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Saturday, 11 March 2017

Semiconductor Electronics - II


Holes:
The vacancy or absence of electron in the bond of a covalently bonded crystal is called a hole. A hole serves as a positive charge carrier.

Mobility:
a) The drift velocity acquired by a charge carrier in a unit electric field is called its electrical mobility and is denoted by m.

b) The mobility of an electron in the conduction band is greater than that of the hole (or electron) in the valence band.

Electrical conductivity of a Semiconductor:
a) If a potential difference V is applied across a conductor of length L and area of cross-section A, then the total current I through it is given by,
where ne and nh are the electron and hole densities, and veand vh are their drift velocities, respectively.
b) If mh  are the electron and hole mobilities, then the conductivity of the semiconductor will be,
and the resistivity will be,

c) The conductivity of an intrinsic semiconductor increases exponentially with temperature as,
Forward Biasing of a pn-junction:
If the positive terminal of a battery is connected to the p-side and the negative terminal to the n-side, then the pn-junction is said to be forward biased. Both electrons and holes move towards the junction. A current, called forward current, flows across the junction. Thus a pn-junction offers a low resistance when it is forward biased.

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