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1. Main Concepts To Learn:
(A Five Minute Tutorial)
This applet visualizes the
following processes of excess minority carriers in a semiconductor:
2. A Quick Step-By-Step For Each Topic:
generation of excess carriers, by a laser beam,
diffusion, due to the concentration gradient,
drift, by the applied bias (or electric filed),
Topic-1: Generation of excess carriers. The start button initiates
a laser pulse, which falls onto the Ge semiconductor and generates excess
concentration of electron-hole pairs.
Topic-2: Diffusion process of excess minority carriers. Once
the excess carriers are generated in the center portion of the Ge bar,
the excess minority carriers diffuse, or spreads, due to the concentration
nonuniformity. As always, red color means p-type and holes. Blue
color means n-type and electrons. The width of the concentration
profile increases due to the diffusion. You can experiment on this
concept by varying the carrier mobility (which is related to the
diffusion coefficient by Einstein's relation).
Topic-3: Drift process of excess minority carriers. The nonzero
applied voltage (or electric field) causes the excess carriers to move
in one direction of the semiconductor. The direction depends on the
polarity of applied voltage (or electric field) and on the sign of minority
carrier charge. The peak position of the concentration profile moves
due to the drift process. Experiement on this concept by varying the bias
value, mobility value, or the sample length (electric field
= voltage / sample length).
Topic-4: Recombination process of excess minority carriers.
Once the laser pulse (which is the external stimulus) is not present, the
excess carriers recombine and eventually disappear completely, thus returning
the carrier concentration back to the thermal equilibrium value.
The total number of excess carriers will therefore decrease with
time. Experiement on this concept by varying the carrier lifetime.