Schottky diode technology at Rutherford Appleton Laboratory
Most parts of the electromagnetic spectrum are well understood and exploited, but the terahertz region between the microwave and infrared is still relatively under developed. Potential receiver applications are wide-ranging and cross-disciplinary, spanning the physical, biological, and medical sciences. In this spectral region, Schottky diode technology is uniquely important. InP MMIC amplifiers are generally limited to frequencies less than ∼200 GHz, above which their noise performance rapidly deteriorates. Superconducting circuits, which require cooling, may not always be practical. Either as varistor diodes (heterodyne mixing), or varactor diodes (sub-millimetre power generation), Schottky technology underpins terahertz receiver development. Two important developments have occurred in recent years. First, the underpinning technology base has demonstrably matured. Planar Schottky diode technology has been shown to be practical at frequencies as high as 2,500 GHz, and frequency multipliers have been shown to be capable of generating 100s of mW at frequencies around W-band. Secondly, circuit designs can now be optimised theoretically with CAD electromagnetic structure simulators and non-linear analysis programs. New high-speed computer controlled mills, improved lithographic capabilities and micro-machining techniques also offer exciting new options for cavity and circuit manufacture. This paper describes the Schottky diode technology currently being developed at the Rutherford Appleton Laboratory. Discrete diode components are described as well as integrated diode/filter circuits. Frequency multiplier diode structures are reported which include novel substrate transfer techniques to reduce the effects of dielectric loading and self-heating.