Browsing by Author "Wismayer, A.C."

Now showing 1 - 5 of 5

Current gain increase in AlGaAs/GaAs heterojunction bipolar transistors using AlGaAs layer overgrowth
(Applied Physics Letters, 1994) Kalingamudali, S.R.D.; Wismayer, A.C.; Woods, R.C.; Roberts, J.S.
Reduction of the surface recombination current components in Npn AlGaAs/GaAs heterojunction bipolar transistors has been achieved by overgrowing the emitter?mesas with an AlGaAs layer approximately 0.5 ?m thick. It was observed that the n=2 recombination current was reduced by ?90%, to about 10% of the original value, with a corresponding 13?fold increase in current gain for 270?20 ?m2 devices. In addition, devices with the same emitter?base area, but significantly different perimeters, were observed to have similar current gains and n=2 recombination current values. This was in contrast to devices fabricated without an overgrowth layer. These results suggest that the overgrowth layer causes a very significant reduction in the perimeter recombination current.
Experimental evaluation of separate contributions to ideality factor for the base surface recombination current in heterojunction bipolar transistors
(Materials Science and Engineering: B, 1994) Kalingamudali, S.R.D.; Wismayer, A.C.; Woods, R.C.
In this paper it is shown experimentally, for the first time, that the ideality factor n for the extrinsic base surface recombination current in AlGaAs/GaAs heterojunction bipolar transistors (HBTs) is unity and is bias independent. The devices with various emitter radii have been fabricated and the n = 1 and n = 2 current components of the emitter-base junction current were calculated. It was observed experimentally that the n = 1 current component was proportional to the total area of the emitter-base junction plus the exposed extrinsic base surface, both at lower biases such as emitter-base voltage VBE=0.60V (when the n = 2 current component dominates), and at higher biases such as VBE=1.0V (when the n = 1 current component dominates). The ideality factor value for the diffusion current is 1. Therefore, these results suggest that the ideality factor of the extrinsic base surface recombination current in HBTs is unity and is independent of the emitter-base bias.
New Experimental Interpretation for the Emitter-size Effect of AIGaAs/GaAs heterojunction bipolar transistors.
(1995) Kalingamudali, S.R.D.; Woods, R.C.; Wismayer, A.C.
AIGaAs/GaAs heterojunction bipolar transistors (HBTs) have an excellent frequency performance. One of the major limitations of these devices is current gain degradation with decreased emitter-base junction size, which is known as the emitter-size effect. Several experimental and theoretical studies have been reported on the emitter-size effect in AIGaAs/GaAs HBTs. These papers suggest that it is due to the extrinsic base surface recombination current. However, in thts paper. experimental evidence is reported which suggests that it is the perimeter recombination current which causes the emitter-size effect. Further analysis or previously published results, from rectangular geometry AIGaAs/GaAs HBTs fabricated with and without an overgrown A1GaAs layer approximately 0.5 um thick around the emitter-mesas, has been carried out. This analysis suggests that the perimeter recombination current rather than the extrinsic base surface recombination current causes the emitter-size effect. Devices with the overgrown layer and with the same emitter-base area. but significantly different perimeters, had similar current gains and n=2 recombination current values. In addition, for similar geometry devices without the overgrowth layer, the n=2 recombination current was roughly proportional to the device perimeter and due to the emitter-size effect current gains were decreased when the perimeter was increased. Since the reduction in the perimeter recombination current leads to a reduction of the emitter-size effect, it is suggested that the emitter-size effect is not due to the extrinsic base surface recombination current. Consequently, these results suggest that the emitter-size effect is due to the perimeter recornbination current.
Perimeter and bulk recombination currents in GaAs homojunction diodes and heterojunction bipolar transistors after surface processing
(Solid State Electronics, 1997) Kalingamudali, S.R.D.; Wismayer, A.C.; Woods, R.C.; Wight, D.R.
The emitter-mesa surface (perimeter) and emitter-base bulk recombination currents, in heterojunction bipolar transistors (HBTs), are known to degrade device performance at low applied voltages, where these currents dominate. To study the effects of the etchant (used in defining the diode mesas) on the perimeter recombination current, GaAs homojunction diodes were given a short time etch in a selection of wet chemical etchants and the effect on the edge leakage current investigated. It was observed that the edge leakage current could be modified by the process used to fabricate the mesa. Following encouraging results, the work was extended to include HBTs, and an investigation was undertaken of the effects on recombination at the perimeter of the emitter-mesa and on bulk recombination current, with surface treatments. A simple model was used to separate the bulk and perimeter contributions from the recombination current. The surface recombination current was strongly dependent on the process used to define the mesa, and can therefore, by appropriate processing, be reduced.
Recombination current reduction in AlGaAs/GaAs heterojunction bipolar transistors with polyimide deposition
(Solid State Electronics, 1994) Kalingamudali, S.R.D.; Wismayer, A.C.; Woods, R.C.
N-p-n AlGaAs/GaAs heterojunction bipolar transistors of various emitter-mesa diameters have been fabricated to examine the effects on the recombination current with polyimide deposition. The recombination current in these devices (when the ideality factor is close to 2) was proportional to the device perimeter before and after polyimide deposition. This suggests that the dominant component of the recombination current in these devices is the perimeter recombination current. A simple model was developed which allowed the contribution from the perimeter and bulk recombination currents to be calculated. The common-emitter d.c. current gains of these devices increased correspondingly as recombination current decreased due to the polyimide deposition.