▁di ode ▁designed ▁to ▁explo it ▁the ▁voltage - dependent ▁capac it ance ▁of ▁a ▁reverse - bi ased ▁p – n ▁j unction . ▁ ▁App lications ▁ ▁Var act ors ▁are ▁used ▁as ▁voltage - control led ▁capac itors . ▁They ▁are ▁commonly ▁used ▁in ▁voltage - control led ▁oscill ators , ▁paramet ric ▁ampl ifiers , ▁and ▁frequency ▁multip liers . ▁ ▁Vol t age - control led ▁oscill ators ▁have ▁many ▁applications ▁such ▁as ▁frequency ▁mod ulation ▁for ▁FM ▁transmit ters ▁and ▁phase - locked ▁loops . ▁Ph ase - locked ▁loops ▁are ▁used ▁for ▁the ▁frequency ▁synth es izers ▁that ▁t une ▁many ▁rad ios , ▁television ▁sets , ▁and ▁cell ular ▁tele ph ones . ▁ ▁The ▁var ic ap ▁was ▁developed ▁by ▁the ▁Pacific ▁Sem icon duct or ▁subs idi ary ▁of ▁the ▁Ram o ▁Wo old ridge ▁Corporation ▁who ▁received ▁a ▁pat ent ▁for ▁the ▁device ▁in ▁June ▁ 1 9 6 1 . ▁The ▁device ▁name ▁was ▁also ▁tra dem ark ed ▁as ▁the ▁" Var ic ap " ▁by ▁TR W ▁Sem icon duct ors , ▁the ▁successor ▁to ▁Pacific ▁Sem icon duct ors , ▁in ▁October ▁ 1 9 6 7 . ▁This ▁helps ▁explain ▁the ▁different ▁names ▁for ▁the ▁device ▁as ▁it ▁came ▁into ▁use . ▁ ▁Operation ▁▁ ▁Var act ors ▁are ▁operated ▁in ▁a ▁reverse - bi ased ▁state , ▁so ▁no ▁DC ▁current ▁flows ▁through ▁the ▁device . ▁ ▁The ▁amount ▁of ▁reverse ▁bias ▁controls ▁the ▁thick ness