Design and fabrication of ultra-small GaAs Schottky barrier diodes for low-noise tetrahertz receiver applications

The optimization of GaAs Schottky barrier mixer diodes for use in heterodyne receivers in the frequency range from 1-3 THz is investigated. The principal design rules are: (1) the substrate doping must be as high as possible to minimize series resistance; (2) the diode cutoff frequency must be increased as much as possible, which requires a small anode diameter and a higher epilayer doping density than has previously been used; and (3) the diode´s RF impedance must be made to match the impedance of the corner cube antenna, which requires a low junction capacitance. To achieve these goals, a nominal epilayer doping density of 5×10¹⁷ cm^-3, an anode diameter of 0.5 μm, and the most highly doped substrate available (4.5×10 ¹⁸ cm^-3) were used. The diodes were fabricated with UV lithography and reactive ion etching. The resulting figure-of-merit cutoff frequency (10.6 THz) and zero-bias junction capacitance (0.4-0.5 fF) are both state of the art. The diodes have been RF-tested at 1.5 THz and yielded significant improvements in receiver sensitivity and reduced LO (local oscillator) power requirements