MOSFET channel length: extraction and interpretation

This paper focuses on MOSFET channel length: its definition, extraction, and physical interpretation. After a brief review of the objectives of channel length extraction and previous extraction methods, the principle and the algorithm of the latest 'shift and ratio' (S&R) method are described. The S&R method allows the channel mobility to be an arbitrary function of gate voltage and, at the same time, provides a way to determine the threshold voltage of short-channel devices independent of their parasitic resistances. Accurate and consistent results are obtained from nMOSFET and pMOSFET data down to 0.05 μm channel length. By applying the S&R method to model generated current-voltage (I-V) curves, it is shown that the extracted channel length should be interpreted in terms of the injection points where the MOSFET current spreads from the surface layer into the bulk source-drain region. This implies significant degradation of short-channel effects (SCE's) if the lateral source-drain doping gradient is not abrupt enough. Several remaining issues, including errors due to channel-length-dependent mobilities, difficulties with lightly-doped drain (LDD) MOSFET's, and interpretation of capacitance-voltage (C-V) extracted channel lengths, are discussed in Section VIII.