SEMICONDUCTORS

What is Semiconductor?

- Is a material which has an electrical conductance which is between that of an insulator and a conductor.

A semiconductor behaves as an insulator at very low temperature, and has an appreciable electric conductance at room temperature.

It can be distinguished from a conductor by the fact that, at absolute zero, the uppermost filled electron energy band is fully filled In a semiconductor, but only partially filled in a conductor.

A semiconductor has a band gap which is small enough such that its conduction band is appreciably thermally populated with electrons at room temperature

An insulator has a band gap which is too wide for there to appreciable thermal electrons in its conduction band at room temperature.

Theory of Semiconductors

The operation of semiconductors is best understood using band theory. When a large number of atoms combine to form a solid, the electrons e − in the solid are distributed into energy bands among all the atoms in the solid. Each band has a different energy, and the electrons fill these bands from the lowest energy to the highest, similar to the way electrons occupy the orbitals in a single atom.

The variation in properties between electrical insulators, conductors ( metals ), and semiconductors stems from differences in the band structures of these materials.

Valence Band - the highest energy band that contains electrons

Conduction Band

- the lowest energy empty band

Band gap

- the difference in energy between the valence and conduction bands

• In a metal, the valence band is only partially filled with electrons (Figure 1a&b). This means that the electrons can access empty areas within the valence band, and move freely across all atoms that make up the solid. A current can therefore be generated when a voltage is applied.

• In general, for electrons to flow in a solid, they must be in a partially filled band or have access to a nearby empty band.

•In an electrical insulator, there is no possibility for electron flow (Figure 1d), because the valence band is completely filled with electrons, and the conduction band is too far away in energy to be accessed by these electrons (the band gap is too large).

A semiconductor (Figure 1c) is a special case in which the band gap is small enough that electrons in the valence band can jump into the conduction band using thermal energy. That is, heat in the material (even at room temperature) gives some of the electrons enough energy to travel across the band gap.

Thus, an important property of semiconductors is that their conductivity increases as they are heated up and more electrons fill the conduction band.

Fig 1. Schematic of the electronic band structures of different types of solids. (Electrons are represented in red)

DOPING OF SEMICONDUCTORS

intentionallly introduces impurities into an extremely pure (intrinsic) semiconductor fro the purpose of modulating its electrical properties.

One of the main reasons that semiconductors are useful in electronics is that their electronics properties can be greatly altered in a controllable way by adding small amounts of impurities.

Intrinsic & Extrinsic Semiconductors

Intrinsic Semiconductor- Is one that is pure enough that impurities do not appreciably affect its electrical behavior.

Extrinsic Semiconductor- Is one that has been doped with impurities to modify the number and type of free charge carriers.

TYPE OF DOPING

N – Type Doping

- Is to produce an abundance of mobile or “carrier” electrons in the material.

P – Type Doping

- It is to create an abundance of holes.

P-n Junctions

It is maybe created by doping adjacent of the semiconductor with p-type and n-type dopants.

Fig. 2. Schematic diagram of the band structures of (a) p-type semiconductors, (b) n-type semiconductors, & (c) a p-n junction

SEMICONDUCTORS IN ELECTRONICS

Semiconductors used in electronics perform a variety of tasks from enabling communication to speeding up processing.

• Semiconductors are used extensively in solid-state electronic devices and computers.

•An important property of p-n junctions is that they allow electron flow only from the n side to the p side. Such one-way devices are called diodes.(Figure 2c )

•If a positive voltage (also called a forward bias) is applied that lowers the energy barrier between n and p, then the electrons in the conduction band on the n side can flow across the junction (and holes can flow from p to n ).

•A reverse bias, however, raises the height of the barrier and increases the charge separation at the junction, impeding any flow of electrons from p to n.

Diodes have several important applications in electronics. The power supplied by most electrical utilities is typically alternating current (AC); that is, the direction of current flow switches back and forth with a frequency of sixty cycles per second. However, many electronic devices require a steady flow of current in one direction (direct current or DC).

Since a diode only allows current to flow through it in one direction, it can be combined with a capacitor to convert AC input to DC output. For half the AC cycle, the diode passes current and the capacitor is charged up. During the other half of the cycle, the diode blocks any current from the line, but current is provided to the circuit by the capacitor. Diodes applied in this way are referred to as rectifiers.

REPORTERS:

Rotchil A. Casurra & Ma. Diana R. Coñado