Chapter 7 - Experimental Results for WFSRD
Devices
7.1 - Introduction
In this chapter experimental results for the diodes fabricated by the process described in the
previous chapter, and described theoretically in Chapters 4 and 5, are presented. DC measurements are presented to
confirm the basic diode operation in section 7.2. The switching transients for the series-connected
pulse-sharpening circuit configuration (see Figure 1.1) are presented in section 7.3. The switching transients for
the shunt-connected pulse-sharpening circuit configuration (see Figure 1.1) are presented in section 7.4. A
discussion of the results obtained is presented in section 7.5.
7.2 - DC Measurements
Figure 7.1 shows the reverse breakdown characteristics of a typical diode (designated "A8.6")
fabricated in Chapter 6, as measured on a Fairchild 6200-A curve tracer. As described in Section 5.6, the diode was
conservatively chosen to have an operating voltage of 300 V and an ideal theoretical breakdown voltage of 529 V,
based on the predictions of Chapter 4. Figure 7.1 confirms that the breakdown voltage is indeed around 500 V, if we
arbitrarily define breakdown as occurring at I = - 200 μA.
Figure 7.2 shows the forward bias I-V curve for the diode A8.6. It is obvious that the forward
conduction characteristics are less than ideal. For instance, a large forward bias of V = 6 V is required for I =
100 mA. This poor forward characteristic is not surprising, due to the low surface doping, and hence poor ohmic
contacts in the device. Additional dopant implants at the wafer surface after the drive-in could fix this problem
very easily in future devices. However, the next section will show that the forward bias currents required for good
pulse-sharpening behavior are small, resulting in low power dissipation despite the high
on-voltages.
7.3 - Series-Connected Pulse-Sharpening Operation
Figure 7.3 shows the "series-connected" pulse-sharpening circuit used to obtain the waveforms of
Figure 7.4 and 7.5. Figure 7.4 shows seven different waveforms. The widest pulse is the 11 ns wide, 300 V input
waveform, measured with the diode shorted out. The remaining six waveforms show the operation of the diode A8.6 for
IBIAS = 2, 4, 6, 8, 10, and 12 mA, in order of increasing pulse width. In each case, the output fall
time is about 1.7 ns. During this time, the diode switches 300 V and 6.0 A of current. In Figure 7.5, the input
pulse width has been greatly extended, and output waveforms are shown for IBIAS = 6, 12, 18, 24, and 30
mA. Clearly, the pulse sharpening action diminishes for output pulse widths above 10 ns. Both the ramp voltage and
the fall time increase.
Figure 7.3 - Series-connected pulse sharpening test
circuit.
Figure 7.6 shows the circuit output for the diode with the added lifetime-killer impurities
(A8.PT.850.1). The widest pulse is the 38 ns wide, 300 V input waveform, measured with the diode shorted out. The
remaining ten waveforms show the operation of the diode A8.6 for IBIAS = 20, 40, 60, 80, 100, 120, 140,
160, 180, and 200 mA, in order of increasing pulse width. In each case, the output fall time is about 1.5 ns. The
pulse sharpening action diminishes for output pulse widths above 30 ns. Both the ramp voltage and the fall time
increase.
7.4 - Shunt-Connected Pulse-Sharpening Operation
Figure 7.7 shows the "shunt-connected" pulse-sharpening circuit used to obtain the waveforms of
Figure 7.8. Figure 7.8 shows five different waveforms. The earliest (i.e., farthest to the left) pulse is the 300 V
input waveform, measured with the diode removed from the circuit. The remaining four waveforms show the operation
of the diode A8.6 for IBIAS = 2, 4, 6, and 8 mA, in order of increasing delay. The first sharpened pulse
has an extremely fast rise time of about 0.9 ns. The remaining three have longer rise times, on the order of 2
ns.
Figure 7.7 - Fast input shunt-connected pulse
sharpening test circuit.
