In this letter, we report the fabrication and performance of a novel, high gain photodetector. Basically, the device is a modulation-doped field effect transistor (MODFET) structure with its channel region made of low-temperature MBE-grown GaAs. It exhibits an excellent responsivity of 65 A/W at ~0.87 μm and 6.5 A/W at ⊥1.0 μm. In the sub-bandgap range (0.9-1.3 μm) the responsivity of this device is the highest ever reported, to our knowledge, for any GaAs-based device. Thus, the device appears to be ideally suited for applications requiring a high photodetection sensitivity, especially in the 1.3 μm wavelength region. Charge separation by the built-in field normal to the heterojunction plane is attributed to be responsible for the gain in the device.