Question

 

(i) Semiconductors are useful as electronic devices because

       They are very abundant in nature

       They are all good conductors

       They have a very high melting point

        Their electrical conductivity can be controlled by the addition of impurities

        Their electrical conductivity can be controlled by a variation in the temperature

Explain why you chose this answer:      

(ii) The magnitude of the electronic bandgap in a solid is primarily determined by

      The temperature

       The number of free electrons in the material

       The nature and strength of the chemical bond in the material  

        The type of doping in the material

        Will decrease when we apply a positive voltage

Explain why you chose this answer:

(iii) The number of intrinsic carriers in a semiconductor   

      Will be determined by the temperature and the bandgap of the material

       Is always greater than the extrinsic concentration

       Is always less than the extrinsic concentration  

       Is determined by the doping level in the material

       Will determine the conductivity of the material  

Explain why you chose this answer:

iv) When doping a semiconductor n type, the bandgap of the material will:

      Increase

       Decrease

       Remain the same

        Will decrease when we apply a negative voltage

        Will decrease when we apply a positive voltage

Explain why you chose this answer:      

Answer 1

(i) Ans: Their electrical conductivity can be controlled by the addition of impurities.

Semiconductor materials are useful because their behavior can be easily manipulated by the deliberate addition of impurities, known as doping. A semiconductor material has an electrical conductivity value falling between that of a conductor, such as metallic copper, and an insulator, such as glass. Its resistance falls as its temperature rises; metals are the opposite. Its conducting properties may be altered in useful ways by introducing impurities ("doping") into the crystal structure. When two differently-doped regions exist in the same crystal, a semiconductor junction is created. The behavior of charge carriers, which include electrons, ions and electron holes, at these junctions is the basis of diodes, transistors and all modern electronics.

Semiconductor devices can display a range of useful properties, such as passing current more easily in one direction than the other, showing variable resistance, and sensitivity to light or heat. Because the electrical properties of a semiconductor material can be modified by doping, or by the application of electrical fields or light, devices made from semiconductors can be used for amplification, switching, and energy conversion. The conductivity of silicon is increased by adding a small amount (of the order of 1 in 10⁸) of pentavalent (antimony, phosphorus, or arsenic) or trivalent (boron, gallium, indium) atoms. This process is known as doping and resulting semiconductors are known as doped or extrinsic semiconductors.

(ii)Ans: The number of free electrons in the material.

This formation of bands is mostly a feature of the outermost electrons (valence electrons) in the atom, which are the ones involved in chemical bonding and electrical conductivity. The inner electron orbitals do not overlap to a significant degree, so their bands are very narrow.

Band gaps are essentially leftover ranges of energy not covered by any band, a result of the finite widths of the energy bands. The bands have different widths, with the widths depending upon the degree of overlap in the atomic orbitals from which they arise. Two adjacent bands may simply not be wide enough to fully cover the range of energy. Hence, the more number of free electrons, the lesser the energy bandgap.

(iii) Ans: Will be determined by the temperature and the bandgap of the material.

The number of intrinsic carriers depends on Temparature and bandgap, if lesser the band gap easier for electrons to jump from valence band to coduction band and creat electron hole pairs. if the temparature is increased, energy absorbed by semiconductor due to this again electron hole pairs are generated.

In an intrinsic semiconductor, the number of electrons generated in the conduction band is equal to the number of holes generated in the valence band. Hence the electron-carrier concentration is equal to the hole-carrier concentration.

The hole concentration in the valence band is given as    

    E_F- E_V= (Band gap)/2

The electron concentration in the conduction band is given as

E_C- E_F=(Band gap)/2

(iv) Ans: Decreases

The addition of donor impurities contributes electron energy levels high in the semiconductor band gap so that electrons can be easily excited into the conduction band. This shifts the effective Fermi level to a point about halfway between the donor levels and the conduction band.

Electrons can be elevated to the conduction band with the energy provided by an applied voltage and move through the material. The electrons are said to be the "majority carriers" for current flow in an n-type semiconductor.