Since SRDs tend to work best with waveforms that already have a fast rise time, the input waveform of Figure 7.8 is a best-case waveform. Figure 7.9 shows the output of another shunt-connected circuit using diode A8.6 which uses a slower input waveform (generated by a commercially-available Avtech AVR-3-PW-C-OP1 pulse generator.) This particular unit had a rise time of about 20 ns for a maximum output amplitude of 200 V. Since the pulse source in this circuit has a very low output impedance (compared to the 50 Ω sources used above), the pulse sharpening circuit has been inductively coupled. (Details on coupling techniques are available in [HP918]). Figure 7.10 shows that a 4:1 improvement in rise time is easily obtained with the pulse sharpener. (To improve the rise time further, multiple stages can be used).
Figure 7.9 - Slower input shunt-connected pulse
sharpening test circuit
Figure 7.11 shows six different waveforms from the circuit of Figure 7.7. The earliest (i.e.,
farthest to the left) pulse is the 300 V input waveform, measured with the diode removed from the circuit. The
remaining five waveforms show the operation of the diode A8.PT.850.1 for IBIAS = 15, 30, 45 and 60 mA,
in order of increasing delay. The first sharpened pulse has an extremely fast rise time of about 0.6
ns.
7.5 - Discussion
Overall, excellent experimental results have been obtained. The series-connected SRD
configuration is very useful in pulse generators for varying the pulse width of a fast input, while also realizing
fast fall times. Fall times as fast as 1.7 ns were obtained for 300 V pulses into 50 Ω loads, using A8.6 This agrees reasonably well with the theoretical estimate of 1.1 ns (see section 5.6). It was
especially pleasing to note that relatively wide pulses (on the order of 10ns) could be obtained with very low
forward bias currents. For instance, in Figure 7.4, a pulse width of 9 ns was obtained with a bias current of 12
mA. As a comparison, Figure 3.13 shows that a pulse width of 4 ns was obtained (with a similar fall time) with an
extremely large bias current of 500 mA for the commercial diode designated "no. 88", making power dissipation
prohibitively high for wider pulse widths. Evidently, the fabricated diode A8.6 had a much longer effective
lifetime - indeed, Tables 1.1 and 3.2 show that the 4500ns effective lifetime of A8.6 is at least an order of
magnitude of greater than nearly all of the commercial-available SRDs (Table 1.1) and parasitic SRDs (Table
3.2).
The effective carrier lifetime of A8.PT.850.1 was deliberately decreased by adding platinum
impurities. The reduced lifetime of 850 ns (calculated from equation (3.4)) resulted in slightly faster switching
times, and significantly higher bias currents, as one would expect. Interestingly, the shorter
τEFF also allowed longer storage times to be used, without significant rise-time
degradation. This is due to the fact that the stored charge in the diode is stored closer to the junctions than in
a longer-lifetime diode, so less charge diffuses into the middle regions during a given storage
time.
The shunt-connected SRD configuration is useful in pulse generators for improving the rise time
of input pulses. Figure 7.8 shows that a fast 300 V pulse, with a 2 ns rise time, was sharpened using A8.6 to
obtain a rise time of 0.9 ns, with a bias current of 2 mA. This agrees very well with the predicted 1.1 ns
switching time. (The rise time increased as the bias, and hence storage time, increased, to a maximum of about 2
ns). Even more impressively, Figure 7.11 shows a 300 V pulse, with a 2 ns rise time, that was sharpened using
A8.PT.850.1 to obtain a rise time of 0.6 ns. Table 1.1 shows that this time-rate-of-change is superior to that of
all of the listed commercial SRDs.
Figure 7.10 shows a slower 200 V input, with a 20 ns rise time, that has been sharpened to a 5 ns
rise time using A8.6, a 4:1 improvement. Multiple sharpening stages can be used to improve this
[HP918].
These experimental results clearly indicate that the new WFSRD exceeds the capabilities of
currently available SRDs. They also show that by controlling the carrier lifetimes, either the storage times or the
switching times can be optimized.
