High electric field packaging of silicon carbide photoconductive switches

Photoconductive semiconductor switches (PCSS) made from semi-insulating (SI) silicon carbide (SiC) are promising candidates for high frequency, high voltage, and low jitter switching. However, existing switches fail at electric fields considerably lower than the intrinsic dielectric strength of SiC (3 MV/cm) because of the field enhancements near the electrode-semiconductor interfaces. Various geometries were identified which could reduce the electric field near the contact regions. The switches were simulated with various parameters and compared. In all cases, it was determined that a high dielectric constant (high-k) encapsulant is a crucial requirement that reduces high fields within the bulk material while inhibiting surface flashover. Assorted high-k encapsulants were evaluated and a portion was subsequently tested in the lab. The observed dielectric strength and relative permittivity of the encapsulants are presented. Pseudo switches, employing sapphire substrates, were constructed and biased to electrical breakdown. The dielectric strength of the interface between the semiconductor and the encapsulant was tested and improvements were discussed.