A varactor is a nonlinear device that provides a capacitance that varies responsive to an applied control voltage. A varactor is, essentially, a variable voltage capacitor. The capacitance of a varactor, when within its operating parameters, decreases as a voltage applied to the device increases. Variable reactors (varactors) are essential for the design of key radio frequency (RF) CMOS and BiCMOS circuits, and are specifically used as tuning elements in voltage controlled oscillators (VCOs), phase shifters, and frequency multipliers. Oscillator circuits, including voltage controlled oscillator circuits, are well known. Typically, an oscillator circuit includes at least one active device and a resonator circuit coupled to the active device. The resonator is coupled to the active device such that the active device is capable of oscillating only at a specific frequency determined by the resonator. A voltage controlled oscillator is an oscillator that generates an output signal with a frequency that varies in response to an input control voltage. In a VCO, the frequency of oscillation is controlled by electronically altering the point of resonance of the resonator circuit. Typically, a variable resonator circuit includes at least one variable capacitance diode (varactor). The frequency of operation is set by applying a bias voltage to the varactor to alter the capacitance of the varactor diode. This change in capacitance alters the net reactance of the resonator and alters the frequency of operation of the oscillator circuit. Varactors often comprise multiple voltage-tunable capacitor cells, with each cell having a capacitive range, wherein the net capacitive range of the varactor is substantially equal to a sum of the capacitive ranges of the individual capacitor cells. Varactor diodes are operated in a reverse-bias operating mode. When the diode is reverse-biased, the capacitance of the pn junction decreases with an increasing reverse voltage. An important characteristic of the varactor diode is that the diode presents a capacitance which varies as a result of applying a variable bias voltage to a depletion region of the diode.
INTRODUCTION to Varactors in CMOS VCO
varactors are most commonly used LC-tank voltage controlled oscillators inside the synthesizers. This passive components should feature a good Q-factor and, perhaps more importantly, should enable an electrically controlled tuning sufficient to compensate the technology spreads. Inductors integrated on Si-technology, though featuring poor s up to now, are already widely used. The tuning element, commonly used in these applications, is a reverse biased p–n junction whose capacitance value is controlled by the reverse voltage. The technology scaling lowers the maximum circuit supply voltage and the maximum usable diode reverse voltage. For example, the maximum supply for the 0.35-, 0.25-, and 0.18- um technologies are, respectively, 3.3, 2.5, and 1.8 V. Typically, the achievable frequency tuning range of filters, in which p–n junctions varactors are used, is less than +/- 10% of the center value, if a control voltage smaller than 2 V is used. This is, however, not enough to compensate for the capacitance and inductance variations with respect to their nominal values. The basic idea in this work is to tune the capacitance of a metal–oxide–semiconductor (MOS) structure, by changing the operation from accumulation to depletion . With the technology scaling, the oxide thickness is reduced and correspondingly the oxide capacitance is increased. On the other hand, the value of the depletion capacitance underneath the gate, for a given biasing condition, increases at a lower rate. This means that the tuning range is expected, to a first order, to increase with scaling. Moreover, scaled technologies enable to realize MOS varactors with better quality factors because the parasitic resistance scales with the channel length.
A physical model of a CMOS varactor with high capacitance tuning range in multi-finger layout is presented. The model describes the voltage dependent capacitances and resistances and the parasitics due to connecting wires. Simulations of frequency tuning of a LC-oscillator employing the varactor are verified against measurements. More details at Free research paper on cmos varactor modelling THE MOS VARACTOR
The cross section of the proposed device is shown in Fig. 1. A metal oxide structure is built on top of an n-well diffusion. The gate and the two n contacts inside the n-well are the controlling electrodes. The device capacitance is given by Ccap= CWL
where 1/C=inverse of Cox + inverse of Csi
in which cox and Csi are, respectively, the oxide capacitance and the capacitance of the depletion layer under the gate, per unit area. By applying a positive voltage between the gate and the n-well the surface is accumulated and the device capacitance equals the oxide capacitance. If the applied voltage is reversed, the surface layer is depleted and the series capacitance decreases. The maximum capacitance, per unit area, of the device corresponds to a heavily accumulated surface and equalsCox = epsilon/tox On the other side, a minimum value (Cdmin) is reached when the voltage difference between the electrodes equals the threshold voltage. Beyond this point, an inversion layer is formed under the gate. At low frequency this effect brings the value of the device capacitance close to the oxide one. At high frequency, where the varactor is assumed to be operated, this effect is not seen and the capacitance remains at its minimum value. The ratio between Cox and Cdmin defines the tuning range. As the technology scales Cox, scales inversely with the technology feature, while Cdmin shows a relatively small increase. This means that scaled technologies should enable a wider tuning range. As an example, going from 0.5- to 0.35- m CMOS technologies, the oxide thickness changes from 12 to 7 nm and the oxide capacitance increases by a factor of about 1.7. For a constant substrate doping level a corresponding increase in the tuning range would result. In reality, a lower increase is obtained due to both an increase in the doping level of the substrate and to parasitic effects that scale slower than the technology feature. Nonetheless, a tuning range increase of 1.3 is obtained.
A 80GHz voltage controlled oscillator is presented. A passive impedance transformation enables the transformation of a varactor capacitance into negative capacitance varactor is proposed, partially canceling the capacitance of the differential pair. This enables the transmission line resonator to be longer and have a higher impedance. Design criterion for achieving negative varactor capacitance is also presented, and design issues of millimeter wave voltage controlled oscillators are discussed. The voltage controlled oscillator is fabricated in 90nm CMOS technology and achieves a phase noise of When a reverse voltage is applid to a PN junction , the holes in the p-region are attracted to the anode terminal and electrons in the n-region are attracted to the cathode terminal creating a region where there is little current.This region , the depletion region, is essentially devoid of carriers and behaves as the dielectric of a capacitorUnderstanding the varactor operation