The physical properties of a new high-sensitive photodetector developed on the base of Avalanche heterostructures with Negative Feedback (ANF) are considered. The main difference between such a structure and a conventional avalanche photodiode (APD) is non- stationarity of the electrical field strength in an avalanche region caused by the feedback. It is shown that when the non-stationarity is manifested at amplification of a single photocarrier, it changes radically the main characteristics of the avalanche process. A qualitative physical model of ANF process and the results of numerical simulation and experiments are presented. They exhibit an ability of ANF-based device to provide a unique for solid state photosensor a combination of low noise, high gain and low response time. It is also shown that at the same time the negative feedback enables an avalanche device to be manufactured in multi-element design without serious difficulties. The experimental distribution of gain resulting from single- electron-initiated avalanche events was obtained on a SiC-Si ANF heterostructure, and confirmed the main model predictions. The results, illustrating the process of a few-photon light pulse registration are also presented. The opportunities of ANF-based devices further development are discussed.© (1995) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.