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Heterojunction phototransistors (HPTs) with several Ge/Si nano-dot layers as the absorption region are fabri cated to obtain improved light detectivity at 1.55 μm. The HPT detectors are of n-p-n type with ten layers of Ge(8ML)/Si(45nm) incorporated in the base-collector junction and are grown by an ultrahigh-vacuum chemical vapor-deposition system. The detectors are operated with normal incidence. Because of the good quality of the grown material and fabrication process, the dark current is only 0.71 pA/μm2 under 5 V bias and the break down voltage is over 20 V. Compared to the positive-intrinsic-negative (PIN) reference detector with the same absorption layer, the responsivity is improved over 17 times for normal incidence at 1.55 μm.