This paper presents an analytical model of bipolar-mode static induction transistor (BSIT) aiming to elaborate the saturation behavior of its device characteristics. Based on new ideas and analyses, it is pointed out that the saturation behavior is due to the shrinking of the plasma, which is composed of large magnitude of holes and electrons resulting from carrier injection. The density and profile of the plasma are mainly governed by the gate biasing voltage. A transition voltage and a saturation voltage are defined, respectively, where the saturation voltage is expressed as a function of the gate voltage. It is also shown that the transconductance changes nearly exponentially with the gate voltage in the saturated segment of current. The analytical results are shown in good accordance with experiments.