David Prutchi has published a detailed study on the Shockley 4-layer diode, a largely forgotten two-terminal NPNP semiconductor device invented by William Shockley. The device functions as a voltage-controlled switch, activating when voltage across it reaches a threshold via avalanche breakdown triggering a complementary transistor pair. Historical applications included relaxation oscillators, switches, and logic circuits built with minimal components — valuable when semiconductors were expensive. Though superseded by more versatile devices like the PUJT, silicon-controlled switch, and eventually ICs, its thyristor cousin remains in use today. Prutchi's investigation was sparked by finding the diode used in an early heart pacemaker circuit.
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