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What is an extrinsic semiconductor?

Intrinsic semiconductor like pure silicon and germanium are characterized by high resistivity because of low free carrier charge density e.t.c. Hence in order to increase conductivity trivalent and pentavalent impurities are added. The added impurities are called dopants and the process is termed as doping. The resulting semiconductor after doping is called extrinsic semiconductor.

 What is N type Semiconductor?

 If the dopant added to an intrinsic semiconductor is a pentavalent impurity such as phosphorous, Arsenic, Antimony the resulting semiconductor is termed as N-type conductor. The added impurity atoms will displace some of silicon atoms in the crystal lattice. Four of the valence electrons of dopant will occupy covalent bonds with silicon; fifth electron will be nominally free and can be used as carrier of current. The energy required to detach this fifth electron is of the order of 0.01eV for Germanium and 0.05eV for Silicon.

This can be understood alternatively as follows, due to replacement of some of the silicon atoms with pentavalent atoms from the crystal the energy band structure of the crystal lattice gets altered. The consequence of this alteration is the introduction of new allowable energy levels in the band structure in the forbidden gap close to conduction band. The energy required to excite the electrons occupying this new energy level to conduction band is of the order of 0.01 eV for Germanium and 0.05 for Silicon.

What is P- Type Semiconductor?

If the dopant added to an intrinsic semiconductor is a trivalent impurity such as Boron, Indium, Gallium the resulting semiconductor is termed as P-type conductor. The added impurity atoms will displace some of silicon atoms in the crystal lattice. Three of the valence electrons of dopant will occupy covalent bonds with silicon; vacancy in the fourth bond constitutes a hole which effectively acts as positive charge carrier and can accept an electron.

Due to replacement of some of the silicon atoms with trivalent atoms from the crystal the energy band structure of the crystal lattice gets altered. The consequence of this alteration is the introduction of new allowable energy levels in the band structure in the forbidden gap close to valence band. The energy required to excite the electrons from valence band to this new energy level is of the order of 0.01 eV for Ge and 0.05 for Si generating holes in valence band.

 What are passive and active elements?

Active component is one which is capable of delivering energy independently. Examples are voltage and current sources, transistors, Opamp e.t.c. If the element is not capable of delivering energy then it is termed as Passive element.

Bohr’s model?

Bohr’s model is one of the proposed model of atom and was able to explain the emission and absorption spectra of hydrogen and single electron ions such as He2+. Some of the main postulates of Bohr’s atomic model are:

a) Electrons revolve around the nucleus of atom much like planets revolve around the sun in only definite circular paths called as orbits. Each orbit corresponds to a definite energy. The energy values are quantized and are allowed to posses only certain values.

b) As long as the electron is in this path it will not absorb or emit radiation and the atom will be stable. Hence these are called stationary orbits.When an electron drops from higher energy orbit to lower energy orbit, the difference in the energy of those orbits will be emitted as radiation.

                                               E_{2} - E_{_1} =h*\nu

whereh*\nuis the energy of emitted photons.

c) Each energy level is assigned a number which is a integer and is called principal quantum number.

d) Angular momentum of electron in each energy level is quantized in terms of h/ (2*\pi ) i.e.

                                                  M*v*r = n* h/ (2*\pi )

Where M is the mass of electron, v, and n are velocity and principal quantum number of electron corresponding to that particular orbit respectively, r is the radius of orbit, h is Planck’s constant.

 What is a conductor?

Conductor is a material which offers less resistance (ideally zero) for current flow. In conductors even a small applied field generates large currents. Most of the metals are conductors due to presence of loosely bound valence electrons. Conductance is a material property which quantifies how good the material fits as a conductor. It is measured in units of Siemens/metre. Ideally the conductance should be infinity for a perfect conductor (such is the case of super conductor). Silver is the best known conductor with conductance value of 6.30×107 S/m at 20 Deg C succeeded by copper 5.96×107 S/m which is widely used conductor due to its less cost compared to silver.


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