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Variable capacitor (varactor)
Monday, 08 January 2007
Capacitors are one of the three basic electronic elements along with resistors and inductors that make up all passive electrical circuits. Capacitors are circuit devices for storing electric energy, which are almost essentially employed in diverse electric and electronic circuits. Such circuits usually utilize a great number and various types of capacitors, especially the circuits associated with frequency domain signal or AC signals power supply device and filtering circuits. Capacitors have two main types, fixed and variable. A fixed capacitor has a pre-defined capacitance that is fixed during manufacturing. Variable capacitors are devices in which a change in a control voltage charge or current causes a change in capacitance. Variable capacitors do not have their capacitance fixed during manufacturing. Instead, these capacitors are designed so that they afford a range of capacitance values. Numerous capacitors are applied in filtering circuits, power supplying devices, and wireless communication circuits. The capacitors serve as vital elements with the critical specifications on their accuracy and performance. Along with the development of wireless communication through digital and analog signals, capacitors applied in filtering circuits, power supply, and wireless communication device and circuits are vital elements, with the critical specifications on the performance of capacitors. Variable capacitors or varactors play a fundamental role in high-frequency and radio-frequency (RF) circuits. Variable capacitors are important elements of a variety of electrical circuits including variable-frequency oscillators, tuned amplifiers, parametric amplifiers, phase shifters, equalizers, and impedance-matching circuits. Variable capacitors are used in RF power match networks to match the impedance of the electrode and the plasma, constituting an electrical load, to the impedance of a source which delivers RF power to the plasma. Variable capacitors are widely used in RF communication applications for low-noise paramteric amplifiers, harmonic frequency generators, and frequency controllers such as voltage-controlled oscillators. In a mobile communication system, a radio frequency (RF) block is designed to support many frequency bandwidths. A variable capacitor having a different capacitance for each frequency bandwidth should be used as a capacitor used in a filter having a direct relation to a frequency bandwidth. In addition, a voltage controlled oscillator (VCO) adjusts a voltage applied to a variable capacitor to vary the capacitance thereof, and changes a resonance frequency thereby. In this way, the variable capacitor is a very important device for a tunable filter or a VCO for the RF block.

A voltage variable capacitor, also known as a varactor, a variable capacitance diode or varacap, is a semiconductor device characterized by voltage sensitive capacitance which resides in the space charge region at the surface of a semiconductor bounded by an insulating layer. A typical capacitor has a pair of electrode layers with a dielectric material therebetween. Voltage is applied across the electrode layers in order to store a charge in the capacitor, with the amount of charge being storable in the capacitor being proportional to the opposing areas of the electrodes and to the dielectric constant of the dielectric material. A variable capacitor includes an interelectrode spacing between a pair of electrodes that can be controllably varied in order to selectively vary the capacitance between the electrodes. Variable capacitors commonly employ moveable plates that vary the capacitance by varying the amount of overlap of adjacent parallel plates or the distance separating two opposed parallel plates. The variable capacitor operates by applying a voltage across the first electrode and the movable plate so that the plate is deflected towards the first electrode by electrostatic attraction. As the movable plate moves, the spacing between the second electrode and the movable plate changes, thus changing the capacitance value between the second electrode and the plate. In order to form a high performance voltage variable capacitor a dielectric film having a very thin cross section and an extremely high integrity is required to be deposited on a semi conductive substrate. A variable capacitor can be made by including several capacitors in parallel on the integrated circuit (IC) chip. Conventional semiconductor manufacturing processes are unable to form a variable capacitor within the integrated circuits. Advances in thin film technology have enabled the development of sophisticated integrated circuits. This advanced semiconductor technology has also been leveraged to create MEMS (micro-electromechanical system) structures. MEMS structures are typically capable of motion or applying force. MEMS devices are being developed for a wide variety of applications because they provide the advantages of low cost, high reliability and extremely small size. Using MEMS technology, variable capacitors can be realized by various silicon IC-compatible, micron-sized electromechanical structures. MEMS variable capacitors have been fabricated that include a movable, conductive plate that is suspended above first and second coplanar electrodes. MEMS capacitors offer better range and other advantages. MEMS capacitors can achieve higher quality factor compared to CMOS capacitors. Integration of MEMS capacitors provides lower interconnection and parasitic related losses. In addition, the MEMS capacitors can reduce complexity due to monolithic integration. Variable capacitors using MEM technology can be easily implemented in standard semiconductor devices for applications in aerospace, consumer electronics and communications systems.

One well known implementation of the variable capacitor is the varactor, typically realized as a p-n junction diode. A varactor is a nonlinear device that provides a capacitance that varies responsive to an applied control voltage. The capacitance of a varactor, when within its operating parameters, decreases as a voltage applied to the device increases. A diode-type of device is often used as a varactor. 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. The diode is used for a rectification circuit or the like by utilizing that an electric current flows when a bias is applied to a PN junction plane in a forward direction. The PN junction plane has a region referred to as a depletion layer in which either electrons or holes do not exist. The diode junction is commonly used to form a varactor in an integrated circuit (IC), since the depletion region of the diode acts as a capacitor dielectric between two capacitor plates. Semiconductor material in the depletion region is typically doped either uniformly or decreasing with depth. When a reverse bias is applied to the diode, the depletion layer becomes thicker because both the electrons and the holes are pulled in a direction away from the PN junction plane, and the thickness of the depletion layer changes depending on the magnitude of the reverse bias. A transistor can also be operated as a varactor. By reverse biasing a junction of a transistor, the capacitance of the junction decreases as with the increasing reverse voltage. Varactors are widely used in electrical and electronic circuits in a variety of applications, including voltage-variable tuned circuits, voltage controlled oscillators, phase shifters, frequency multipliers, etc. Varactors are commonly used in the voltage-controlled oscillator (VCO) circuits of phase-locked loops (PLLs) which, in turn, are commonly used in high-frequency applications, such as with cellular phones. The varactor diode can be supplied at a low price because of a structure suitable for mass production using a semiconductor process, and furthermore, a variable amount of capacitance is large.