Shockley diodes are very similar to tunnel diodes in that they have a negative resistance characteristic. They are actually classified as a thyristor because they operate in only two states, on or off. The Shockley diode is constructed by joining four semiconductor layers together to form a PNPN junction, Figure 1. The Shockley diode exhibits a very high resistance, like that of an open switch, until the voltage across it reaches a point called the breakover voltage. When breakover voltage is reached, the diode suddenly exhibits a very low resistance and begins to conduct, as in Figure 2. Once the Shockley diode begins to conduct , the voltage across it will suddenly drop to about 1 volt. Once conduction begins, it will continue to conduct until the current falls below a certain level called the holding current level.
The negative resistance characteristic permits the Shockley diode to be used as a simple relaxation oscillator, Figure 3. In this example circuit, the battery voltage is greater than the breakover voltage for the Shockley diode, but the value of the series resistor is high enough to limit the maximum circuit current to a value that is below the diode's holding current level. When switch S1 is closed, the capacitor will begin to charge through the series resistor. When the voltage across the capacitor reaches the breakover value for the Shockley diode, it will turn on and discharge the capacitor. When the current level falls below the holding current value, the diode will turn off and the capacitor will begin charging again.
