Varactor Diode

Varactor Diode is a reverse biased p-n junction diode, whose capacitance can be varied electrically. As a result these diodes are also referred to as varicaps, tuning diodes, voltage variable capacitor diodes, parametric diodes and variable capacitor diodes. It is well known that the operation of the p-n junction depends on the bias applied which can be either forward or reverse in characteristic. It is also observed that the span of the depletion region in the p-n junction decreases as the voltage increases in case of forward bias. On the other hand, the width of the depletion region is seen to increase with an increase in the applied voltage for the reverse bias scenario. Under such condition, the p-n junction can be considered to be analogous to a capacitor (Figure 1) where the p and n layers represent the two plates of the capacitor while the depletion region acts as a dielectric separating them.varactor diode analogous to a parallel plate capacitor Thus one can apply the formula used to compute the capacitance of a parallel plate capacitor even to the varactor diode.

Hence, mathematical expression for the capacitance of varactor diode is given by Where, Cj is the total capacitance of the junction. ε is the permittivity of the semiconductor material. A is the cross-sectional area of the junction. d is the width of the depletion region. Further the relationship between the capacitance and the reverse bias voltage is given as Where, Cj is the capacitance of the varactor diode. C is the capacitance of the varactor diode when unbiased. K is the constant, often considered to be 1. Vb is the barrier potential. VR is the applied reverse voltage. m is the material dependent constant. In addition, the electrical circuit equivalent of a varactor diode and its symbol are shown by Figure 2. This indicates that the maximum operating frequency of the circuit is dependent on the series resistance (Rs) and the diode capacitance, which can be mathematically given as In addition, the quality factor of the varactor diode is given by the equation Where, F and f represent the cut-off frequency and the operating frequency, respectively. varactor diode As a result, one can conclude that the capacitance of the varactor diode can be varied by varying the magnitude of the reverse bias voltage as it varies the width of the depletion region, d. Also it is evident from the capacitance equation that d is inversely proportional to C. This means that the junction capacitance of the varactor diode decreases with an increase in the depletion region width caused to due to an increase in the reverse bias voltage (VR), as shown by the graph in Figure 3. Meanwhile it is important to note that although all the diodes exhibit the similar property, varactor diodes are specially manufactured to achieve the objective. In other words varactor diodes are manufactured with an intention to obtain a definite C-V curve which can be accomplished by controlling the level of doping during the process of manufacture. Depending on this, varactor diodes can be classified into two types viz., abrupt varactor diodes and hyper-abrupt varactor diodes, depending on whether the p-n junction diode is linearly or non-linearly doped (respectively). characteristic curve of a varactor diode These varactor diodes are advantageous as they are compact in size, economical, reliable and less prone to noise when compared to other diodes. Hence, they are used in

  1. Tuning circuits to replace the old style variable capacitor tuning of FM radio
  2. Small remote control circuits
  3. Tank circuits of receiver or transmitter for auto-tuning as in case of TV
  4. Signal modulation and demodulation.
  5. Microwave frequency multipliers as a component of LC resonant circuit
  6. Very low noise microwave parametric amplifiers
  7. AFC circuits
  8. Adjusting bridge circuits
  9. Adjustable bandpass filters
  10. Voltage Controlled Oscillators (VCOs)
  11. RF phase shifters
  12. Frequency multipliers

Closely Related Articles Vacuum Diode History Working Principle and Types of Vacuum DiodePN Junction Diode and its CharacteristicsDiode | Working and Types of DiodeDiode CharacteristicsHalf Wave Diode RectifierFull Wave Diode RectifierDiode Bridge RectifierWhat is Zener Diode?Application of Zener DiodeLED or Light Emitting DiodePIN Photodiode | Avalanche PhotodiodeTunnel Diode and its ApplicationsGUNN DiodeLaser DiodeSchottky DiodePower DiodesDiode ResistanceDiode Current EquationIdeal DiodeReverse Recovery Time of DiodeDiode TestingMore Related Articles Op-amp | Working Principle of Op-ampAmplifier Gain | Decibel or dB GainIntegrated Circuits | Types of ICRegulated Power SupplyLaser | Types and Components of LaserWork FunctionMobility of Charge CarrierWhat are Photo Electrons? Electron volt or eVEnergy Quanta | Development of Quantum Physics Schottky EffectHeisenberg Uncertainty PrincipleSchrodinger Wave Equation and Wave FunctionCyclotron Basic Construction and Working PrincipleSinusoidal Wave SignalCommon Emitter AmplifierRC Coupled AmplifierDifferential AmplifierWave Particle Duality PrincipleSpace ChargeInverting AmplifierMOSFET | Working Principle of p-channel n-channel MOSFETMOSFET CircuitsMOS Capacitor | MOS Capacitance C V CurveApplications of MOSFETMOSFET as a SwitchMOSFET CharacteristicsPower MOSFETHalf Wave RectifiersFull Wave RectifiersBridge RectifiersClamping CircuitTheory of SemiconductorIntrinsic SemiconductorExtrinsic SemiconductorsEnergy Bands of SiliconDonor and Acceptor Impurities in Semiconductor Conductivity of SemiconductorCurrent Density in Metal and Semiconductor Intrinsic Silicon and Extrinsic SiliconP Type SemiconductorN Type SemiconductorP N Junction Theory Behind P N JunctionForward and Reverse Bias of P N JunctionZener BreakdownAvalanche BreakdownHall Effect Applications of Hall EffectGallium Arsenide SemiconductorSilicon SemiconductorTypes of TransistorsBipolar Junction Transistor or BJTBiasing of Bipolar Junction Transistor or BJTTransistor BiasingTransistor CharacteristicsCurrent Components in a TransistorTransistor Manufacturing TechniquesApplications of Bipolar Junction Transistor or BJT | History of BJTTransistor as a SwitchTransistor as an AmplifierJFET or Junction Field Effect Transistorn-channel JFET and p-channel JFETApplications of Field Effect TransistorDIAC Construction Operation and Applications of DIACTRIAC Construction Operation and Applications of TRIACPhototransistorNew Articles Ring CounterDischarging a CapacitorCharging a CapacitorElectric PotentialParity GeneratorElectric Flux