▁is ▁still ▁used ▁to ▁generate ▁much ▁higher ▁frequencies , ▁in ▁the ▁ 1 0 0   G Hz ▁– ▁ 1   TH z ▁range , ▁where ▁even ▁the ▁fast est ▁Ga As ▁trans ist ors ▁are ▁still ▁in ade qu ate . ▁ ▁Sub st it utes ▁for ▁var ic ap ▁di odes ▁All ▁sem icon duct or ▁j unction ▁devices ▁exhib it ▁the ▁effect , ▁so ▁they ▁can ▁be ▁used ▁as ▁var ic aps , ▁but ▁their ▁characteristics ▁will ▁not ▁be ▁controlled ▁and ▁can ▁vary ▁widely ▁between ▁batch es . ▁ ▁Popular ▁makes h ift ▁var ic aps ▁include ▁LED s , ▁ 1 N 4 0 0 X ▁series ▁rect ifier ▁di odes , ▁Sch ott ky ▁rect ifiers ▁and ▁various ▁trans ist ors ▁used ▁with ▁their ▁collect or - base ▁j unction s ▁reverse ▁bi ased , ▁particularly ▁the ▁ 2 N 2 2 2 2 ▁and ▁BC 5 4 7 . ▁Re verse ▁bias ing ▁the ▁em itter - base ▁j unction s ▁of ▁trans ist ors ▁also ▁is ▁quite ▁effective ▁as ▁long ▁as ▁the ▁AC ▁amplitude ▁remains ▁small . ▁Max imum ▁reverse ▁bias ▁voltage ▁is ▁usually ▁between ▁ 5 ▁and ▁ 7 ▁Vol ts , ▁before ▁the ▁a val anche ▁process ▁starts ▁conduct ing . ▁High er - current ▁devices ▁with ▁greater ▁j unction ▁area ▁tend ▁to ▁possess ▁higher ▁capac it ance . ▁The ▁Phili ps ▁B A   1 0 2 ▁var ic ap ▁and ▁a ▁common ▁z ener ▁di ode , ▁the ▁ 1 N