PNPN diode :-
The Shockley diode or PNPN diode is a four layer (P-N-P-N), two terminals (namely anode and cathode) semiconductor switching device. It is also called as four layer diode. It functions like a normal diode without any trigger inputs, in reverse biased condition, no current flows through it and in forward biased condition current flows through it when the voltage across it is more than the break over voltage of it.
Shockley was the first semiconductor devices invented which was a “pnpn” diode. ( Types of Diode and symbol) It is equivalent to a thyristor without a gate. Shockley Diode is not widely because of of many alternate devices like Diac, Triac, SCR. These devices works on the principle of Shockley diode.
It is a two terminal device hence it is called as diodes. It is used as a switch, it operates in two states either ON or OFF. because of its construction Its also called PNPN diode. It is a two terminal device hence it is called as diodes.