Based on the procedure of Pierret and Shields [1, Solid-St. Electron. 26, 143, 1983] for the long-channel bulk MOSFET, a new single-integral expression is obtained to describe the current-voltage characteristics for the silicon-on-insulator (SOI) MOSFET. This expression is valid for: any degree of inversion, all back-gate bias conditions and any semiconductor film thickness. Our single-integral expression, applied to a given back-gate bias condition and using the appropriate approximations, can be simplified to the results of the previous models.