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Space-charge-limited currents in germanium Nichol, Dennis William


A study has been made of space-charge-limited hole flow in germanium by investigating the current-voltage-temperature characteristics of selected p-n-p transistors used as diodes with the base open-circuited. These transistors were selected so as to minimize the effect of avalanche multiplication. These diodes pass hole current through the base after a voltage designated as the punchthrough voltage has been applied to deplete the n type base of electrons. The resulting space-charge-limited current above punchthrough has been closely studied and also its temperature dependence. To explain the form of these characteristics, published data have been used for the relationship between electric field and carrier drift velocity for holes in germanium in order to consider the hole flow through the high field region of the base. It was further found necessary to consider the variation of effective emitting area as the applied voltage is increased past punchthrough. For high applied voltages and hence high applied fields in the base, a constant differential resistance is obtained of magnitude about equal to that expected theoretically for a constant drift velocity of holes in the base. The temperature dependence of this current can be satisfactorily explained by the temperature variation of the base-generated current and of the punchthrough voltage itself. A satisfactory model of the latter variation has been made by considering the temperature variation of the potential barrier at the emitter junction. If the observed characteristics are corrected for these variations, there is found to be negligible variation of the space-charge-limited hole -flow. Capacitance measurements were made on both junctions for these diodes. From these measurements the assumption of a step junction and of uniform impurity distribution in the base were justified. No abrupt change of capacitance was observed as the voltage was increased through the punchthrough voltage contrary to the findings of Barker. The original theories of Shockley-Prim-Dacey were extended to include the effects of (i) mobility variation over a wide range of-fields (ii) non-planar geometry and (iii) the potential distribution near the emitter junction.

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