Substrate voltage and accumulation-mode MOS varactor capacitance

To adjust the capacitance of an accumulation-mode MOS varactor, a voltage is applied to the drain/source with the gate grounded. In this novel arrangement, a voltage is applied to the gate V_gate and another to the substrate V_sub with the drain/source grounded. Applying V_sub between the p⁺-type substrate and the varactor´s n-well adjusts the minimum capacitance C_min and flattens the overall capacitance-voltage (C-V) curve over a wide range of negative V_gate. Measurements show the tuning range using V_sub to be as wide as that using V_gate. By tuning with V_sub, the capacitance C_min is constant over a range of negative V_gate. In this region, it may be possible to apply a large ac signal on V_gate without modulating the capacitance. To illustrate this technique, C-V measurements of a MOS varactor are simulated in a differential LC-tank VCO circuit. Simulations show how adjusting both V_gate and V_sub alters the oscillation frequency.