SEMICONDUCTORS

                                                     SEMICONDUCTORS

Introduction to Semiconductors: Properties, bonds and types of semiconductors.

• Semiconductor Diodes and Special Purpose Diodes: The pn junction diode: formation,

properties and V-I characteristics, Basic constructions, characteristics, operations and uses of

special diodes: Light-emitting diode (LED), Zener diode etc.

• Diode Application: Half-wave and full-wave rectifiers – operation and efficiency, Ripple factor,

Filter circuits – capacitor input filter, LC filter and Π-filter, Clipping and Clamping circuits,

Voltage regulation and regulator circuits - Zener diode and transistor voltage regulator.

• Bipolar Junction Transistors: npn and pnp transistors, amplifying and switching actions of

transistor, transistor characteristics in CB, CE & CC configurations, transistor load line and

Operating point.

• BJT Biasing: Faithful amplification, inherent variation of transistor parameters and thermal

runway, stabilization and stability factor, methods of BJT biasing, analysis and design of biasing

circuits.

• Single Stage Transistor Amplifier: Single stage amplifier circuit, phase reversal, dc and ac

equivalent circuits, load line analysis, voltage gain and power gain, classification of amplifiers,

amplifier equivalent circuits.

Introduction to Semiconductors

Semiconductors

Semiconductors are a special class of elements having a conductivity between that of a good conductor and

that of an insulator.

Semiconductor materials fall into one of two classes (based on the structure):

1. Single-crystal semiconductors such as germanium (Ge) and silicon (Si) have a repetitive crystal

structure.

2. Compound semiconductors such as gallium arsenide (GaAs), cadmium sulfide (CdS), gallium

nitride (GaN), and gallium arsenide phosphide (GaAsP) are constructed of two or more

semiconductor materials of different atomic structures.

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

GaAs.

A brief history

Initially Germanium was the choice to construct devices. Then came silicon which gained popularity due

to being less sensitive to temperature changes. Finally came the issue of speed with the introduction of

computers and communication devices. GaAs transistors were introduced which had speed of operation up

to 5 times of Si. But Si still remained popular with the advantage of cheaper manufacturing and efficient

design process. Today GaAs has finally emerged as a base material for new high-speed, very large scale

integrated (VLSI) circuit design. However, Si is still the fundamental building block for Intel’s new line of

processors.

Covalent Bond

To fully appreciate why Si, Ge, and GaAs are the semiconductors of choice for the electronics industry

requires some understanding of the atomic structure of each and how the atoms are bound together to form

a crystalline structure. The fundamental components of an atom are the electron, proton, and neutron. In

the lattice structure, neutrons and protons form the nucleus and electrons appear in fixed orbits around the

nucleus. The Bohr model for the three materials is provided in Fig. 1.3.

As indicated in Fig. 1.3 , silicon has 14 orbiting electrons, germanium has 32 electrons, gallium has 31

electrons, and arsenic has 33 orbiting electrons (arsenic that is a very poisonous chemical agent). For

germanium and silicon there are four electrons in the outermost shell, which are referred to as valence

electrons . Gallium has three valence electrons and arsenic has five valence electrons. Atoms that have four

valence electrons are called tetravalent , those with three are called trivalent , and those with five are called

pentavalent . The term valence is used to indicate that the potential (ionization potential) required to remove

any one of these electrons from the atomic structure is significantly lower than that required for any other

electron in the structure.

In a pure silicon or germanium crystal the four valence electrons of one atom form a bonding arrangement

with four adjoining atoms, as shown in Fig. 1.4 .

This bonding of atoms, strengthened by the sharing of electrons between neighboring atoms, is called

covalent bonding.

Because GaAs is a compound semiconductor, there is sharing between the two different atoms, as shown

in Fig. 1.5 . Each atom, gallium or arsenic, is surrounded by atoms of the complementary type. There is still

a sharing of electrons similar in structure to that of Ge and Si, but now five electrons are provided by the

As atom and three by the Ga atom.

Although the covalent bond will result in a stronger bond between the valence electrons and their parent

atom, it is still possible for the valence electrons to absorb sufficient kinetic energy from external natural

causes to break the covalent bond and assume the “free” state. The term free is applied to any electron that

has separated from the fixed lattice structure and is very sensitive to any applied electric fields such as

established by voltage sources or any difference in potential. The external causes include effects such as

light energy in the form of photons and thermal energy (heat) from the surrounding medium.

One important and interesting difference between semiconductors and conductors is their reaction to the

application of heat. For conductors, the resistance increases with an increase in heat. This is because the

numbers of carriers in a conductor do not increase significantly with temperature, but their vibration pattern

about a relatively fixed location makes it increasingly difficult for a sustained flow of carriers through the

material. Materials that react in this manner are said to have a positive temperature coefficient.

Semiconductor materials, however, exhibit an increased level of conductivity with the application of heat.

As the temperature rises, an increasing number of valence electrons absorb sufficient thermal energy to

break the covalent bond and to contribute to the number of free carriers. Therefore: Semiconductor materials

have a negative temperature coefficient.

Semiconductors can be classified as (based on purity):

• Intrinsic Semiconductors

• Extrinsic Semiconductors

Intrinsic materials are those semiconductors that have a very low level of impurities , whereas extrinsic

materials are semiconductors that have been exposed to a doping process. Doping is a technique used to

vary the number of electrons and holes in semiconductors.

Energy Levels

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.

Fig: conduction and valence bands of an insulator, a semiconductor, and a conductor.

Silicon semiconductors

n-type and p-type materials

The characteristics of a semiconductor material can be altered significantly by the addition of specific

impurity atoms (doping) to the relatively pure semiconductor material (intrinsic).

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

There are two extrinsic materials of immeasurable importance to semiconductor device fabrication: n -type

and p -type materials.

n-type material

An n -type material is created by introducing impurity elements that have five valence electrons (

pentavalent ), such as antimony , arsenic , and phosphorus. The effect of such impurity elements is indicated

in Fig. 1.7 (using antimony as the impurity in a silicon base). Note that the four covalent bonds are still

present. There is, however, an additional fifth electron due to the impurity atom, which is unassociated with

any particular covalent bond. This remaining electron, loosely bound to its parent (antimony) atom, is

relatively free to move within the newly formed n -type material.

Diffused impurities with five valence electrons are called donor atoms. The net result, therefore, is that the

number of electrons far outweighs the number of holes. For this reason: In an n-type material the electron

is called the majority carrier and the hole the minority carrier.

p-type material

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 , gallium , and indium.

The effect of one of these elements, boron, on a base of silicon is indicated in Fig. 1.9 . Note that there is

now an insufficient number of electrons to complete the covalent bonds of the newly formed lattice. The

resulting vacancy is called a hole and is represented by a small circle or a plus sign, indicating the absence

of a negative charge. Since the resulting vacancy will readily accept a free electron: The diffused impurities

with three valence electrons are called acceptor atoms. For the p -type material the number of holes far

outweighs the number of electrons. Therefore: In a p-type material the hole is the majority carrier, and the

electron is the minority carrier.