  Ultra-compact spectrometers are becoming increasingly popular for their
promising applications in biomedical analysis, environmental monitoring, and
food safety. In this work, we report a novel single-pixel-photodetector
spectrometer with a spectral range from 480 nm to 820 nm, based on the
AlGaAs/GaAs p-graded-n junction with a voltage-tunable optical response. To
reconstruct the optical spectrum, we propose a tailored method called Neural
Spectral Fields (NSF) that leverages the unique wavelength and bias-dependent
responsivity matrix. Our spectrometer achieves a high spectral wavelength
accuracy of up to 0.30 nm and a spectral resolution of up to 10 nm.
Additionally, we demonstrate the high spectral imaging performance of the
device. The compatibility of our demonstration with the standard III-V process
greatly accelerates the commercialization of miniaturized spectrometers.
