Why is the conductivity of insulatorsnegligible, compared to semiconductor ? It depends on mainly two factors , conductivity Atomic bond Energy band structure
conductivityInsulator semiconductor• valence electrons are tightly • Mostly covalent bonding bound to (or shared with) the somewhat weaker bonding individual atoms – strongest ionic • Electrons can reach the (partially covalent) bonding. conduction band at ordinary temperatures.• The energy gap is too large when • An electron promoted into the compared to semiconductor. conduction band leaves a Hole (positive charge) in the valence band, that can also participate in conduction., • The conductivity increases with increasing temperature.
• The highest filled state at 0 K Fermi Energy (EF) Band structure• The two highest energy bands The energy difference are: between the bottom of the• Valence band – the highest band Conduction and the top of where the electrons are present at 0 the Valence bands is called K the Band Gap• Conduction band - a partially filled or empty energy band where the electrons can increase their energies by going to higher energy levels within the band when an electric field is applied
Band modelInsulators: Semiconductors:wide band gap (> 2 eV) narrow band gap (< 2 eV)
When enough energy issupplied to the e- sitting at the Emptytop of the valance band, e- can conductionmake a transition to the bottom bandof the conduction band.When electron makes such atransition it leaves behind amissing electron state.This missing electron state iscalled as a hole. Hole behaves Energy e- e- e- e- + + + +as a positive charge carrier. FullMagnitude of its charge is the valance bandsame with that of the electronbut with an opposite sign.
Electron mobility• Characterizes how quickly an electron can move through a metal or semiconductor, when pulled by an electric field, in semiconductors .• When an electric field E is applied across a piece of material, the electrons respond by moving with an average velocity called the drift velocity V, Then the electron mobility μ is defined as |v| = μE
Vd E E: applied field : mobility of charge carrier cm 2 Vd is a proportionality factor V Sec E So is a measure how easily charge carriers move under the influence of an applied field or determines how mobile the charge carriers are.
How mobility depend on doping?• Mobility is dependent on the drift velocity. The main factor determining drift velocity (other than effective mass) is scattering time. How long the carrier is accelerated by the electric field until it scatters (collides) with something that changes its direction and/or energy.• The most important sources of scattering in typical semiconductor materials, discussed below, are ionized impurity scattering.
DopingDoping is the incorporation of [substitution] impurities into asemiconductor according to our requirements.In other words, impurities are introduced in a controlledmannerImpurities change the conductivity of the material so that it can be fabricated into a device Doped crystals are extrinsic semiconductors. “adding minute amounts of suitable impurities to the pure crystals” Crystals are doped to be n type or p type n type semiconductors have few minority carriers (holes). p type semiconductors have few minority carriers (electrons).
• The purpose of semiconductor doping is to increase the number of free charges that can be moved by an external applied voltage..• So the crystal has no resistance to current flow and behaves as a superconductor. The perfect periodic potential does not impede the movement of the charge carriers.• However, in a real device or specimen, the presence of impurities, interstitials, subtitionals, temperature , etc. creates a resistance to current flow.
probability of occupation• The Fermi level or Fermi energy is the energy, at which the probability of occupation by an electron (or hole) is exactly ½. In semiconductor, usually, Fermi level is in the band gap.•• F(E )=1 ⁄ 1+exp[(E-EF) /KT ] Where• K = Boltzmann constant• E F =Fermi energy or Fermi level• T =0k• F(E)=The probability that an electron state having energy E is occupied