Semiconductor Diodes

MUHAMMAD UMAR

Introduction

• In this Chapter will learn about• SEMICONDUCTOR MATERIALS: Ge, Si, AND

GaAs• COVALENT BONDING AND INTRINSIC

MATERIALS • ENERGY LEVELS• n -TYPE AND p -TYPE MATERIALS• SEMICONDUCTOR DIODE• IDEAL VERSUS PRACTICAL

What are SemiConductors

• Semiconductors are a special class of elements having a conductivity between that of a good conductor and that of an insulator.

• There are two types of semiconductors: Single Crystal Semiconductor:• Examples : Germanium (Ge) and Silicon (Si)

have a repetitive crystal structure Compound Semiconductor• Examlpes : Gallium Arsenide(GaAs),Cadmium

Sulfide (CdS),are constructed of two or more semiconductor materials of different atomic structures.

Uses of Semiconductor

• The three semiconductors used most frequently in the construction of electronic devices are Ge, Si, and GaAs.

COVALENT BONDING• A covalent bond, also called a molecular bond,

is a chemical bond that involves the sharing of electron pairs between atoms.

• Semiconductors also form Covalent bonds

INTRINSIC MATERIALS

• An intrinsic semiconductor is an undoped semiconductor.The term intrinsic is applied to any semiconductor material that has been carefully refined to reduce the number of impurities to a very low level—essentially as pure as can be made available through modern technology.

ENERGY LEVELS• Electrons are found in areas called shells.

A shell is sometimes called an energy level.• The farther an electron is from the nucleus,

the higher is the energy state, and any electron that has left its parent atom has a higher energy state than any electron in the atomic structure.

• An electron in the valence band of silicon must absorb more energy than one in the valence band of germanium to become a free carrier. Similarly, an electron in the valence band of gallium arsenide must gain more energy than one in silicon or germanium to enter the conduction band.

ENERGY LEVELS

n -TYPE AND p -TYPE MATERIALS

• A semiconductor material that has been subjected to the doping process is called an extrinsic material.

• There are two types of Extrinsic material in Semiconductors.

• n -TYPE AND p -TYPE

n -TYPE AND p -TYPE

n -TYPE p -TYPE• An n -type material is created

by introducing impurity elements that have five valence electrons (pentavalent) such as antimony , arsenic , and phosphorus.

• The p -type material is formed by doping a pure germanium or silicon crystal with impurity atoms having three valence electrons. The elements most frequently used for this purpose are boron , g allium , and indium

SEMICONDUCTOR DIODE

• A semiconductor diode is a device typically made from a single p–n junction. At the junction of a p-type and an n-type semiconductor there forms a depletion region where current conduction is inhibited by the lack of mobile charge carriers.

• This region of uncovered positive and negative ions is called the depletion region due to the “depletion” of free carriers in the region.

SEMICONDUCTOR DIODE

Types of Semiconductor Diode

Forward Biased Reverse Biased• A forward-bias or “on” condition is

established by applying the positive potential to the p -type material and the negative potential to the n -type material as shown in Fig

• If an external potential of V volts is applied across the p – n junction such that the positive terminal is connected to the n -type material and the negative terminal is connected to the p -type material as shown in Fig

IDEAL VERSUS PRACTICAL

Ideal Diode Practical Diode• An ideal diode is a diode that acts

like a perfect conductor when voltage is applied forward biased and like a perfect insulator when voltage is applied reverse biased.

In a practical diode there is very little forward current until the barrier voltage is reached. When reverse bias only a small amount of current flows as long as the reverse voltage is less than the breakdown voltage of the device.