Velocity overshoot in a modulation-doped Si/Si_1-xGe_x structure

The in-plane transport properties of a strained (100) Si layer on a relaxed Si_1-xGe_x substrate are studied for an ungated modulation-doped structure. We use an ensemble Monte Carlo technique. These results are then used to study a gated device structure with a moment equation method. Similar velocity-field characteristics are found for ungated strained Si with any valley splitting energy Delta E>or=0.1 eV. These phonon-limited electron mobilities reach 4000 cm²V^-1s^-1. at 300 K, and 23 000 cm²V^-1s^-1 at 77 K. There is only a slight increase in the saturation velocity at both temperatures. However, a significant overshoot peak transient velocity is found to depend upon Delta E, and for Delta E=0.4 eV it reaches 4.1*10⁷ cm s^-1 at 300 K, and 5.2*10⁷ cm s^-1 at 77 K. Impact ionization increases with Delta E, due to the reduction in the bandgap. For the gated device structure, our numerical simulation of a deep submicrometre modulation-doped device shows velocity overshoot with a peak velocity of 2.6*10⁷ cm s^-1 in the quantum well at 300 K, which is important in achieving a high transconductance of about 300 mS mm ^-1.