Variable capacitance diode

Variable capacitance diode (Varicap diode also called Varactor diode) – a type of semiconductor diode with a special construction. Its ability to use junction barrier capacity has been improved. Capacitance of “p-n diode junction” is changed depending on changes in the voltage applied in the reverse-bias. Varicap diodes are mostly manufactured from Silicon (Si) and Gallium Arsenide (GaAs). With its manufacture it is strived to achieve the highest value of voltage sensitivity (re-tuning diode factor).

Its capacitance can change in a narrow range, usually from about 6 pF (picofarads) to 20 pF with the voltage change from 2 to 20 V. The main parameters describing the varicap diode are its two limiting capacities: C_tmin – minimum capacity and C_tmax – maximum capacity. As mentioned above, these parameters are dependent on voltage. At high voltage, capacity has minimal value, whereas at small voltage – maximum. 

There are two variants of capacitance diode:

• Varicap (Variable Capacitance) – low power component, characterized by capacity varying from 10-500 pF, used specially in automatic tuning systems of resonant circuits, usually in amplifiers systems,
• Varactor (Variable Reactance) – high power component, characterized by capacity varying from 0.2 to 20 pF. They are mainly used in high frequency devices, e.g. to reproduce frequencies.

Technical properties and permissible limits for the varicap:

• Cj – junction diode capacitance at reverse bias and reverse frequency (usually at a voltage value close to maximum),
• Rs – series diode resistance or its Q value at a determined reverse bias and reverse frequency,
• U_Rmax – the maximum constant reverse voltage,
• U_RMmax – the maximum peak constant reverse voltage,
• I_Fmax – the maximum constant bias,
• Tj – permissible junction temperature.

Technical properties and permissible limits for the varactors:

• Cj – junction diode capacitance at determined reverse bias and reverse frequency (usually for UR = 0, so for maximum capacity),
• I_R – reverse current at determined U_R reverse voltage (usually at U_R -U_rmax)
• The maximum frequency at a given reverse voltage,
• U_Rmax – the maximum constant reverse voltage,
• P_MAX – the maximum value of dissipation power,
• Tj – permissible junction temperature or ambient temperature.