Negative photoconduction in fully fabricated InP based HEMTs

We studied the photocurrents in InP-based HEMTs as a function of bias voltages and photon energy. The devices had excellent electrical characteristics. In the deep subthreshold regime the drain current stems from the gate current. In this region, the photo-drain-current also is caused by the photo-gate current, as evidenced by its magnitude and spectral characteristics. In the subthreshold regime, the photocurrent rises rapidly at a photoenergy of approximately 0.7 eV (which corresponds to the energy gap of the InGaAs channel). Spectral analysis indicates that the response stems from a multitude of transitions. In this regime, transitions to the first electron subband are dominating. When the device is strongly turned on, this dominance disappears. At small drain voltages (i.e. in the linear region) the photoconduction behavior is similar to that observed in GaAs based HEMTs. The transistor amplifies the photogenerated charge due to hole storage in the channel. In the saturation regime we observe a drastically different behavior: The photocurrent is negative. In this case, the drain current becomes smaller when the device is illuminated. We also observe a pronounced change in spectral response in this region. We explain this effect by storage of electrons rather than of holes.