We demonstrate a photodetector with a narrow spectral linewidth (approximately 2 Å) and a useful electrical-tuning range of 35 Å. The photodetector features a horizontal resonant cavity in which feedback is achieved using distributed feedback reflectors. The optical and electrical paths in this photodetector are orthogonal. Consequently, this normal incidence horizontal resonant cavity photodetector is completely integrable into wavelength division demultiplexing and other optoelectronic integrated circuits with a potential for high speed operation.