# be5

## What is the effect of Temperature on Metals ?

In metals the valence electrons are almost unbound and are available as carriers of current at room temperatures. An increase in the temperature increases the vibrations of crystal lattice. The increased lattice vibrations in turn increase the collisions between conducting electrons and crystal lattice .The result is the decrease in mobility and hence in conductivity.

## What is the effect of Temperature on Semiconductors resistance?

ANS:  As the temperature the density of electron and hole pairs increases which varies as T^3 *exp(-Ego/KT). Also mobility decreases with increasing temperature which varies as T^-m where m is from 1.5 to 3 due to increased collisions of electrons with positively charged lattice ions. On a whole the effect of former dominates the latter and hence the conductivity increases..

## What is Mobility?

The ratio of drift velocity of electron to the applied electric field is a constant for wide range of electric fields at constant temperature. The constant of proportionality is termed as Mobility denoted by µ.the units of mobility are m^2/sec/volt.

Mobility µ = Vd/E where Vd is drift velocity of electron and E is the applied electric field.

## What is Hall Effect?

If a specimen carrying a current is placed in transverse magnetic field an electric filed is induced in a direction perpendicular to both electric current and magnetic field. This phenomenon is known as Hall Effect. Let us consider a semiconductor bar of breadth ‘l’ in the in the direction of magnetic field and width ‘d’ carrying a uniform current ‘I’ travelling in X direction and placed in uniform magnetic field ‘B’ in Y direction. Assume the semiconductor current is composed of flow of charge carrier each carrying an electric charge ‘Q’, the Lorentz force on each charge carrier is Fl = Bo*Q*Vo where Vo is velocity of charge carrier and is given by Vo = I/ (l*d*Φ), Φ is charge concentration (or) charge density, the force due to electric field produced by displaced charge is Fe = E*Q. At equilibrium these two forces balances each other

Bo*Q*Vo = E*Q

Electric field produced by displaced charges is equal to E = VH/d, where VH is hall voltage. Hence   VH = (Bo* I)/ (l*Φ), VH = (Bo*I*RH)/ l where RH is hall coefficient given by RH = (1/ Φ).

## What are the Applications of Hall Effect?

1. Hall Effect is used in measuring magnetic fields (The hall voltage induced in a material is proportional to magnetic field provided the current, carrier density, breadths are kept constant).

2. It is also used in Hall Effect multipliers which provide output proportional to the product of two signals. If the current is made proportional to one of the inputs and if B is linearly related to the second signal, then induced Hall voltage is proportional to the two inputs.

## Why hole mobility is less compared to electron?

ANS: Mobility of charge carrier is inversely proportional to the mass of charge carrier. As the hole is having higher effective mass compared to electron for this reason hole mobility is less compared to electron mobility.

## Relation between intrinsic carrier concentration with temperature and band gap?

Intrinsic concentration in a semiconductor is given by

Where ni is intrinsic concentration, A is a constant independent of temperature, T is absolute temperature in Kelvin, Ego is band gap at zero degree Kelvin, K is Boltzmann constant.

## What is mass action law?

Mass action law states that at equilibrium temperature the product of concentrations of free holes and electrons is equal to the square of intrinsic concentration at that temperature.

No*Po = Ni²,

where No is concentration of electrons in doped semiconductor’s conduction band, Po is concentration of holes in valence band, Ni is intrinsic carrier concentration.

## Explain Electrical neutrality based on law of conservation of charge?

Law of conservation of charge states that electric charge can be neither created nor destroyed, total charge is always conserved. Accordingly in an N-type semiconductor the free electron density will increase due to excess charge carriers donated to the conduction band by dopant. After donating the excess electron the dopant atoms will be positively charged (similarly after accepting an electron from valence band acceptor impurity will be negatively charged). Obviously the positive charge of dopant atom exactly balances the excess electron charge by donor which is negative. This is termed as electric neutrality.

## What are the types of photo excitation?

There can be two types of photo excitations they are a) intrinsic excitations b) Extrinsic excitations

Intrinsic excitations occur when an electron in valence band is excited by a high energy photon to conduction band. Alternatively a photon may excite an electron in donor level to conduction band or a valence band electron may go into acceptor state. Such excitations are termed as extrinsic excitations.

## What is ohms law?

ANS: Ohm’s law states that in a conductor current density is directly proportional to applied electric field i.e J α E where J is the current defined as ratio of current to the cross section area trough which the current is flowing and E is the electric field applied. The constant of proportionality is called conductance.

Hence ohm’s law is defined as J = σ*E.

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