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 aconductor by the fact that, atabsolute zero, the uppermostfilled electron energy band is fullyfilled In a semiconductor, but onlypartially filled in a conductor.
A semiconductor has a band gap which issmall enough such that its conduction band isappreciably thermally populated withelectrons at room temperatureAn insulator has a band gap which is toowide for there to appreciable thermalelectrons in its conduction band at roomtemperature.
Theory of Semiconductors The operation of semiconductors is bestunderstood using band theory. When a largenumber of atoms combine to form asolid, the electrons e − in the solid aredistributed into energy bands among all theatoms in the solid. Each band has a differentenergy, and the electrons fill these bandsfrom the lowest energy to the highest, similarto the way electrons occupy the orbitals in asingle 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 nopossibility for electron flow (Figure1d), because the valence band is completelyfilled with electrons, and the conduction bandis too far away in energy to be accessed bythese 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 thatsemiconductors are useful in electronicsis that their electronics properties can begreatly altered in a controllable way byadding small amounts of impurities.
Intrinsic & ExtrinsicSemiconductorsIntrinsic 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-typesemiconductors, (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 thatthey allow electron flow only from the n side tothe p side. Such one-way devices are calleddiodes.(Figure 2c )
•If a positive voltage (also called a forward bias)is applied that lowers the energy barrierbetween n and p, then the electrons in theconduction band on the n side can flow acrossthe junction (and holes can flow from p to n ).•A reverse bias, however, raises the height of thebarrier and increases the charge separation atthe junction, impeding any flow of electronsfrom p to n.
Diodes have several important applicationsin electronics. The power supplied by mostelectrical utilities is typically alternating current(AC); that is, the direction of current flow switchesback and forth with a frequency of sixty cycles persecond. However, many electronic devices requirea steady flow of current in one direction (directcurrent or DC).
Since a diode only allows current to flowthrough it in one direction, it can be combinedwith a capacitor to convert AC input to DC output.For half the AC cycle, the diode passes current andthe capacitor is charged up. During the other halfof the cycle, the diode blocks any current from theline, but current is provided to the circuit by thecapacitor. Diodes applied in this way are referredto as rectifiers.
REPORTERS: Rotchil A. Casurra & Ma. Diana R. Coñado
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