Advertisements
Advertisements
Question
In a semiconductor,
(a) there are no free electrons at 0 K
(b) there are no free electrons at any temperature
(c) the number of free electrons increases with temperature
(d) the number of free electrons is less than that in a conductor.
Advertisements
Solution
(a) there are no free electrons at 0 K
(c) the number of free electrons increases with temperature
(d) the number of free electrons is less than that in a conductor
In semiconductors, the valence band is full at 0 K, but the conduction band is empty. So, no free electron is available for conduction at 0 K.
As the temperature increases, covalent bonds that provide free charge carriers for conduction in a semiconductor break.
As the conduction band in metals is already partially filled at 0 K, many free electrons below the Fermi level acquire energy from an external source or temperature, jump to the conduction band and start behaving like free electrons. Hence, metals contain more free electrons than semiconductors.
APPEARS IN
RELATED QUESTIONS
Draw energy band diagrams of an n-type and p-type semiconductor at temperature T > 0 K. Mark the donor and acceptor energy levels with their energies.
There are energy bands in a solid. Do we have really continuous energy variation in a band ro do we have very closely spaced but still discrete energy levels?
What is the resistance of an intrinsic semiconductor at 0 K?
Electric conduction in a semiconductor takes place due to
Let np and ne be the number of holes and conduction electrons in an intrinsic semiconductor.
In a pure semiconductor, the number of conduction election 6 × 1019 per cubic metre. How many holes are there in a sample of size 1 cm × 1 mm?
The band gap between the valence and the conduction bands in zinc oxide (ZnO) is 3.2 eV. Suppose an electron in the conduction band combines with a hole in the valence band and the excess energy is released in the form of electromagnetic radiation. Find the maximum wavelength that can be emitted in this process.
Suppose the energy liberated in the recombination of a hole-electron pair is converted into electromagnetic radiation. If the maximum wavelength emitted is 820 nm, what is the band gap?
(Use Planck constant h = 4.14 × 10-15 eV-s, Boltzmann constant k = 8·62 × 10-5 eV/K.)
Find the maximum wavelength of electromagnetic radiation which can create a hole-electron pair in germanium. The band gap in germanium is 0.65 eV.
(Use Planck constant h = 4.14 × 10-15 eV-s, Boltzmann constant k = 8·62 × 10-5 eV/K.)
The conductivity of a pure semiconductor is roughly proportional to T3/2 e−ΔE/2kT where ΔE is the band gap. The band gap for germanium is 0.74 eV at 4 K and 0.67 eV at 300 K. By what factor does the conductivity of pure germanium increase as the temperature is raised from 4 K to 300 K?
The product of the hole concentration and the conduction electron concentration turns out to be independent of the amount of any impurity doped. The concentration of conduction electrons in germanium is 6 × 1019 per cubic metref conduction electrons increases to 2 × 1023 per cubic metre. Find the concentration of the holes in the doped germanium.. When some phosphorus impurity is doped into a germanium sample, the concentration o
The energy of a hydrogen atom in the ground state is −13.6 eV. The energy of a He+ ion in the first excited state will be:
For germanium crystal, the forbidden gas energy gap
A semiconductor is cooled from T.K to T2K its resistance will
In a common-base circuit calculate the change in the base current if that in the emitter current is αmA and a = 0.98
In a common base configuration Ie = 1 mA α = 0.95 the value of base current is
The reaction between α and β parameter of a transistor is given by
Draw the energy band diagrams for conductors, semiconductors and insulators. Which band determines the electrical conductivity of a solid? How is the electrical conductivity of a semiconductor affected with rise in its temperature? Explain.
Which one of the following elements will require the highest energy to take out an electron from them?
Pb, Ge, C and Si
