Variation of Junction Breakdown Voltage by Charge Trapping

Abstract

A variation of the avalanche breakdown voltage $V_b$ resulting from charge trapping as predicted by Shockley was experimentally verified for silicon $p-n$ junctions. The breakdown voltage of a $p-n$ junction is determined by the space-charge density within the depletion layer, e.g., for the case of an $n^{+}p$ step junction, by the density of ionized acceptors and ionized traps. Consequently, any variation in the amount of trapped charge resulting either from carrier capture or impact ionization during breakdown will lead to a corresponding variation of $V_b$. If the sign of the charge trapped during breakdown is such as to decrease the space-charge density, the space-charge layer widens and $V_b$ is increased. If, however, the trapped charge increases the space-charge density, then the breakdown voltage is lowered and a "lock-on" mechanism prevents random on-off fluctuations. Both effects were observed in silicon $p-n$ junctions at -196°C. A detailed analysis of this effect for the case of small uniform avalanche diodes with known dimensions allows the determination of trap density, integrated capture cross section for hot holes, and average capture time from the experimental results.