Shockley Diode 

A Shockley diode is a PNPN device having two terminals as shown in fig. This device acts as a switch and consist of four alternate P-type and N-tpye layers in a single crystal.

WORKING OF SHOCKLEY DIODE
That Shockely diode behaves like a switch. So long as the forward voltage is less than breakover voltage, Shockley diode offers very high resistance (i.e., Switch is open ) and practically conducts no current.

At voltages above the break-over value, Shockley diode presents a very low resistance (i.e., Switch is closed ) and Shockley diode conducts heavily. 
Note :- It may  be noted that Shockley diode is also known as PNPN diode or four layer diode or reverse-blocking diode thyristor