Modeling of I_ds- V_ds characteristics of short-channel double-gate (DG) MOSFETs
In this dissertation, an above-threshold I-V model framework is constructed for short-channel double-gate (DG) MOSFETs. This is a non-GCA model that takes the effect of the lateral field gradient into account. By applying the model to the low drain and high drain bias cases, useful physical insights are obtained. At low-drain bias voltages, the effect of encroachment of the source-drain bands into the channel (i.e., the source-drain encroachment effect) appears as a reduction of the channel resistance, which is gate-voltage dependent. This effect is stronger in subthreshold region and weaker in above threshold region. At high-drain bias voltages, a point of “virtual cathode” (or minimum potential) at a small distance from the source is caused by the intersection of source band encroachment with the gate-controlled channel potential. This is in contrast to the drain region non-GCA model which is only applicable to device with a channel length of L>20nm. The current model is also extended to incorporate the effect of band bending caused by the depletion of carriers in the source and drain regions and the source-drain series resistance effect. By implementing the velocity saturation in the current continuity equation, the I_ds-V_ds and I_ds-V_gs characteristics generated by the model are verified by TCAD simulations. Moreover, the model is applied to the bulk and the ground plane MOSFETs. It shows that as an additional parameter affecting short-channel effect of bulk MOSFETs, the junction depth of the source and drain is not taken into account in the short-channel non-GCA model for DG MOSFETs. The charge sheet model is not accurate enough to fit parameters like C_inv,V_t for long channel MOSFETs and a modified charge sheet mode is developed. Moreover, the short-channel non-GCA model tends to underestimate the short-channel effect for practical source/drain junction depths like x_j=25nm.