A varactor is a nonlinear reactive device used for harmonic generation, parametric amplification, mixing, detection, and voltage-variable tuning. However, present applications of varactors are mostly for harmonic generation at millimeter and submillimeter wave frequencies, and as tuning elements in various microwave applications. Varactors normally exhibit a voltage-dependent capacitance and can be fabricated from a variety of semiconductor materials.

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Stake, J. (Jan), East, J. (Jack), Trew, R.J. (Robert J.), Cowles, J.C. (John C.), Liu, W. (William), MacEachern, L, … Dunleavy, L.P. (Lawrence P.). (2000). Active device technologies. In The RF and Microwave Handbook (pp. 7‐1–7‐203).