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Punchthrough Voltage Analyses and its Effect on Bipolar Device Performance

The paper deals with boundary case of punchthrough effect when emitter-base and collector-base depletion area edges on the base side are close to each other. The bipolar devices with uniform doping profile and step junctions are analyzed. Both non punchthrough case and punchthrough case are numerically simulated and the results are compared with analytical equations. Punchthrough effect decreases the maximal value of potential in the base and thus, lowers the potential barrier for the emitter electrons injected toward collector. The effect is greater at lower base-emitter voltages, because the emitter-base depletion area width is higher at lower base-emitter voltages. At higher base-emitter voltages emitter-base depletion area is decreased and punchthrough case tends to non punchthrough case. The decrease of potential barrier results in increase of emitter and thus, collector current. As the effect occurs just at lower base-emitter voltages the collector current still depends on base-emitter voltage and bipolar transistor can be used for some digital applications where there is a need for lower base width devices.

bipolar device; punchthrough; collector current; numerical simulation

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