Appendix D - The Relationship Between t_R and

D.1 - Introduction

As mentioned in Chapter 5, the expression for calculating the intrinsic switching time given in equation (5.4.3.4) is a fairly simple estimate of t_R. The results presented in Chapter 7 show that this equation is quite reasonable as an engineering estimate. However, it would be more useful if it established the relationship between t_R and the carrier lifetime τ. Regrettably, this relationship is far too complex to be dealt with analytically. This appendix presents the results of several computer simulations, and shows that the switching time varies weakly with carrier lifetime.

Intuitively, one would expect that the switching time would decrease with lower carrier lifetimes, since this would concentrate charge at the edges of the middle region, which is the goal of most conventional SRD designs.

D.2 - Computer Simulations

Several simulations were run for the structure described in Section 5.6 in the series-connected circuit configuration described in Section 7.3, with different values for . Since τ >> τ_n + τ_p at high injection levels (see equation (2.3)), the actual simulation parameters τ_n and τ_p were each set at /2. The forward bias was adjusted in each case such that the storage time t_S was approximately 5 ns in all cases. In other words, each device had a similar amount of stored charge. The results of the simulations are summarized in Figure D.1.

Figure D.1 - Variation of simulated switching time with lifetime, for equal stored charge, with the structure described in Section 5.6 and the circuit described in Section 7.3.

Evidently a 50:1 change in lifetime only induces a 1.8:1 change in switching time. As an experimental check on this relationship, one can compare the results obtained with the two WFSRDs in section 7.3. Diode A8.PT.850.1 had an effective lifetime about five times lower than that of diode A8.6. This 5:1 lifetime difference corresponded to a 1.13:1 difference in switching times, which agrees well with the simulated results. (The other commercial diodes had higher switching times despite their lower carrier lifetimes; evidently these commercial structure were further from the optimum structure than diode 89.)

Based on these results, equation (5.4.3.4) can be applied with the expectation that the result will be accurate within a factor 2.