Chapter 9 - Concluding Remarks

9.1 - Summation and Conclusions

Two approaches to realizing high-voltage step recovery diodes have been considered in this thesis. By examining switching transients in commercially available power diodes, and by considering the results of computer simulations, a new step recovery mechanism has been proposed and demonstrated. It has been shown that these high voltage SRDs can be fabricated by using a diffused structure on a lightly-doped p-type substrate.

Diodes based on this principle were successfully fabricated and demonstrated in several different common circuit configurations. Switching times as low as 0.6 ns were shown for a 300 V transient into a 50 Ω load. This is considerably better than what is possible with commercially available devices. It has also been shown that these devices can be fabricated with very long effective carrier lifetimes, which makes them very easy to use in practical pulse generation circuits. Indeed, this author is pleased to report that these devices were commercialized even before the defense of this thesis. These diodes have been incorporated into shipped units of the Avtech Electrosystems Ltd. line of AVRF pulse generators.

These new diodes have been termed "wide field step recovery diodes (WFSRDs)". Theoretical expressions were developed to permit the optimum design of these devices. In particular, expressions for predicting the transition time and estimating the ramp voltage of WFSRDs were obtained. A straightforward, two-step method of empirically estimating the breakdown voltage of WFSRDs, and diffused rectifiers in general, was also presented. This theory resulted from the unexpected observation that there appears to be an approximate one-to-one relationship between the diode breakdown voltage and the critical electric field of the diode, regardless of the exact details of the diffused structure.

The design theory for the previously-proposed "drift step recovery diode (DSRD)" was also examined, and found to be incomplete. Previous design theories had not considered the nature of the forward transient used to bias the DSRD. By considering the nature of the forward transient, a critical current density related to the substrate doping has been derived. For forward transient currents below this level, the injected charge tends to remain close to the p+ n- junction, which is desirable for DSRD operation. Currents above this level tend to inject charge at both junctions, which is undesirable. This provides an additional design constraint, and allows a definitive choice of the optimum bias current. This also permits an estimate of the maximum stored charge consistent with step recovery action. It is shown that previous designs reported in the literature agree well with the new theories. The new theory justifies the previously unjustified choices of bias current and the lightly-doped layer width, and agrees well with experimental observations of the maximum stored charge consistent with step recovery action.

Also, in considering the general nature of the forward transient, a new expression for the charge carrier density evolution with time during the forward transient in a pin diode has been derived. This expression is considerably more compact than the conventional expression.

It was found experimentally that WFSRDs offer the possibility of very long carriers lifetimes. Indeed, the experimentally measured lifetimes were orders of magnitude better than those of conventional SRDs. WFSRDs are not expected to be useful above 1 kV. In contrast, the DRSDs have been found to be useful primarily above 1 kV, and offer lifetimes only somewhat better than conventional SRDs. As such, these two devices have been found to occupy different application niches.

9.2 - Alternative Approaches to High Speed Semiconductor Switching

It is worth considering the avenues to high-speed diode switching that have not been explored in the main body of this thesis. For instance, other semiconductor materials might be considered for use in these devices. However, a cursory examination of the properties of other readily available semiconductors shows that silicon offers the best comprise between critical breakdown fields and hole mobility. Silicon carbide offers high breakdown voltages, but much lower mobilities. Gallium arsenide offers comparable hole mobility, but lower breakdown fields. No reasonable semiconductor offers both improved breakdown and mobility. Furthermore, direct bandgap semiconductors (like GaAs) suffer the serious disadvantage of inherently lower carrier lifetimes, which is highly undesirable in SRDs.

However, interesting possibilities may arise from the clever use of heterostructures. For instance, one might envisage a step recovery diode band diagram like that shown in Figure 9.1. In this case, a silicon-germanium alloy layer exists in the lightly-doped middle region, adjacent to the p+ s junction. The injected charge would accumulate in the valence band pedestal next to the junction, due to the favorable energy conditions. This would ensure that the charge remains right at the junction, as is desired in a SRD, while maintaining a wide lightly-doped layer capable of withstanding a large reverse voltage. However, incorporation of SiGe layers into Si lattices still represents state-of-the-art laboratory technology [Meye92], which has certainly never been tried at the reverse biases required here. The question of whether or not reasonable lifetimes could be obtained in view of the likely dislocations and other lattice defects also arises.

