanism.

Importance of pn Junction

  • Fundamental to modern electronic devices; essential for nearly all semiconductor applications.

Fabrication of pn Junction

  • Cannot be directly joined; requires process of impurity diffusion.

  • Alloy Junction Method:

    • Early method using aluminum (Al) on n-type silicon at 580 °C.

    • Formation of molten Al-Si mixture followed by cooling to create p-type region.

  • Modern Methods:

    • Current techniques use gaseous compounds like Arsenic trihydride (AsH3) for doping n-type silicon wafers at high temperatures (1050 °C).

2.1.1 Depletion Region Formation in pn Diode Without Bias

Mechanism of Depletion Region Formation

  • Before Diffusion: pn junction has excess holes in p side and excess electrons in n side.

  • Diffusion Process:

    • Carriers migrate across the junction due to charge attraction, forming an electric field and voltage (Barrier Potential - VBP).

Characteristics of Depletion Region

  • Composed of immobile charge ions, high resistance, insulating layer (< 1 µm thick).

  • Charge Representation:

    • CA region: holes as majority, electrons as minority.

    • AJ region: immobile acceptor ions (+ve charge).

    • DB region: electrons as majority, holes as minority.

    • BJ region: immobile donor ions (-ve charge).

2.1.2 Reverse Biasing and Minority Current

Reverse Biasing Explanation

  • Reverse Bias: Positive terminal connected to n-type and negative to p-type.

  • Effects:

    • Increases depletion width and potential barrier, preventing majority carrier flow.

    • Minor leakage current (µA level) flows due to minority carriers.

  • Energy Band Diagram Changes:

    • Fermi levels shift according to applied voltage, combining barrier potential and reverse bias (VBP + VR).

2.1.3 Forward Biasing and Majority Current

Forward Biasing Description

  • Forward Biasing Setup: Positive terminal to p-type, negative to n-type.

Current Flow Mechanism

  • Behavior:

    • Opposes built-in voltage; low forward bias (VBP) allows significant current flow (mA levels).

  • Key Points:

    1. Majority carriers pushed into depletion region.

    2. Reduced depletion width as neutralizing potential barrier.

    3. Increased majority carrier diffusion boosts current flow.

    4. Diode shows low resistance during forward bias.

2.1.4 Total V-I Characteristics of a pn Junction Diode - Experimentally

V-I Characteristics Procedure

  • Comparison of voltage across a diode and current through it, forming V-I curves.

Key Observations

  • At zero voltage: build-up of diffusion current equals zero, leading to a potential barrier.

  • Under reverse and forward bias, study of saturation current and relation to temperature effects.

2.1.5 Dependence of Reverse Leakage Current (I0) on Temperature

Temperature Effects Summary

  • Reverse saturation current (I0) rises at 7% for each °C increment.

  • Doubling occurs with every 10°C increase, affecting breakdown and cut-in voltage.

2.1.6 Calculations of Built-in Potential (VBP) and Depletion Layer Width

Built-in Potential Calculations

  • Relation to doping densities NA and ND, intrinsic carrier concentration ni.

Key Factors

  • Built-in potential depends on temperature; doping levels influence energy band diagrams and Fermi levels.

2.2 Breakdown Diodes - Avalanche and Zener

Overview of Breakdown

  • Reverse-bias breakdown leads to excessive current unless controlled.

  • Specialized diodes (Avalanche and Zener) can operate safely in breakdown regions.

2.2.1 Breakdown Mechanisms

Types of Breakdown

  • Avalanche Breakdown:

    • Occurs at lower doping (<5 x 10^17/cc); involves collision-induced carrier multiplication resulting in high currents.

  • Zener Breakdown:

    • Happens in heavily doped diodes (>>5 x 10^17/cc); characterized by tunneling of electrons due to very thin depletion layers.

  • Comparison of Mechanisms:

    • Both mechanisms involve carrier multiplication but differ in doping levels and depletion layer widths.

2.2.2 Comparison of Zener and Avalanche Breakdown Diodes

Parameter

Zener Breakdown Diode

Avalanche Breakdown Diode

Breakdown Voltage

Decreases with temperature

Increases with temperature

Doping Densities

Heavy doping (>5 x 10^17/cc)

Light doping (<5 x 10^17/cc)

Depletion Layer Width

Narrow (~0.1 µm)

Wide (~1.0 µm)

2.2.3 Zener Diode Characteristics and Specification

Key Points on Zener Diodes

  • V-I characteristics consistent with rectifier diodes but features specified Zener voltage.

  • Operates mainly in the breakdown region, avoiding burn out if current is controlled.

2.2.4 Zener Diode as a Voltage Regulator

Application in Regulation

  • Commonly used for providing stable output voltage from variable input.

  • Operates on the principle of Zener voltage being maintained despite load current fluctuations.

2.2.5 Effect of Temperatures on Zener Diodes

Impact of Temperature Changes

  • Zener voltage exhibits a negative temperature coefficient; important in designing circuits with Zener diodes.