*Conduction in Semiconductors*Conduction in Semiconductors

Conduction in Semiconductors

*Conduction in Semiconductors*Electrical Conduction in Good Conductors

Conduction in Semiconductors

*Conduction in Semiconductors*In addition to the random motion, the electrons drift towards the positive terminal. This drift of the charge carriers results in an electric current, called drift current. Drift Current

Conduction in Semiconductors

*Conduction in Semiconductors*Drift VelocityIt is the average velocity of charge carriers in a definite direction under the influence of electric field.Its value depends upon The nature of charge carriers.The applied electric field.

Conduction in Semiconductors

*Conduction in Semiconductors*MobilityThe constant is called mobility. It can be defined as the drift velocity (in m/s) per unit applied field (in V/m). Thus,

Conduction in Semiconductors

*Conduction in Semiconductors*Conductivity

Conduction in Semiconductors

*Conduction in Semiconductors*Let n be the concentration of charge carriers.The time to cross the length ist = L / vdThe drift current is given asThe current Density is

Conduction in Semiconductors

*Conduction in Semiconductors*Putting = qn, we getJ = EThis is simply Ohms Law, put in another form. The constant is called Conductivity.Conductivity is the inverse of resistivity, = 1 / (S/m)

Conduction in Semiconductors

*Conduction in Semiconductors*Conduction in Intrinsic Semiconductors

Conduction in Semiconductors

*Conduction in Semiconductors*Drift of electrons and holes in an external fieldThe electrons in the conduction band and the holes in the valence band move in a random fashion within a crystal due to their thermal energy.When an external voltage is applied to the semiconductor, a drift velocity is superimposed on the random thermal motion of the electrons and holes.The drift of the electrons in the conduction band and that of the holes in the valence band produce an electric current.

Conduction in Semiconductors

*Conduction in Semiconductors*The electrons move towards the positive electrode and the holes towards the negative electrode.Conventional current flows in semiconductors from the positive electrode to the negative electrode.When the charge on an electron is same as the charge on a hole, why is the electron-drift current In is greater than the hole-drift current Ip ?

Conduction in Semiconductors

*Conduction in Semiconductors*Conductivity of Intrinsic Semiconductors

Conduction in Semiconductors

*Conduction in Semiconductors*Charge density = qn Qn = (qn)(Avn)

Conduction in Semiconductors

*Conduction in Semiconductors*Total Current, But

Conduction in Semiconductors

*Conduction in Semiconductors*Applying Ohms Law,

Conduction in Semiconductors

*Conduction in Semiconductors*Since in an intrinsic semiconductor, n = p = ni,

Conduction in Semiconductors

*Conduction in Semiconductors*Intrinsic ConcentrationWith increasing temperature, the density of electron-hole pairs increases and correspondingly conductivity increases.It is found that intrinsic concentration ni varies with T as,

EG0 = Energy gap at 0 K in electron voltsk = Boltzman constant in eV/KA0 = a constant independent of T.

Conduction in Semiconductors

*Conduction in Semiconductors*In case Germanium, 1 atom in about 109 contributes a free electron (and also a hole).In case Silicon, 1 atom in about 1012 contributes a free electron (and also a hole).

Conduction in Semiconductors

*Conduction in Semiconductors*Energy Gap or Band GapThe forbidden energy gap in semiconductors depends upon temperature.Experimentally it has been found that,For Germanium,Eg = 0.785 2.23 X 10-4TAt 300 K, Eg = 0.72 eV.For Silicon,Eg = 1.21 - 3.60 X 10-4TAt 300 K, Eg = 1.12 eV.

Conduction in Semiconductors

*Conduction in Semiconductors*ReviewElectrical Conduction in Good ConductorsDrift Current.Drift Velocity.Mobility.Conductivity.Conductivity of Intrinsic Semiconductors .Intrinsic Concentration.

Conduction in Semiconductors