5.5 Transient and A-C Conditions Time Variation of Stored Charges Reverse Recovery Transient Switching Diodes Capacitance of p-n Junctions The varactor Diode 5.6 Deviations from the Simple Theory Effect of Contact Potential on Carrier Injection Recombination and Generation in the Transition Region Ohmic Losses Graded Junctions 5.7 Metal-semiconductor Junctions Schottky Barriers Rectifying Contacts Ohmic Contacts Typical Schottky Barriers 5.8 Heterojunctions

was neglected at low injection

An electron at E r is thermally excited to the conduction band (G n ) and a valance band electron is subsequently excited thermally to the empty state on the recombination level, leaving a hole behind in the valance band (G p ). Normally, these emission processes are exactly balanced by the corresponding capture processes R n and R p. However, in the reverse-bias transition region, generated carriers are swept out before recombination can occur, and net generation results.

A level near the middle of the band gap is most effective, since for such centers neither G n nor G p requires thermal excitation of an electron over more than about half the band gap. In most materials recombination centers exist near the middle of the gap due to trace impurities or lattice defects. If no recombination level is available, this type of generation is negligible. Generation from centers within W is most important in materials with large band gaps, for which band-to-band generation in the neutral regions is small. Generation within W naturally increases linearly with W as reverse bias increases. (The saturation current due to generation in the neutral regions was found to be essentially independent of reverse bias)

The reduction in junction voltage V lowers the level of injection so that the current increases more slowly with increased bias. However, the conductivity of each neutral region increases with increasing carrier injection. (conductivity modulation)

In a graded junction the usual depletion approximation is often inaccurate. If the grade constant G is small, the carrier concentration (p-n) can be important. The usual assumption of negligible space charge outside the transition region is questionable for small G. Nevertheless, junction theory is qualitatively applicable to the graded junction, with some alterations in the functional form of the resulting equations.

5.5 Transient and A-C Conditions Time Variation of Stored Charges Reverse Recovery Transient Switching Diodes Capacitance of p-n Junctions The varactor Diode 5.6 Deviations from the Simple Theory Effect of Contact Potential on Carrier Injection Recombination and Generation in the Transition Region Ohmic Losses Graded Junctions 5.7 Metal-semiconductor Junctions Schottky Barriers Rectifying Contacts Ohmic Contacts Typical Schottky Barriers 5.8 Heterojunctions

5.5 Transient and A-C Conditions Time Variation of Stored Charges Reverse Recovery Transient Switching Diodes Capacitance of p-n Junctions The varactor Diode 5.6 Deviations from the Simple Theory Effect of Contact Potential on Carrier Injection Recombination and Generation in the Transition Region Ohmic Losses Graded Junctions 5.7 Metal-semiconductor Junctions Schottky Barriers Rectifying Contacts Ohmic Contacts Typical Schottky Barriers 5.8 Heterojunctions