Development and characterization of varactor diodes for millimeter wavelenghts

The GaAs varactors described here are P+NN+ mesa devices with base widths (distance between P-+ and N+regions) made as narrow as possible to achieve high cutoff frequency. Routine dimensions are now 1 to 2 microns using as thin a P+ diffusion as the epitaxial layer thickness permits. Diffusion depth is typically 1.5 to 2.0 microns. Metallization processing has contacted P-+ regions thinner than one micron deep without shorting. Epitaxial layer doping level (ND) is controlled to achieve high (25-45v) or low (10-25v) voltage breakdown devices.