Variable Capacitance Diode

Variable Capacitance Diode (Varicap Diode, also called Varactor Diode) – a special type of semiconductor diode. It is designed to exploit the changes in voltage-dependent capacitance of a reverse-biased “p-n junction”, therefore its ability to use junction barrier capacity has been improved. In other words, Varicap can be described as a voltage-controlled capacitor. 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. 

Fig. 1. Variable capacitance diode symbol

Variable Capacitance Diode – Component division

• Varicap (Variable Capacitance) – low power component, characterized by capacity varying from 10-500pF, 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 20pF. They are mainly used in high frequency devices, e.g. to reproduce frequencies.

Variable Capacitance Diode – Limiting properties

Technical properties and permissible limits for the varicaps:

• C_j – junction diode capacitance at reverse bias and reverse frequency (usually at a voltage value close to maximum),
• R_s – series diode resistance or its Q value at a determined reverse bias and reverse frequency,
• V_Rmax – the maximum constant reverse voltage,
• V_RMmax – the maximum peak constant reverse voltage,
• I_Fmax – the maximum constant bias,
• T_j – permissible junction temperature.

Technical properties and permissible limits for the varactors:

• C_j – junction diode capacitance at determined reverse bias and reverse frequency (usually for UR = 0, so for maximum capacity),
• I_R – reverse current at determined UR reverse voltage (usually at UR -Urmax)
• Maximum frequency at a given reverse voltage,
• V_Rmax – the maximum constant reverse voltage,
• P_max – the maximum value of dissipation power,
• T_j – permissible junction temperature.

Variable Capacitance Diode – Characteristics

Varicap characteristics are shown on Fig. 2. below.

Fig. 2. Variable Capacitance Diode characteristics

Variable Capacitance Diode – Applications

Varicaps are most commonly used in parametric amplifiers, voltage-controlled oscillators or frequency multipliers.