A3 Short Channel Effects (MOSFET)

Topic

As the MOSFET's channel length shortens to the point where it is comparable to the depletion regions of the source and drain junctions, a "roll-off" in the threshold voltage occurs: as the source and drain depletion regions encroach upon an increasingly large fraction of the shortened channel, they themselves assume a portion of the charge that—in a long-channel device—would be entirely controlled by the gate; consequently, a lower threshold voltage is required for the gate to invert the remainder of the channel. Further shortening of the channel causes these depletion regions to encroach upon an even greater fraction of the channel, thereby making the threshold voltage roll-off more pronounced.

Additionally, when the drain-to-source voltage across this shortened channel is sufficiently high, it induces a further reduction in the potential barrier that controls channel formation at the source; this effect—drain-induced barrier lowering—causes the drain depletion region to directly lower the source-side potential barrier as it approaches the source depletion region within the shortened channel, without requiring any additional gate voltage to achieve this reduction. A higher drain-to-source voltage across the shortened channel results in a correspondingly greater degree of drain-induced barrier lowering. Consequently, the combined effect of threshold voltage roll-off and drain-induced barrier lowering determines the effective threshold voltage of this short-channel MOSFET. This corrects the simple quadratic model—which applies to the same MOSFET but is valid only when the channel is long enough for neither short-channel effect to be significant. The shorter the channel and the higher the applied drain-source voltage, the further the effective threshold voltage—driven by the combined action of roll-off and drain-induced barrier lowering—deviates from the fixed threshold voltage predicted by the simple long-channel quadratic model. Because the effective threshold voltage of this short-channel MOSFET depends on both channel length and applied drain-source voltage in this manner, the simple long-channel quadratic model ceases to be a valid approximation for describing the device.

ID:4509