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A new unified device model of a polysilicon emitter transistor (PET) is proposed for various emitter structures. More factors influencing the base current are considered in this model and the general expressions for the common emitter current gain (hFE) and the emitter Gummel number (Ge) are derived in an analytical form. From these general expressions we can classify five different PET device structures of PET by specifying some structure parameters of the devices. Ning's theoretical equation for the current gain enhancement factor K and De Graaff's theoretical equation for Ge can be deduced directly from this model. This model is in good agreement with experimental results. The value for the minority hole mobility in polysilicon is also deduced to be in the range of 4.5-6.1 cm2 V-1 s-1 from fitting to the experimental data.