ANS:D - Varactor diode

Gunn diode, Tunnel diode and Impatt diodes are negative resistance devices. The limitation of solid state devices at high frequencies include those associated with transit time and junction capacitances. The devices used are : Transferred electron oscillators (Gunn diode), Avalanche diode oscillators (Impatt diode, Trapatt diode, Masters, Lasers, Tunnel diode, Varactor etc).