Structure-Determined Gain-Band Product of Junction Triode Transistors

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Abstract
This paper discusses some fundamental frequency limitations of the junction triode. It also describes briefly practical accomplishments with germanium diffused base transistors of the type reported by Lee. Finally, the frequency limitations of the junction triode are compared with those of the field effect transistor and the analog transistor. For transistors of the mesa type with linear emitter and base electrodes (i.e., an emitter stripe with parallel base contact stripes a fraction of the emitter width distant on each side), the (power gain)½(bandwidth) product is found to be about 7.5 × 106/s cps where s is emitter stripe width in centimeters. This is an order of magnitude better than the corresponding figures for field effect and analog transistors. For operation at or below the alpha cutoff fre quency, the gain-band product is shown to be nearly independent of the particular alpha cutoff frequency selected. This independence arises from the reciprocity between collector depletion layer capaci tance per unit area and collector depletion layer transit time. This transit time effectively determines alpha cutoff frequency in optimunm gain-band designs.