A Minimal Photocurrent Analog Multiplexer Using Edge-On Diodes

An edge-on diode (EOD) is a dielectrically-isolated lateral PN junction diode formed in a thin (2 μm) layer of single-crystal silicon formed on an insulating substrate. EODs formed on sapphire have extremely low primary photocurrents because the junction areas and minority carrier lifetimes can be made very small. A 16-input analog multiplexer silicon on sapphire integrated circuit was fabricated to evaluate the EOD concept. The multiplexer met all electrical specifications and proved very fast, settling within 60 ns. While circuit radiation testing is not yet complete, individual diodes tested at a dense plasma focus x-ray source showed individual diode photocurrents of only about 100 μA/1011 rads (Si)/s.