Abstract
Gallium arsenide photoconductive semiconductor switches (PCSS) are being studied as enabling technologies for a variety of applications. High gain PCSS can be triggered with small laser diodes or laser diode arrays. Some of the applications require low temporal jitter of the switches relative to the trigger laser. The purpose of this study was to compare the temporal switch jitter times for different systems: we varied the type of trigger laser and its risetime, the type of pulse charger and transmission line that was discharged through the PCSS, and the geometry of PCSS used. One of the PCSS was an opposed contact PCSS geometry used by the Air Force Research Laboratory (AFRL). The other was a coplanar geometry switch made by Sandia National Laboratories (SNL). It is found that the optical trigger laser characteristics are dominant in determining the PCSS jitter while the nature of the contact geometry (opposed or coplanar) is not as important.
INTRODUCTION
Gallium arsenide (GaAs) photoconductive semiconductor switches (PCSS) have been used for some time in high power ultra-wideband source applications."2'3 In order to achieve high power, many switches are closed at the same time. For the powers of interest and under high repetition rate conditions, this is only possible ifthe jitter among individual switches can be kept below approximately 40 ps. This number is determined from the rule-of-thumb empirically determined jitter of about 10% of the rise time achieved by the PCSS. The rise time of the Air Force Research Laboratory's (AFRL) switches is approximately 350 ps. Our two research groups have pursued slightly different paths to achieve each organization's specific needs. At AFRL, high peak power results are tantamount; therefore, keeping the jitter to a minimum is essential. On the other hand, SNL also has switching needs that have focused on reliability and/or slower switching times. Consequently, the number of switching events before failure (lifetime studies and measurement of the mean time before failure) also has been an important yardstick for SNL4 '5. Our switches are made differently and are tested under different conditions. Therefore, it was not surprising that the performance of these switches were substantially different when simply comparing achieved values reported from each laboratory. However, a careful comparison had never been attempted. Recently, we decided to select one parameter, in this case switch jitter, to study what differences and similarities exist between our two switches and the operating conditions. In order to do this, an opposed-contact switch was tested for jitter at AFRL with a parallel plate transmission line and a fast risetime, high pulse energy laser, and then a coplanar switch was tested in the same configuration. Next, jitter measurements were made on the same two switches at the SNL laboratory. In this case, a cylindrical transmission line was used that is charged with a slower risetime than the suarezrobert@example.net. Two lasers were used: a slower risetime, lower pulse energy laser and the previously used fast risetime, high pulse energy laser.
EXPERIMENTAL PROCEDURES

Switches under test
The AFRL switch is made from a 0.5-mm thick GaAs wafer and has 0.25 cm opposed contact gap spacing, as shown in Figure 1 . They are formed on a 5.08-cm diameter semi-insulating, LEC GaAs wafer from Bertram Laboratories. The Army Research Laboratory (ARL) processed this bulk material for AFRL in the following manner.' The wafer was polished and passivated with a 1000 A layer of silicon nitride. The wafer was then coated with photoresist, and a Rogowski profile contact pattern was photolithographically defmed and developed. The wafer was then placed in a reactive ion etcher, and the silicon nitride layer was removed in the patterned area. The contacts are formed by placing the wafer in an E-beam evaporator and depositing 50 A Ni, 750 A Ge, 750 A Au, 750 A Pd, and a 2000 A Au cap onto the wafer. The wafer wasthen soaked in warm acetone to facilitate metal lift-off. The entire process was then repeated for the opposite side of the wafer, and a fmal rapid thermal anneal was performed at 4800 C for 10 seconds. The result obtained after dicing the wafer into individual components is a set of n-i-n semiconductor switches made from through the bulk of GaAs that we use in our laboratory experiments. AFRL has recently started to receive switches from the Naval Surface Warfare Center (NSWC) at Dahigren, 6 Future plans will include comparison of the jitter of these switches with the others discussed here.
Switches under test
Switches made at SNL are processed from both LEC and VGF GaAs wafers, using both 2 and 3 inch diameter wafers. P and N ohmic contacts are made from Au-Be and Ni-Ge-Au-Ni-Au metallizations respectively. Such contacts have much greater longevity operating under forward bias conditions. Both contacts are made to the top surface of the wafer, as shown in Figure 1 , in contrast to the opposed-contact geometry employed in the AFRL switches. Like the AFRL switches, the contacts are patterned using photolithographic lift-off and a -4000 A layer of silicon nitride is used to passivate the GaAs surface in the switch gap. 2.5 mm gap switches were used as a direct comparison to the AFRL switches.
