Observation of electron velocity overshoot in sub-100-nm-channel MOSFET's in Silicon

n-channel MOSFET's with channel lengths from 75 nm to 5 µm were fabricated in Si using combined X-ray and optical lithographies, and were characterized at 300, 77, and 4.2 K. Average channel electron velocities υ
e
were extracted according to the equation
\upsilon_{e}=g_{mi}/C_{ox}
, where g
mi
is the intrinsic transconductance and C
ox
is the capacitance of the gate oxide. We found that at 4.2 K the average electron velocity of a 75-nm-channel MOSFET is 1.7 × 10
⁷
cm/s, which is 1.8 times higher than the inversion layer saturation velocity reported in the literature, and 1.3 times higher than the saturation velocity in bulk Si at 4.2 K. As channel length increases, the average electron velocity drops sharply below the saturation velocity in bulk Si. These experimental results strongly suggest velocity overshoot in a 75-nm-channel MOSFET.