SOLID_STATE_ELECTRONIC_DEVICES_2 - 1.1.1 Drift and...

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1.1.1 Drift and Diffusion Currents The two basic processes which cause electrons and holes to move in a semiconductor are: (a) drift , which is the movement caused by electric fields; and (b) diffusion , which is the flow caused by variations in the concentration, that is, concentration gradients. Such gradients can be caused by a non-homogeneous doping distribution, or by the injection of a quantity of electrons or holes into a region. To understand drift, assume an electric field is applied to a semiconductor. The field produces a force that acts on free electrons and holes, which then experience a net drift velocity and net movement. Consider an n-type semiconductor with a large number of free electrons. An electric field E applied in one direction produces a force on the electrons in the opposite direction because of the electrons’ negative charge. The electrons acquire a drift velocity v dn (cm/s) . E v n dn (1.7) where n is a constant called the electron mobility and has units of cm 2 /V-s. For low-doped silicon, the value of n is typically 1350 cm 2 /V-s. The mobility can be thought of as a parameter indicating how well an electron can move in a semiconductor. The negative sign in Equation (1.7) indicates that the electron drift velocity is opposite to that of the applied electric field. The electron drift produces a drift current density J n (A/cm 2 ) given by: E J e - v dn
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Consider a p-type semiconductor with a large number of holes. An electric field E applied in one direction produces a force on the holes in the same direction, because of the positive charge on the holes. The holes acquire a drift velocity v dp (in cm/s) which can be written as: E v p dp (1.9) where p is a constant called the hole mobility, and again has units of
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This note was uploaded on 02/23/2011 for the course ELETRONICS P24L taught by Professor Learymyers during the Spring '11 term at University of the West Indies at Mona.

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SOLID_STATE_ELECTRONIC_DEVICES_2 - 1.1.1 Drift and...

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