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/sub gate/ and another to the substrate V/sub sub/ with the drain/source grounded. Applying V/sub sub/ between the p/sup +/-type substrate and the varactor's n-well adjusts the minimum capacitance C/sub min/ and flattens the overall capacitance-voltage (C-V) curve over a wide range of negative V/sub gate/. Measurements show the tuning range using V/sub sub/ to be as wide as that using V/sub gate/. By tuning with V/sub sub/, the capacitance C/sub min/ is constant over a range of negative V/sub gate/. In this region, it may be possible to apply a large ac signal on V/sub 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/sub gate/ and V/sub sub/ alters the oscillation frequency.

I. Introduction

Accumulation-Mode metal–oxide–semiconductor (MOS) varactors have become increasingly popular in silicon RF CMOS phase-locked loop (PLL) circuits [1], [2]. They are used in the voltage-controlled oscillator (VCO) where a wide oscillation range is desired. Requirements for a CMOS VCO are low phase noise, low power consumption, and a wide tuning range [3]. On semiconductive silicon substrates, electrical disturbances can travel between subcircuits on the die, impacting the VCO's noise performance. While a substrate voltage normally has a deleterious effect, in this case it is used to improve the performance. This paper details the effect of using a substrate voltage to tune an accumulation-mode MOS varactor. Section II covers the device design while Section III discusses the varactor's operation. In Section IV, measurements of a substrate voltage-tuned varactor are incorporated into a VCO design. Section V summarizes the results. Fig. 1. Cross section of the accumulation-mode MOS varactor.