Ch 1c 2006

Ch 1c 2006 - Carrier Action From a device point of view...

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Carrier Action From a device point of view nothing interesting happens under equilibrium conditions (i.e. we can’t get any net current flow). Only when a semiconductor is perturbed, giving rise to carrier action can currents flow within and external to the semiconductor. Under normal operating conditions the 3 primary types of carrier action that occur within a semiconductor are drift, diffusion and recombination-generation (R-G). We will look at each of these individually. Drift Drift is charged particle motion in response to an applied electric field. When an electric field is applied across a semiconductor as shown below the resulting force on the charge carriers accelerates the positively charged holes IN THE SAME DIRECTION AS THE electric field and e- in the opposite direction (as long as there are available energy states in the CB or VB). Because of frequent collisions of these particles with lattice atoms and ionized impurity atoms, this carrier acceleration is constantly interrupted (called scattering events). Result is carrier motion in the general direction with / against the electric field but not in a straight, direct path/ EE 329 Introduction to Electronics 30
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The drift motion of each particle is complicated but we can simplify things by only considering macroscopic quantities reflecting the average motion of the carriers. If we average the motion of all the e- or h+ at any given time, we find that the resultant motion of each carrier type can be described in terms of a constant drift velocity (v d ). Now we simplify things by assuming that all the carriers move in the appropriate direction with respect to the electric field at a CONSTANT velocity. Remember that this drift velocity is IN ADDITION to the random thermal motion of the carriers which has no net directional flow. The thermally related random carrier velocity at room temperature is about 1 / 1000 speed of light, but since they have no net directional motion, this motion can be ignored when calculating device current transport. Now we develop the drift current equations; first define current as the charge per unit time crossing any
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This note was uploaded on 09/10/2011 for the course EE 3114 taught by Professor Moon during the Spring '10 term at NYU Poly.

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Ch 1c 2006 - Carrier Action From a device point of view...

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