Current Gain Of Bipolar Transistors Research Articles

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

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.

Auger recombination in heavily doped shallow-emitter silicon p-n-junction solar cells, diodes, and transistors

A rigorous analytic evaluation of an emitter model that includes Auger recombination but excludes bandgap narrowing is presented. It is shown that such a model cannot explain the experimentally observed values of the open-circuit voltage V OC in p-n-junction silicon solar cells. Thus physical mechanisms in addition to Auger recombination are responsible for the experimentally observed values of V OC in silicon solar cells and the common-emitter current gain in bipolar transistors.

Some aspects of current gain variations in bipolar transistors

This paper deals with the study of the main mechanisms involved in the bipolar transistor current-gain variations. In the first part, we describe some properties of characteristic parameters N S and I SR of the surface “diode”; an attempt is made to explain the strong correlation between N S and I SR. In a second part, we analyse the influence of emitter current crowding, high injection in the base region, and base widening effects on the high level bias current gain value. Critical current densities beyond which each of these effects takes place are defined, allowing us to evaluate their comparative importance at a given collector current value. Our study shows that, on the one hand, for transistors affected by emitter-current crowding, the surface currents play an essential part in the high level bias current gain fall off and, on the other hand, the base widening effect has a very great influence on the static ( I C, V EB) characteristic and on the emitter injection efficiency value. In the third part, the experimental results obtained are reported and compared with previously proposed analytical relations.

Current Gain Mechanism in Arsenic Emitter Transistors

The effect of arsenic and arsenic‐phosphorous mixed emitters on current gain in bipolar transistors is discussed. Experimental data for doping profiles and output admittance tests are correlated with emitter efficiency. It is conclusively shown that emitter efficiency is greater for arsenic‐type devices, and that base recombination limits the gain when arsenic is used as the emitter dopant.

Effects of ionizing radiation on oxidized silicon surfaces and planar devices

This paper examines in detail the effects of high and low energy electron, X-ray, and ultraviolet radiation on oxidized silicon surfaces and planar devices. Two permanent effects of ionizing radiation on oxidized silicon surfaces are distinguished: 1) The buildup of a positive space charge within the oxide, and 2) The creation of fast surface states at the oxide-silicon interface resulting in increased surface recombination velocity. The dependence of these effects on dose and dose rate, on bias applied during irradiation, and on structural parameters is discussed and a theory is presented which accounts for the observed features of the space-charge buildup. This theory involves trapping of holes which are generated within the oxide by the radiation. It is shown that all details of the experimental observations can be accounted for by assuming a high density of hole traps near the oxide-silicon interface which decays rapidly with distance into the oxide. Radiation-induced changes in the characteristics of MOS and junction field-effect transistors, p-n junction diodes, and p-n-p and n-p-n transistors are reported and examined in terms of the above two effects. It is shown that the charge buildup causes shifts in the operating point of MOS transistors, catastrophic increases in the reverse current of p-n junctions, and variations in their breakdown voltage. The increase in fast surface-state density is responsible for the lowering of the transconductance of MOS transistors and, in combination with the space-charge buildup, for the degradation of the current gain in bipolar transistors. It is shown that junction field-effect transistors are relatively insensitive to both effects of ionizing radiation and therefore offer the most promise for use in ionizing radiation environments.