The fabrication and performance of planar doped barrier diodes as 200 GHz subharmonically pumped mixers

The PDB (planar doped barrier) diode consists of a p⁺ doping spike between two intrinsic layers and n⁺ ohmic contacts. Such devices can have an anti-symmetric current vs. voltage characteristic. The capacitance is approximately constant with the applied voltage, and the barrier height and device capacitance are easily adjustable. These characteristics make the PDB a candidate for millimeter- and submillimeter-wave subharmonic mixers. We have fabricated 2 and 4 μm diameter diodes with different barrier designs using GaAs epi-layers. The devices are planarized using an air-bridge and a surface channel etch. After completely removing the substrate, the devices are mounted on a quartz substrate to reduce parasitic effects. Diced diodes were tested as subharmonic mixers around 200 GHz in both a quasi-optical planar wideband subharmonic receiver and a planar-diode waveguide-mixer. The quasi-optical measurements show that a 0.23 V (and 0.4 V) barrier height GaAs diode with 2.0 μA (and 5 nA) of saturation current gives a DSB conversion loss of 10.8 dB (and 9.5 dB) and a DSB noise temperature of 3795° K and 2450° K). The waveguide mixer measurements were made with a similar 0.23 V barrier height PDB. Such a mixer has a minimum conversion loss of 10.2 dB and noise temperature of 3570° K, and requires only 1.2 milliwatts of available LO power