High Energy Optically Controlled Kilovolt Semiconductor Switch

This invention relates in general to a high energy, optically controlled, kilovolt semiconductor switch, and in particular to such a switch for use in microwave and laser transmitters. By optically controlled is meant either carrier generation due entirely to an optical signal or carrier generation due to a "seed" optical signal and avalanche multiplication of the "seed" carrier.

In recent years, major advances in kilovolt, photoconductive semiconductor switches have been achieved, using semiinsulating high resistivity gallium arsenide as the semiconductor material As the name implies, switching is achieved when the semiconductor is illuminated, usually in the form of light energy emanating from a laser as for example, Nd:YAG laser and laser diodes. An important concern in such switches is the high voltage blocking capability. Depending on the gap length between electrodes of 0.5 to 10 mm, the semiconductor must hold off one to fifty kilovolts and conduct currents ranging from tens to thousands of amperes. The combination of electric field stress and current often results in a destructive avalanche or breakdown of the device. One of the factors contributing to the destructive avalanche is the intense electric field that exists at or very near the electrodes. Any micro- protrusion at the electrode as caused for example, by a fissure in the material, serves as a potential initiator of a filamentation channel, arising from the field re-enforcement at the site of the electrode defect. Typically, the large current densities at the microprotrusion melts the electrode material, and this produces a filamentary, conducting channel in the semiconductor. Thereafter, the switch loses its voltage hold-off capability and the semiconductor can no longer function as a switch.

The general object of this invention is to provide an improved high energy, optically controlled, kilovolt semiconductor switch for use in microwave and laser transmitters. A further object of the invention is to provide such a switch that provides improved high voltage capability and efficiency, while maintaining sub-nanosecond switching speed and complete high voltage isolation

It has now been found that the aforementioned objects can be attained by the introduction of thick, doped epitaxial layers into the photoconductive semiconductor.

More particularly, according to the invention, thick highly doped epitaxial layers are grown on opposite sides of a bulk piece of high resistivity semiconductor as for example, gallium arsenide, using vapor or liquid deposition techniques. A typical thickness for the epitaxial layer is in the 10 to 100 micron range. One side of the gallium arsenide is doped with a P+ impurity while the opposite side is doped with N+ impurity. Typical doping densities fall in the 10.sup.14 to 10.sup.17 cm.sup.-3 range The exact values of both the epitaxial thickness and the doping density depend on voltage bias and other parameters. Metallic electrodes are then deposited on the epitaxial layers using standard ohmic contact procedures for gallium arsenide. As a result of the introduction of the epitaxial layers, the electric field profiles change dramatically.

FIG. 1 shows a semi-insulating GaAs switch structure and field profile.

FIG. 2 shows a GaAs switch structure with the addition of epitaxial layers and the corresponding field profile. The thickness of the epitaxial layers are exaggerated.

FIG. 3 shows a photoconductive GaAs switch with both epitaxial layers and gridded electrode.

Referring to FIG. 1, without the epitaxial layers, the field is practically uniform throughout the semiconductor 11.

Referring to FIG. 2, the addition of the doped layers 13, 15 causes the field to drop close to zero near the boundary of the epitaxial layer. The epitaxial regions therefore succeed in removing the fields a substantial distance away from the electrodes 17, 19. The light 23 is shown entering perpendicular to the current flow in the semiconductor 11.

Referring to FIG. 3, one of the electrodes 17' is gridded, and the light 23 enters the semiconductor 11 through the grid apertures 21. Such an arrangement allows the transverse dimension D of the semiconductor 11 (perpendicular to the current flow) to extend well beyond the electrode 17, 19 dimensions. Such a design helps to prevent surface breakdown, particularly for gap lengths 155 1.0 mm.

The epitaxial layers also serve an additional purpose. Photo-conductive carrier generation in semi-insulating gallium arsenide arises from "EL-2" traps, which are quite numerous (.about.10.sup.16 cm .sup.-3) The traps are produced during the growth process (either Czchrolski or Bridgeman). In epitaxial grown layers, however, the traps are far less numerous. The carrier generation thus is restricted to the semi-insulating region, which is also the region supporting the field This is generally desirable from an efficiency standpoint.

The introduction of thick, doped epitaxial layers in photo-conductive gallium arsenide switches significantly improve the operation of the switch. The removal of the fields away from the electrodes enables the switch to operate at higher electric fields without causing electrode sputtering, and subsequent filamentation breakdown. By confining the fields to the interior of the semiconductor, chances are improved of producing a controllable non-destructive avalanche. This allows the switch to operate with a minimum of light energy, thus improving the overall efficiency. Also, the selective absorption and carrier generation in the semi- insulating region, which coincides with the electric field drift region, makes the best use of the light energy, and the efficiency.

In summary, the new switch structure results in improved voltage capability and greater efficiency.

We wish it to be understood that we do not desire to be limited to the exact details of construction shown and described for obvious modifications will occur to a person skilled in the art.