A millimeter wave monolithic VCO with an integrated HEMT as a varactor

A voltage controlled oscillator (VCO) is one of the key components in millimeter wave systems for FM or FSK modulation, frequency tuning and phase locked sources, etc. MMIC VCOs with integrated varactor for frequency control or modulation is useful for such functions. The main problem is to integrate the varactor with a standard technological process. One possibility is the use of a cold HEMT as a diode. Drain and source are connected and the non-linearity Cgs is only depending upon the voltage Vgs (=Vds). Capacitance ratio and quality factor can then be optimized in relation to the gate length. A short gate length provides a low capacitance ratio, but high quality factor. The purpose of this work is to develop a monolithic VCO with improved frequency variation linearity and with simplified fabrication process, by applying high f _max 0.1 μm gate HEMT both as an active element and as a varactor