Physical Limitations and Exploratory Devices

Summary

Regardless of the specific type of semiconductor power device, the two stationary states “on” and “off” and the transients to high voltage dV/dt and to high current dl/dt can be identified. It is shown that between field-effect and bipolar devices a considerable difference exists in the relation of maximum forward-current density to breakdown voltage. This difference results from the characteristics of the semicon­ductor material, especially charge-carrier mobility and maximum field strength. The high conductivity of current bipolar devices is combined with limitations of the switch­ing speed.

The maximum current density is usually not limited by the semiconductor but by heat removal. New packaging concepts seem to allow an improvement of surge-current capability and average on-state current by a factor of 2 to 3. The maximum breakdown voltage may also be raised further. But in this case progress is combined with an enhanced sensitivity to overload and stricter requirements on the perfection of the crystals .

The dV/dt transient depends strongly on the structure of the device and on the mode of operation. Since no principal limitation exists and the maximum ratings of present bipolar devices are relatively low, progress here will depend on the requirements of the circuit. The same holds for the dI/dt transient. The ultimate limit is several orders of magnitude away from today's general needs, and can be approached only at high cost .

A new device is presented which is controlled by electron irradiation. It exhibits high conductivity in the on-state , but the switching speed does not depend on charge-carrier lifetime. Safe operation at switching frequencies up to 200 kHz are demonstrated. This device, however , requires the integration of an electron gun and a semiconductor diode in a vacuum-tight package .

Presented at the symposium by R. Sittig.