The proportional difference operator (PDO) method is presented to study the subthreshold behavior of MOSFETs. By applying the PDO, acting on an expression for the drain current in the subthreshold region, a new proportional difference characteristic of MOSFET can be obtained. The analytical results show that the proportional difference of the subthreshold equation is a peak function. Its peak position, being dependent on the measurement voltage, is related to the characteristic parameters, so the most important parameters (such as thermal and threshold voltages) can be easily obtained. A surface potential approximation has been developed and implemented in studying a MOSFET's proportional difference subthreshold behaviors. The relationships between the classical threshold voltage and the Lindner threshold voltage are also discussed. Values of voltage constants are shown to agree well with those obtained by standard methods.