In discussing rectifiers we emphasized the importance of minimizing the reverse-bias current and the power losses under forward bias. In many applications,
time response can be important as well. If a junction diode is to be used to switch rapidly from the conducting to the nonconducting state and back again, special consideration must be given to its charge control properties.
- We have discussed the equations governing the turn-on time and the reverse recovery time of a junction.
- From these Eqs, it is clear that a diode with fast switching properties must either store very little charge in the neutral regions for steady forward currents, or have a very short carrier lifetime, or both.
- As mentioned above, we can improve the switching speed of a diode by adding efficient recombination centers to the bulk material.
- For Si diodes, Au doping is useful for this purpose.
- To a good approximation the carrier lifetime varies with the reciprocal of the recombination center concentration.
- Thus, for example, a p -n Si diode may have tp= 1 μs and a reverse recovery time of 0.1 μs before Au doping.
- If the addition of 1014 Au atoms/cm3 reduces the lifetime to 0.1 μs and tsd to 0.01μs, 1015 cm-3 Au atoms could reduce tp to 0.01 μs and tsd to 1 ns (10-9 s).
- This process cannot be continued indefinitely, however.
- The reverse current due to generation of carriers from the Au centers in the depletion region becomes appreciable with large Au concentration.
- In addition, as the Au concentration approaches the lightest doping of the junction, the equilibrium carrier concentration of that region can be affected.
- A second approach to improving the diode switching time is to make the lightly doped neutral region shorter than a minority carrier diffusion length.
- This is the narrow base diode.
- In this case the stored charge for forward conduction is very small, since most of the injected carriers diffuse through the lightly doped region to the end contact.
- When such a diode is switched to reverse conduction, very little time is required to eliminate the stored charge in the narrow neutral region.
- The mathematics involved is particularly interesting, because it closely resembles the calculations.