Identification and measurement of scaling-dependent parasitic capacitances of small-geometry MOSFET´s

The scaling dependence of the gate-to-source capacitance in strong accumulation C_gs^acc is investigated for the first time to study the parasitic capacitances for MOSFETs with different gate-lengths. Results show a gate-length L_gate dependent characteristic for C_gs^acc and is consistent with the results from numerical device simulation. The result is found to be different from the widely used assumption in the literature, and is believed to be reported for the first time. The gate-length dependent C _gs^acc characteristic is due to the top side capacitance, and is verified through careful device characterization and numerical device simulation. A measurement technique is further developed to determine C_top for small geometry MOSFETs. The technique is demonstrated on actual transistors with no special test devices required. Based on the technique, a sub-linear dependent relationship is found for the dependence of C_top on L_gate, and is in close agreement with the results predicted by theory. Results also indicate that C_top is about 8-13% of the measured total C_gs^acc, which corresponds to a nonnegligible portion of the total capacitance, and needs to be considered for future device design, characterization and modeling. The impact of C_top on CMOS inverter gate delay is also investigated. Results indicate C_top adversely impact the gate delay by as much as 5.5% as supply voltage is scaled down to 1 V