Abstract

The emitter photocurrent transient of a double­heterojunction InP/Ga_0.47In_0.53As/InP bipolar transistor has been found to be much faster than its ~15-GHz microwave bandwidth would suggest, since optical injection bypasses the base resistance. Slower components, due to holes residing in the base, can be eliminated with appropriate base biasing; this feature may be useful in optical receiver circuits. At a base bias of +0.5 V, the photocurrent transient has a 12-ps duration in response to both 620- and 1530-nm ultrafast optical pulses. At higher biases a slower transient component appears, as does a transient of opposite sign but with the same ~100-ps time constant at lower base biases. One slow component, a charge-neutrality-preserving collector current, has a base-bias-controlled gain; the other components can therefore be made to cancel. The device does not possess gain when operated in this manner, but the data suggest a design for an ultrafast photodetector with gain. The device studied is as fast a photodetector at visible wavelengths as at infrared, and the additional absorption provided by the InP emitter layer increases its short-wavelength sensitivity.

© 1991 Optical Society of America