Figure 9.1 - Possible heterostructure SRD, using Si-Ge alloys.

This thesis has focused on step-recovery diodes, in the traditional sense. That is, these step-recovery diodes operated primarily due to charge storage effects. Other devices exist, which might be used in SRD-like applications, but which owe their behavior to more exotic effects. For instance, extremely fast transitions have been reported by Grekhov et al. [Grek81], [Grek89] using delayed-avalanche devices. Essentially, a very fast-rising reverse pulse is applied to a diode. The rate of rise is sufficiently fast that the critical electric field of the diode can be greatly exceeded for a few nanoseconds without generating significant ionization current. Suddenly, an ionization wave will develop and engulf the space charge region with carriers, causing a voltage collapse. The propagation speed of the ionization wave is not limited to the carrier saturation velocity, so the voltage transition can be very rapid. Switching speeds of 0.2 ns have been reported for 3000 V, 60 A transitions [Grek81]. Switching times of 50 ps have been reported for switched powers of 100 kW [Grek89]. These devices differ significantly from conventional SRDs in their mode of operation, and also from the fact that once the ionization plasma is extinguished, the diode voltage rises again. Also, this dioderequires an already extremely-fast input waveform.

This thesis has also focused on single devices. Naturally, fast high voltage switching can also be obtained by series-connecting many lower voltage conventional SRDs. One example has been reported experimentally, where eight carefully matched conventional SRDs were connected to obtain a 400 to 500 V, 0.8 ns composite SRD [Brow87]. This careful matching will represent a considerable expense in the fabrication of such devices. Also, mechanical reliability can be a concern in stacked devices. More seriously, commercially available stacked SRDs (such as the M/A-COM 44950 series) are aimed primarily at frequency multiplier operations. Achieving long lifetimes and storage times in multiplier diodes is not a priority, since the diode switches once per cycle, and is typically used at GHz frequencies. The short lifetimes make these diodes essentially useless in the series-connected pulse sharpening configuration. The short storage times will also limit the shunt-connected circuits to sharpening input signals that are already quite fast. These short storage times are an inherent feature in stacked SRDs, as can be seen in equation (3.4), repeated here:

(9.1)

Relative to a single SRD, a stack of N SRDs will have an N-times larger I_R, since the allowable operating voltage and load voltage is N times larger. However, I_F generally can not be increased by N times, due to the increased steady-state power dissipation and thermal considerations. Thus, from (9.1), t_S will be considerably lower for a given τ_eff.

The stacked approach does offer a viable alternative in certain cases. Of course, it should be pointed out that the WFSRDs and DSRDs discussed in this thesis could also be stacked to generate very-high voltage composite devices.

Exceedingly rapid voltage transitions have also been predicted for pin diodes used as photoconductive switches [Sun92]. These diodes are biased in the reverse state, and very few free carriers exist until the diode is illuminated with a rapid, high intensity laser pulse, which generates carriers, collapsing the voltage across the diode. This, however, is a rather complex and expensive approach.

9.3 - Future Work Beyond This Thesis

The theory presented within this thesis for the WFSRD does not present a simple method, aside from simulations, of calculating the maximum storage time (and carrier lifetimes) consistent with good step recovery action. As it is desirable to make the storage times as long as possible for pulse sharpening purposes, it would be desirable to have a simple method of calculating it. Again, this is problematic, since it requires exact knowledge of the carrier distribution in the diffused psn diode during the forward steady state. As discussed earlier, the lack of clear boundary conditions makes this an extremely challenging problem to solve analytically.

This thesis has dealt exclusively with high-voltage SRD device design. The study of SRD circuits is also a worthwhile endeavor. The pulsed biasing of the DSRD represents an increase in circuit complexity over conventional SRD circuits, especially since the DSRD operates at much higher voltages. Considerable opportunities exist to propose and experiment with new practical pulsed-bias DSRD circuits.

One other promising area of study is the examination of the benefits and tradeoffs of connecting arrays of WFSRDs and DSRDs in series and in parallel to create a high-voltage, high-current, ultra-fast composite switch. Dr. Alexei Kardo-Sysoev reports that a circuit consisting of 120 stacked DSRDs has been used to sharpen 100 kV pulses, to rise times on the order of 1 ns, resulting in 100 MW of switched power [Kard96]. Very little has been published on this topic, particular theoretically, even including conventional SRDs.