1. The diffusion length L for carriers in a semiconductor is related to their mobility ต, mean lifetime ? and the temperature T by
typically for crystalline Si
ต= 0.3 m2s- 1 V- 1
? = 10- 4 s
L~ 1 mm
The measurement of carrier lifetimes and diffusion lengths is important for the design and optimisation of semiconductor devices.
2. Diodes have current, voltage and temperature ratings. The current rating is based on its ability to dissipate heat. Since the junction area is small the current density can be quite large so that ratings as low as 10mA are common. If this rating is exceeded the diode may be damaged by overheating.
3. The reverse voltage is also important because breakdown occurs in the range from 10 - 100 volts depending on design. If the reverse breakdown voltage is exceeded then large currents can flow and destroy the junction.
4. All semiconductors have operating ranges of temperature in which saturation occurs. For Si this range is wider than that for Ge.
5. Real Si or Ge devices exhibit a threshold voltage (0.3V for Ge and 0.7V for Si) below which IF falls rapidly to I0. This threshold is due to recombination and ionisation occurring as the carriers move through the depletion zone.
6. Rectifiers : pn junction diodes are often used as half-wave rectifiers because they pass a current readily in one direction but not in the other. They can also be used in combination with a capacitor to produce a filter that removes AC from DC in a circuit