The four-layer diode, also called the Shockley diode after its inventor William Shockley, is essentially a low-current SCR without a gate. It is classified as a diode because it has only two external terminals through anode and cathode. Because of its four doped regions it is often called a P-N-P-N diode. The basic structure, two transistor version, equivalent transistor circuit and schematic symbol of a Shockley diode are
The easiest way to understand how it operates is to visualize it to be formed of two transistors Qxand Q2 placed back to back, as shown in figure.b. Figure shows the diode split-up into two parts, displaced mechanically but connected electrically. The left half is a P-N-P transistor while the right half is an N-PN- transistor. It may be seen that N-type base region of P-N-P transistor forms the collector of N-P-N transistor while P-type base of N-P-N transistor forms the collector of P-N-P transistor. Thus the four-layer diode is equivalent to the latch shown in figure.