The thyristor is a type of diode with four layers of semiconducting material that will not let a current flow in either direction unless a voltage is applied to one of the layers, a layer of p type semiconductor known as the gate.
One type of thyristor is the silicon controlled rectifier. This will not conduct in the reverse direction and will only conduct in the forward direction when a voltage is applied to the gate, when it behaves like a normal diode unless the current falls below a current known as the holding current. A few tens of milliwatts of power applied to the gate of a silicon controlled rectifier can produce an output one million times bigger.