Quick Note

QuickNote
(A Five Minute Tutorial)

1. Main Concepts To Learn:
This applet visualizes the following processes of excess minority carriers in a semiconductor:

generation of excess carriers, by a laser beam,
diffusion, due to the concentration gradient,
drift, by the applied bias (or electric filed), and
recombination loss.

2. A Quick Step-By-Step For Each Topic:

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.