Average drift mobility and apparent sheet-electron density profiles in strained-Si-SiGe buried-channel depletion-mode n-MOSFETs

In this paper, we describe a simple method to extract the average drift mobility and the apparent sheet electron density versus the applied gate voltage and the vertical effective electric field in strained-Si-SiGe buried-channel depletion-mode metal-oxide semiconductor field-effect transistors (n-MOSFETs). For this, we adapted an established technique used in evaluating mobility profiles in Schottky-gate MESFETs, by taking into account the existence in our devices of the gate-oxide capacitance C_ox and by introducing an effective junction capacitance C_D, which follows the ideal Schottky´s depletion approximation. By applying our method on fabricated transistors we were able to obtain the average drift mobility profile versus the applied vertical effective field and monitor values as high as 618 cm²/Vs. We also extracted the apparent sheet electron density profile with values reaching as high as 3.4 × 10¹² cm^-2. Although the layer design had not been optimized, the results show mobility enhancement in the strained silicon channel and, to our view, point to a unique regime of operation for these devices, which should benefit the low-power and low-voltage applications. The proposed method could be used as a nondestructive tool for monitoring the transport properties in Si-SiGe modulation-doped MOSFETs. It could also serve as a useful platform for determining explicit modeling links between the layer design and the device performance.