A depletion-mode MOSFET has been analyzed to evaluate its electrical behavior using a novel 3-D numerical simulation package. The characterizing analysis of the BC MOSFET was performed through short-channel narrow-channel and small-geometry effects that are investigated, in detail, in terms of the threshold voltage. The DIBL effect becomes significant for a short-channel device with a channel length of $<;3({mu}m)$. For narrow-channel devices the variation of the threshold voltage was sharp for $<4({mu}m)$ due to the strong narrow-channel effect. In the case of small-geometry devices, the shift of the threshold voltage was less sensitive due to the combination of the DIBL and substrate bias effects, as compared with that observed from the short-channel and narrow-channel devices. The characterizing analysis of the narrow-channel and small-geometry devices, especially with channel width of $<;4({mu}m)$ and channel area of $<;4{ imes}4({mu}m^2)$ respectively, can be accurately performed only from a 3-D numerical simulation due to their sharp variations in threshold voltages.