Varactor diodes are two-terminal electronic components that are designed to provide voltage-controlled capacitance when operated under reverse bias. These simple semiconductors are PN junctions with a positive or P-region with positive ions and a negative or N-region with negative electrons. Applying voltage to the PN junction causes current to flow in only one direction as electrons from the N-region fill “holes” in the P-region. When the junction is reverse-biased, increasing the applied voltage causes the depletion region to widen, increasing the effective distance between the capacitor plates and decreasing the effective capacitance. By adjusting the doping gradient and junction width, the capacitance range can be controlled and changes applied using reverse voltage. A four-to-one capacitance range is not unusual. In fact, a typical varactor diode can vary from 60 pf at zero bias to 15 pf at 20 V. With careful manufacturing, however, the capacitance range can be increased to ten-to-one. Varactor diodes, which are sometimes referred to as varicap diodes, are used in electronic tuning systems to eliminate the need for moving parts.
Performance specifications for varactor diodes include reverse voltage, reverse current, capacitance ratio, terminal capacitance, junction operating temperature, and power dissipation. Reverse current or leakage current (IR), the current at which the specified reverse voltage is applied, measures the current that flows when reverse bias is applied to a semiconductor junction. Capacitance ratio (CDmin/CDmax) is the ratio of the minimum capacitance and the maximum capacitance. Smaller ratios indicate larger ranges of capacitance. Terminal capacitance or diode capacitance (CD) is the minimum capacitance when the specified reverse voltage and frequency are applied. Junction operating temperature (Tj) is the range of temperatures at which diode are designed to operate. Measured in watts (W), power dissipation (PD) is the maximum power dissipated by the diode while in its “on” state at a specified ambient temperature.
IC Package Types
IC package types for varactor diodes include transistor outline (TO), diode outline (DO), small outline transistor (SOT), and small outline diode (SOD). Other varactor diodes are available in a discrete package (DPAK) or in D2PAK, a large surface-mounted package that includes a heat sink. SC-59, SC-74, and SC-76 are plastic, surface-mounted IC packages with three leads. Metal electrode leadless face (MELF) diodes have metallized terminals at each end of a cylindrical body and are designed to fit the same footprints as flat components. QuadroMELF diodes have a square cross section to provide better on board stability and greater "pick and place" accuracy. MiniMELF is a miniature version of MELF and MicroMELF has the same footprint as the SOD110 and SOD323 packages.
Product Life Stages
Varactor diodes follow product life style stages that are defined by the Electronic Industries Alliance (EIA) in EIA-724. Numbered stages range from zero to eight and cover product introduction, growth, maturity, market saturation, phase out, last shipment, and removal. The first stage, Life Cycle Stage Code 0, describes varactor diodes that are in the planning or early design stages. The last stage, Life Cycle Stage Code 8, describes varactor diodes that are no longer stocked in inventory or available for sale.
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