<p>Spectral imaging is a critical technology for analysing the spectral and spatial information of input light signals for both scientific research and industrial uses. Traditional imaging systems, typically incorporating spectrometers with bulky optical components and moving mechanical parts, hinder miniaturization and on-chip integration, which is crucial for in situ spectroscopy and high-speed spectral imaging. This challenge has driven efforts towards highly integrated imaging devices and small-footprint spectrometers, aiming for portable and integrable spectral imagers. Here, inspired by the success of the digital camera-on-a-chip device concept, we develop a miniaturized on-chip spectral imager design based on a vertically cascaded n–p–n photodiode array. The device incorporates an AlGaN-based n–p diode with a compositionally graded profile in the active region in conjunction with a GaN-based p–n diode. Our proof-of-concept configuration enables electrically tunable spectral measurements from 250 nm to 365 nm, a spectral range previously inaccessible to miniaturized on-chip spectral imagers. The device achieves a high peak wavelength accuracy of 0.62 nm and a sub-10-ns response time. We demonstrate a 10 × 10 cascaded diode array for direct spectral imaging with high-quality spectral-to-spatial mapping. We spatially distinguish thin films of four organic materials on the same substrate through single-shot imaging. Our work establishes a scalable pathway for manufacturing and integrating spectral imagers into portable systems at low cost.</p>