A p-type Ge quantum dot infrared photodetector is investigated in this paper. The intrinsic thick Si layers are utilized to block the dark Current under low bias and adjust the tunneling probability by controlling applied field to tune the responsivity spectrum. The 30 K responsivity spectrum shows broadband from 2 to 9 mum at zero bias with photovoltaic effect and extends its range to 20 mum when the applied bias is increased. For long wavelength detection, our detector is tunable with Si blocking layer. The detectivity in this sample has maximum value toward 2.1 x 10(10) cm (Hz(0.5))/W under 0.2 V for the 6 mum wavelength. That for the long wavelength is similar to10(9) cm (Hz(0.5))/W. (C) 2003 Elsevier Ltd. All rights reserved.