Microwave Silicon Windows for High-Power Broad-Band Switching Applications

Theoretical and experimental results obtained with a novel silicon window utilizing double-carrier injection to achieve high-power broad-band microwave switching are presented. The device consists of a wafer of high-resistivity silicon inserted across a waveguide with thin-line junction structures oriented orthogonal to the RF electric field. These line junctions provide hole-injecting contacts on one face and electron-injecting contacts on the other. With zero bias, the window appears as a thin low-loss dielectric slab, and the RF signal is transmitted. With forward bias, the window appears as a highly conductive slab due to the injected electron-hole plasma, and the RF signal is reflected. Calculations show that X-band windows 8 mils thick have a switching ratio of 0.5 to 15 dB if the resistivity change is from 300/spl Omega//spl dot/cm to 1/spl Omega//spl dot/cm. X-band windows have been fabricated having an insertion loss as low as 0.3 dB and a VSWR below 1.3 in the transmission state and an isolation as high as 18 dB (5 amperes at 1 volt) in the reflecting state. These characteristics are maintained across the complete waveguide band (8.2 to 12.4 GHz). Windows have been tested to 50-kW peak power without degradation in switching characteristics. The main advantages of the window over conventional p-i-n diodes are an order of magnitude or more increase in power handling (50 kW peak measured while 300 kW is predicted compared to 5 kW for a single p-i-n diode X-band high-power switch), full guide bandwidth operation (compared to 10 percent bandwidth for the conventional p-i-n), simpler bias circuitry (no reverse bias is required in the transmission state), and higher temperature operation (since no reverse voltage is needed).