Several next generation military weapons systems rely on pulsed power systems.  Early research efforts in this area were directed towards inertial confinement in nuclear fusion reactors.  More recently, these systems have been applied to advanced military hardware including high power microwave or high power laser directed energy weapons, as well as electromagnetic launchers.  In simple terms, a pulsed power system uses energy stored in a bank of capacitors that is discharged as brief pulses.  Through the use of a pulse forming network, these pulses may be compressed and made shorter to increase the power density and the power in each pulse.  One of the key components of a pulsed power system is the switch used to discharge the capacitor bank.  This switch must be capable of operating in a high voltage environment and switching very high currents, with a very fast switching speed or rise time.  Traditionally, gas discharge or vacuum switches are used in these systems. 

However, solid state switches are expected to provide substantial reliability improvements in a more compact package. OptiSwitch Technology Corporation (OTC) has demonstrated solid state switches that meet the basic requirements for military pulsed power systems.  Traditional  solid state switches are triggered (i.e. “switched”) by the application of a gate voltage.  In a conventional electrically gated device, the current initially flows around the gate area and current diffusion away from this activation point is required to support large currents.  The diffusion process is slow, especially for high voltage devices, which limits the rate of current rise.  In contrast, OTC uses a solid state switch that is triggered by an intense laser light pulse.  The rate of light propagation through the bulk silicon device and the creation of carriers is much faster, compared to the traditional electrically gated solid state switch.  This accounts for switching speeds that can be 100x greater than what is encountered in gate turn off (GTO) devices.  A large area switch can be turned on by an optical pulse width of nanoseconds to microseconds.  This triggering technology allows these switches to fulfill the needs of military pulse power systems.  The use of an optical trigger also makes these switches more readily adaptable to a high voltage environment.  Figure 1-1 shows an example of a light activated solid state switch.

The EMPF and OptiSwitch Technology Corp. are currently working together under a US Navy ManTech program to establish a manufacturing line for light activated semiconductor switches that will meet DoD production requirements and achieve significant cost reductions.  This program will optimize the yield of individual fabrication processes and develop testing procedures to remove defective devices before a significant amount of value has been added by the manufacturing process.

Unlike the more common CMOS integrated circuits, high power solid state switches require long minority carrier lifetimes.  In fact, the performance increases achieved during this program have been directly correlated with greater and more uniform carrier lifetimes.  Contamination is a common source of minority carrier lifetime degradation.  In particular, iron is a common contaminant that must be eliminated.  Under the ManTech program, a thyristor fabrication line was developed for the manufacturing of light activated semiconductor switches.  Special attention was paid to the elimination or reduction of known contamination sources.  This line produced wafers with greatly reduced contamination levels, compared to those manufactured elsewhere.  Carrier lifetimes increased, as well as the high power performance of the device.  Device yields from the new manufacturing line have substantially increased, which is expected to lead to significant cost reductions for the overall component.

Since these switches are designed for operation at high voltages, the devices require a significant amount of grinding and passivation coating.  Processes have been developed to grind wafers accurately that are compatible with automated production equipment.  The coating process has been optimized to produce switches that can withstand 20 kV with a high yield.  Under the ManTech program, a hermetic package will be developed so that light activated switches are available in a presspak design configuration.

Through semiconductor process simulation software, the device defects were reduced and the performance optimized.  The elimination of a particularly malicious process induced defect allowed the optical trigger pulses to be reduced by more than an order of magnitude with no loss in switch performance.  This will allow the laser driver power supply to be considerably reduced, leading to an overall cost reduction of a light activated semiconductor switch. 

For solid state switches, the number of cycles1 to failure (NF) can be conservatively related to the temperature excursion (∆T) for each current pulse through the equation:

In pulsed power systems, a discharge pulse duration is often in the range from 10-6 to 10-3 sec.  In this pulse duration range, the heating of the switch is adiabatic, so one can estimate ∆T from the following equation:

The equation above tells us that switch reliability will be improved by optimizing the semiconductor processing to give the lowest “on-state” resistance for the switch.  This equation also allows the designer to estimate the reliability of the switch based on the “on-state” resistance and the action of the switch.  OTC has demonstrated light activated semiconductor switches having actions in excess of 1 x 106 A2-sec.  A switch designed for greater than 200,000 cycles requires ∆T < 75 °C.

In summary, a US Navy ManTech program is currently underway that will develop a manufacturing line for light activated solid state switches that meet the requirements for pulsed power systems needed for advanced weapons systems.  This manufacturing system relies on a semiconductor wafer fabrication line that is optimized for these devices.  Through process optimization and contamination reduction, the manufacturing yields and the performance of these devices have been substantially improved.  These improvements are expected to lower the cost and improve the reliability of the advanced switches required by DoD pulsed power applications.

References:
1I.L. Somos, et.al, “Power Semiconductors Empirical Diagrams
  Expressing Life as a Function of Temperature Excursion”, IEEE
  Trans on Magnetics, 29, (1993): 517-522.