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 shown in figure.
The easiest way to understand how it operates is to visualize it to be formed of two transistors Qx and 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.
Because there are no trigger inputs, the only way to switch the device on is to increase the anode-to-cathode voltage VAK to the forward switching voltage, and the only way to open it is by low current drop out. With a four layer diode it is not necessary to reduce the current all the way to zero to open the latch. The internal transistors of the device will come out of saturation when the current is reduced to a low value, called the holding current. The forward switching voltage Vs is equivalent of the SCR forward breakover voltage, and the minimum current at which device will switch on is the switching current IS.
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