Testing configurations 2.2.1 AFRL test configuration
We performed experiments at AFRL laboratory to determine the difference in temporal switching jitter characteristics between the two switch types when implemented in the AFRL test fixture. The temporal switching jitter is measured as the rms jitter in the time delay between the electrical signal that triggers the laser diode and the electrical signal generated by the closure of the PCSS. In the test fixture (see Figure 2) , the coarse experiment timing is generated using the two positive going pulse outputs of a Stanford Research Systems DG-535 pulse generator. The first output (AB) was used to trigger the high-voltage modulator that charges the PCSS pulse-forming line. The second output (CD) is delayed by 800 ns and used to trigger a Picosecond Pulse Labs model (PSPL) 4000E pulse generator. The 800 ns delay allows sufficient time for the PCSS pulse forming line (PFL) to be charged and for the HV modulator to completely shut offprior to PCSS closure. The PFL and the load line are both 50 l, rectangular coaxial lines: the one way transit time of the lines is 9OO ps for the PFL and-2 ns for the load line. The load line is terminated with a 50 Q resistive load. The PSPL pulse generator is used to generate a fastrising (-. 100 ps) trigger event for both the PCSS laser diode trigger and the SCD-5000 oscilloscope. For the results reported here, the diode trigger consisted of a 904 nm wavelength Laser Diodes Incorporated (LDI) solid state laser with 15 -20 id/pulse with a risetime of -200 ps. When triggered in this manner, the measured temporal switching jitter includes the laser diode jitter, the SCD-5000 jitter and the PCSS jitter. The temporal trigger jitter of the SCD-5000 has been measured by splitting the PSPL output; using one half to trigger the oscilloscope and using the other half as the input signal. In this manner, the SCD-5000 trigger jitter was measured as approximately 6 ps rms and the laser trigger jitter was not measurable.
Testing configurations 2.2.1 AFRL test configuration
During the temporal switching jitter tests, the PCSS was fired at approximately 1 Hz for 10 shots. At the end of the 10 shot sample, the time delay of each switching event was measured as the time (as measured from scope zero) for the pulse to reach half its maximum voltage. This process was repeated several times to determine an overall temporal jitter average, defmed as the rms standard deviation of the time delay values.
SNL test configuration
Testing at SNL was performed using a very similar apparatus and method for determining the timing jitter between the triggering signal and the PCSS closure. The key differences in the SNL experiment were the use a slower HV modulator to pulse-charge the circuit (-40 is vs. 300 ns rise time), a pulse forming line based on a 50 Q RG 8 cable (cylindrical geometry), and the use of an 880 nm fiber coupled diode laser (EG&G Canada Optoelectronics) for triggering the PCSS. This diode laser supplies pulses of-lpJ energy with a rise time of approximately 5ns. The aforementioned trigger laser used by AFRL was also tested at SNL to afford a direct comparison of system performance using the two lasers.
EXPERIMENTAL RESULTS
The data from the tests performed at AFRL are shown in Table 1 where the spreadsheet calculations are reproduced. All switching times in the table are measured in nanoseconds. The same AFRL switch was used for each of the four sets of data (AFRL1 -AFRL 4). Likewise, the same SNL switch was used for the three sets, SNL1 -SNL3. As can be seen from the table, the average standard deviation measured for the AFRL switch was 34.2 ps. The SNL switch had a similar temporal jitter of 41.6 ps. Figure 3 shows one of the sets of data from the AFRL switch. One of the SNL switch data sets is depicted in Figure 4 . The reason that the pulse widths varied on one shot with the AFRL switch and each shot for the SNL switch as the tests progressed has not been exactly determined, but early breakdown of some part of the load circuit is a likely cause. Both types of switches, when operated in the AFRL test fixture, exhibit similar temporaljitter characteristics.
EXPERIMENTAL RESULTS
The same devices were tested at SNL to reveal the differences in system jitter performance due to the use of slower pulse charging and a slower fiber-coupled trigger laser with less optical energy. The data show conclusively that the trigger laser characteristics are critical in obtaining low (<50 ps) jitter performance. Figure 5 shows the effect on timing jitter between using the fiber coupled laser and the lens imaged laser, both sets of data taken using 17-1 8 kV charge voltage on an AFRL switch. More than a threefold reduction (97 vs. 29 ps) of timing jitter is observed using the faster, more powerful laser trigger. A jitter of4lps was also obtained using this laser and the SNL switch. The fact that this low jitter was obtained with the slower pulse charger and similar performance was also obtained using a coplanar SNL switch shows that the trigger laser characteristics are the dominant factor in timing jitter performance.
EXPERIMENTAL RESULTS
The fast (-200ps) rise time of the trigger laser used by AFRL is a key factor in obtaining low PCSS timing jitter. However, it is also critically important to obtain sufficient optical pulse energy and optimal alignment of the laser image in the switch gap. As shown in Figure 6 , the data indicates that it is necessary to deliver greater than about 3 tJ of optical pulse trigger energy to obtain consistently low jitter. We also characterized the sensitivity of timing jitter performance to laser alignment. As shown in Figure 7 , this sensitivity is very significant, with a displacement of only 0.1 mm (a small fraction of the 2.5 mm gap) causing over a doubling of the timing jitter. It is likely that optimizing these factors results in low timing jitter when the PCSS triggers on the fast leading edge of the laser temporal profile. Once the PCSS triggers, the remainder of the optical pulse energy is of negligible effect. Thus it may be possible to obtain low jitter using a lower energy fiber-coupled laser if sufficient energy can be delivered within 200 ps or less, e.g. a gain-switched pulsed laser.
CONCLUSIONS
Despite the fact that the AFRL switch and the SNL switch are significantly different in construction and were manufactured using different processes, the temporal switch jitter results are strikingly similar when tested under identical conditions. The additional comparison made when the AFRL laser was used in the SNL test configuration confirms the fact that the temporal jitter for such solid state photoconductive switches is primarily a function of the laser trigger turn-on characteristics. In particular, low (4O ps) jitter results when the PCSS triggers on the fast (-2OO ps) leading edge ofthe laser temporal profile. Laser Energy (1jJ) Figure 6 . Variation of timing jitter vs. trigger laser optical energy.
CONCLUSIONS
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