Both. From The Controlled Rectifier Volume 1 published by the International Rectifier Corporation, 1962, they mention of page 39 that it can be turned on by a rapid rise of forward voltage across the SCR. On page 40 they have a chapter about "TURN-ON BY GATE SIGNAL", where they describe "...a forward (gate positive) voltage and current to the gate lead." To turn a SCR off the forward voltage has to be reduced below some threshold.Are SCRs controlled by gate current or voltage?