Transistor equivalent circuits

This paper surveys the history of the electric-circuit representation of the transistor over the past fifty years. During the first two decades after the transistor was announced in 1948, primary emphasis was on small-signal equivalent circuits, which could be used for linear-circuit analysis and design. In addition, parameters of many of these equivalent circuits for the bipolar junction transistor, which are described, were related to the physical construction of the device. Approximately two-thirds of the paper is devoted to this period, when the writer personally contributed to this effort. By the beginning of the third decade, transistor circuits had became more complex, and circuit analysis was carried out with the help of digital computers. Interest then shifted away from small-signal equivalent circuits to “models” for computer-aided circuit design (CACD). This transition, including the models used in the widely used CACD program SPICE, is described. MOS transistors are treated only briefly; by the time MOS transistors became commercially viable devices, emphasis then also had shifted to “models” for CACD. In conclusion, the writer notes that there is still hope for us aficionados of small-signal equivalent circuits; new types of transistors are still being characterized in this manner!

I. INTRODUCTION

When I was first invited to prepare a paper on transistor equivalent circuits for this transistor anniversary issue, I

was delighted—after all, that topic played an important part in my professional life for nearly a decade in the mid-1950’s. Then, on further reflection, I recognized that transistor equivalent circuits do not play nearly so significant a role today as they did in that time period because of changes in transistor and computer technology over the intervening years. So why should we write about equivalent circuits? In addressing that question, I conclude that a historical review of the changing role of the equivalent circuit over these five decades—as presented here—should make an interesting paper for our younger readers, and perhaps provide a bit of nostalgia for the rest of us.

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