US 6,674,116 B1Granted

Variable capacitor using MOS gated diode with multiple segments to limit DC current

AssigneePericom Semiconductor Corp.

InventorMin Cao

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Abstract

A voltage-variable capacitor uses the channel-to-substrate junction from a gated diode formed from a metal-oxide-semiconductor transistor. The transistor gate has at least two contacts that are biased to different voltages. The gate acts as a resistor with current flowing from an upper gate contact to a lower gate contact. The gate-to-source voltage varies as a function of the position. A critical voltage is where the gate-to-source voltage is equal to the transistor threshold. A portion of the gate that has gate voltages above the critical voltage has an inversion layer or conducting channel under the gate. Another portion of the gate has gate voltages below the critical voltage, and thus no channel forms. By varying either the gate voltages or the source voltage, the area of the gate that has a channel under it is varied, varying the channel-to-substrate capacitance. Separate gate arms reduce bias current.

Claims8

Dependent claims

A varactor comprising:

a metal-oxide-semiconductor (MOS) gated diode having a gate, a source, and a substrate;
wherein the gate also comprises a gate resistor between an upper-gate contact and a lower-gate contact;
an upper voltage applied to the upper-gate contact;
a lower voltage applied to the lower-gate contact;
wherein the upper voltage and the lower voltage are not equal, causing a gate current to flow through the gate resistor and producing a voltage gradient along the gate wherein gate-to-source voltages vary along the voltage gradient;
a first capacitor terminal, connected to the source and having a source voltage, the source voltage sufficient to cause formation of an inversion layer under a first portion of the gate but insufficient to cause formation of an inversion layer under all of the gate;
wherein the inversion layer is electrically coupled to the source;
a second capacitor terminal connected to a substrate under the inversion layer;
and
a drain alongside the gate, and coupled to the first capacitor terminal;
wherein the drain and source and both biased to the source voltage,
whereby capacitance between the first and second capacitor terminals is varied by enlarging or reducing an area of the first portion.

9 more elements

Dependencies are inferred from the claim text.