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On a printed wiring board, a guard ring is a trace that surrounds a sensitive via or device pin. It is used to intercept leakage currents, or to reduce capacitive coupling.
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In semiconductors, N type material tends to accumulate holes on the interface between th N type material and the silcon dioxide passivation. A guard ring connected to ground or a negative voltage tends to sweep away the holes, thus preventing the N type material from becoming P type material on its surface. To the best of my knowledge, virtually all PNP transistors use guard rings. The use of a guard ring int the manufacture of PNP transistors was introduced by Motorola Semiconductor in the early 60's. This was a major breakthrough in increasing the yield of PNP transistors.
Regards,
Kral
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