Branch : Computer Science and Engineering
Subject : Fundamental of Electronic Devices
Unit : Junction Properties
Switching Diodes
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.
Switching Diodes:
- 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.