[Al Skierkiewicz]

Wow,
Talk about reviving old threads!
The device used in the Victor is an International Rectifier IRL3103. What makes a MOSFET a better choice for this service is the very low "ON" resistance and the ability to switch inductive loads. (This device is actually a HEXFET, a particular processs for the silicon layers of the device) Since the series resistance is so low (12mohm per device) large currents do not cause the internal temps to rise. This device does contain an internal diode so it's ability to be used as an amplifier is limited. These diodes are what conduct to turn lights on in the robot when it is pushed with the power off. Current generated in the motors pass through the diodes back to the robot electrical distro. The "H" bridge forms a very nice bridge rectifier when fed in reverse from the motor.
An exploding FET has little to do with applied voltage and everything to do with internal heat. A rapid release of magic smoke is what blows the case apart.
The IGBT (insulated gate, bipolar transistor) shares some charachterisitics of the MOSFET. i.e. it is a voltage driven device (very low drive current) but it does not have the low "ON" resistance needed for this application.
The output switching frequency of the Victor is a tradeoff that allows better speed control for a series of motors that share similar brush/commutator spacing specifications. As a brush DC motor rotates, the brushes switch different windings to the power source. For best control the switched input current should be optimized to this motor switching so that the motor still has good start torgue at low speeds and good speed control at high speeds. Very short pulses vs brush/commutator spacing will not allow the motor to start under load and long pulses will not give any control at high motor RPM. The 884 Victors have a designed output of 120 Hz for this reason. You will note that the ouptut frequency is not the same as the input PWM signal. The internal controller translates the 0-255 modulated PWM signal to a 94 step output signal and provides direction switching as well. Remember that an H bridge operates by turning on only two of the legs at a time for direction control and braking. There is a lot of circuitry inside an 884.
Please be aware that insulated gate and MOS devices still are subject to static induced failures and require some special handling. They are not nearly as sensitive as when first introduced but...