Effects of collector epitaxial layer on the switching speed of high-performance bipolar transistors

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The importance of the epitaxial layer in designing the scaled-down high-performance bipolar transistors is well accepted, but its effect on the performance of the conventional switching transistors for logic application is not quite observable, because it is usually overwhelmed by the large parasitics of these devices. For self-aligned transistors having very small parasitics, its effect becomes so significant that it is observable in the power-delay characteristics of the switching circuits. We built self-aligned transistors on epitaxial layers of different thicknesses. The measured switching speed of transistors with thicker epitaxial-collector layers is faster at lower currents due to the lower collector capacitance; but it is slower at higher currents due to the base stretching. The present experimental results provide direct confirmation of the design theory [1].