Interpretation of breakdown results in Ge diodes is frequently complicated by effects associated with surface excess current. When these effects are minimized, breakdown is observed within the junction at a ``breakdown center,'' starting at a definite voltage VB, and is accompanied by the onset of microplasma pulses. In any one diode, there may be a number of centers, each having its characteristic value of VB and producing characteristic microplasma pulses. The minimum value of VB determines the breakdown voltage of the diode and it is possible to increase the latter radically by etching away centers having lower values of VB.

Observations were made at temperatures from −253° to 27°C on Ge alloy junctions (n‐type base resistivities from 0.54 to 5.4 Ω‐cm). The properties of the pulses are discussed in some detail: the effect of raising the voltage above VB, the effect of light, and the temperature coefficient of VB. Values of the last are sufficiently high to suggest that suitable diodes can be used as cryogenic thermometers capable of reading smaller changes than 0.01°C at −253°C.

Various aspects of the microplasma breakdown are discussed: the mechanism for triggering a pulse and that for ``turning it off,'' the role of the spreading resistance, the possible role of a negative resistance at breakdown, and the effect of microplasma breakdown on the measurement of carrier multiplication at voltages in the vicinity of breakdown.

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