Current gain in bipolar transistors with a field plate over the base surface

Abstract

Vertical n-p-n and lateral p-n-p transistor structures of an integrated circuit are studied using an electrolytic tank analogue and it is shown that the presence of a proper field plate extending from the collector-base junction over most of the base surface will improve the current-gain factor considerably. Experimental results of the analogue study, simulating typical carrier lifetimes and typical overall dimensions, are presented with various geometrical dimensions as parameters. Recently, a 2-dimensional electrolytic tank analogue 1 simu- lating volume recombination in the base region of bipolar transistors has been reported. The studies 1 ' 2 have shown that the current gain of transistors is limited by the excess stored charge in the transverse region of the base and also by the transverse diffusion of minority carriers into the base contact. This simulation study has also been adopted for the study of lateral p-n-p transistors 3 and it has been shown that the current gain of these transistors can be increased by providing a gap in the n + buried layer. The purpose of this paper is to study the effect of a field plate over the base surface and extending from the collector- base junction edge, on the current gain of vertical n-p-n transistors of integrated circuits, using an electrolytic tank analogue similar to the one discussed earlier. 1 3 It is shown that a field plate (i.e. m.o.s structure), spanning the major portion of the base surface (see Fig. la) and biased such that the region below is depleted, reduces the stored charge in the transverse region of vertical n-p-n transistors. The presence of this depletion region extending up to the collector-base junction, as shown in Fig. la, aids in improv- ing the collection of carriers injected in the transverse direction. Thus an improvement in current gain is achieved by reducing the recombination current and aiding the collection of carriers in the transverse region. In addition to the above, it is shown that a similar field-plate structure employed in lateral p-n-p transistors of integrated circuits would considerably improve their current gain. Sah 4 has given a detailed account of the effects of a field plate over the emitter-base junction of a vertical transistor and Grove 5 has discussed these results. Experimental measurements 6 on a specially designed, fabricated trans- istor with a field plate over the emitter-base junction for studying the surface effect on the current gain have shown that the base current and hence the current gain of the transistor depends upon the bias voltage on the metal electrode over the oxide protecting the emitter-base junc- tion. The present study of vertical n-p-n transistors is different from the above studies and the field plate extends from the collector-base junction over the major portion of the base surface. It may be noted that in the present study the field plate does not extend over the emitter-base junction (Fig. la) and hence the recombination effects discussed earlier 6 are not present in the proposed structure.