Varactor Diode Capacitance

Formula

Description

A varactor (varicap) diode has a junction capacitance that varies with the applied reverse bias voltage. As reverse voltage increases, the depletion region widens, reducing capacitance. Cj0 is the zero-bias junction capacitance, Vbi is the built-in junction potential (about 0.7 V for silicon), and m is the grading coefficient (0.5 for an abrupt junction, 0.33 for a linearly graded junction, up to 2 for hyperabrupt varactors). Varactors are used in voltage-controlled oscillators (VCOs), phase-locked loops (PLLs), and frequency synthesizers.

Variables

• C_j0 — Zero-bias junction capacitance (F)
• V — Applied reverse bias voltage (V, positive value)
• V_bi — Built-in junction potential (V, typically 0.6-0.8 for Si)
• m — Grading coefficient (0.3-2, dimensionless)

Practical Notes

Hyperabrupt varactors (m ≈ 1-2) provide a more linear frequency vs. voltage characteristic in VCO applications. Typical tuning ratios: abrupt junction 3:1, hyperabrupt 10:1 or more. The Q factor of varactors decreases at higher frequencies due to series resistance. For best Q, use the lowest reverse voltage that achieves the desired capacitance. All pn junction diodes exhibit this effect; varactors are simply optimized for it.