Drain-Induced Barrier Lowering :In a MOSFET device with improperly scaled small channel length, too deep source and drain junctions or too low channel doping, undesired electrostatic interaction between the source and the drain known as drain-induced barrier lowering (DIBL) may take place. It is concerned with the effect that the drain-source voltage has on lowering the barrier for current flow. In unscaled device, the gate-source voltage inverts the surface by . In short-channel devices, the depletion region around the drain increases as V_DS increases, and it extends into the channel region. Thus, the DIBL phenomenon consists in the fact that the drain-source voltage assists the gate voltage in the depletion process. As the channel length is reduced, the potential barrier between the source and the drain is reduced even without the application of a drain-source voltage. However, the potential barrier is reduced even further by applying V_DS because it expands the depletion region in the channel. This results in lowering of the threshold voltage, and the decrease in V_T0 is observed to be almost linear with V_DS .

Sub-threshold region conduction : the cutoff region of operation is also referred to as the sub-threshold region, which is mathematically expressed as I_DS =0               V_GS < V_th
However, a phenomenon called sub-threshold conduction is observed in MOS transistors. The current flow in the channel depends on creating and maintaining an inversion layer on the surface. If the gate voltage is inadequate to invert the surface (that is, V_GS< V_T0), the electrons in the channel encounter a potential barrier that blocks the flow. However, in small-geometry MOSFETs, this potential barrier is controlled by both V_S and V_DS . If the drain voltage is increased, the potential barrier in the channel decreases, leading to drain-induced barrier lowering (DIBL). The lowered potential barrier finally leads to flow of electrons between the source and the drain, even if V_GS <V_T0 (that is, even when the surface is not in strong inversion). The channel current flowing in this condition is called the sub-threshold current . This current, due mainly to diffusion between the source and the drain, is causing concern in deep sub-micron designs. The model implemented in SPICE brings in an exponential, semi-empirical dependence of the drain current on V_GS in the weak inversion region. Defining a voltage V on as the boundary between the regions of weak and strong inversion,