May 1988

Document Type


Degree Name



Dept. of Electrical Engineering


Oregon Graduate Center


Presented herein are the results of an experimental and theoretical investigation of the factors influencing the current gain of a polysilicon-emitter- contacted bipolar transistor (PEC transistor). Specifically, the temperature behavior of gain and its optimization are comprehensively discussed. The results show that the PEC transistor exhibits a stronger temperature dependence, when compared with conventional transistors. This is attributed to the diffusion length of minority carriers (holes) in the polysilicon. The current gain, hfe for a common emitter configuration at low frequencies has been extensively modeled. The analysis reveals that the mobility and the recombination lifetime of the minority carriers in the polysilicon are the key parameters for optimizing hfe in PEC transistors. Electrical measurements were carried out on PEC transistors, with no intentional oxide layer between the polysilicon contact and the monosilicon emitter, over a temperature range from 25°C to 165°C. This data was analyzed by assuming that the increase in current gain is due only to the decelerated transport of the minority carriers in the polysilicon contact. With this assumption a two region model is used to derive a general equation for hfe, from which the temperature dependence and the properties for optimizing current gain were examined. It is shown that the increased temperature dependence in PEC transistors over conventional transistors can be accounted for by the temperature dependence of the hole diffusion length in the n+ polysilicon. Examination of the polysilicon dependent term of hfe uncovered the minority carrier, mobility and recombination lifetime as the parameters responsible for enhanced current gain.





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