Two kinds of silicon samples with distinct surface treatment, i.e., one surface was chemically etched and the other was unetched, were used to investigate the flashover characteristics of semiconductor under impulse voltage in vacuum. Before flashover, the ohmic current was observed for unetched samples and the space charge limited (SCL) current for etched samples, respectively. Meanwhile both samples showed quite different flashover tracks. It was believed that all the phenomena were due to their different density and distribution of surface states. A new model was proposed to describe the development process of surface flashover along semiconductor, i.e., the thermal effect and subsequently the current filament with the applied voltage. It was suggested that the flashover phenomena occurred in the interface layer of silicon butted to electrodes and in its lateral layer near to the ambient.