Influence of the MOS varactor gate doping on the performance of a 2.7GHz - 4GHz LC-VCO in standard digital 0.12 µm CMOS technology

The influence of the gate type of the MOS varactor on the tuning range and phase noise of a fully integrated LC-VCO (voltage controlled oscillator) is presented. Three varactors in multifinger layout with STI (shallow trench isolation) are compared. The polysilicon gate is either entirely n- or p-doped or the fingers have alternating nor p- doping. Differences in capacitance, steepness and quality factor are shown. Two identical VCOs with the varactors having n-gates or np-gates are measured. Homogenous doping increases the VCO tuning range to 1.31GHz (±19.6%) in comparison to 1.06GHz (±14.8%) obtained by mixed doping. Mixed doping however has the advantages of linearized frequency behaviour, lower upconverted flicker noise and reduced maximum sensitivity to variations in supply voltage. With a current consumption of only 1mA from a supply voltage of 1.5V both VCOs show a phase noise of -115dBc/Hz and a figure of merit (FOM) of -183.3dBc/Hz at a 1MHz offset from a 4GHz carrier.