IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. 51, NO. 8, AUGUST 2004 1309 Average Drift Mobility and Apparent Sheet-Electron Density Profiles in Strained-Si–SiGe Buried-Channel Depletion-Mode n-MOSFETs Kostis Michelakis, Member, IEEE, Antonio Vilches, Christos Papavassiliou, Solon Despotopoulos, Member, IEEE, Kristel Fobelets, and Chris Toumazou, Fellow, IEEE Abstract—In this paper, we describe a simple method to ex- tract 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 ac- count the existence in our devices of the gate-oxide capacitance and by introducing an effective junction capacitance , 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 cm /Vs. We also extracted the apparent sheet electron density profile with values reaching as high as cm . 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. Index Terms—Drift mobility, heterostructure, metal–oxide semiconductor field-effect transistor (MOSFET) mobility, MOS-MODFET, SiGe, silicon, strained-si MOSFETs. I. INTRODUCTION S TRAINED-silicon n-type metal–oxide semiconductor field-effect transistors (n-MOSFETs), which make use of the Si/SiGe heterojunction, are known to sustain inversion channels with up to 70% enhanced mobility, compared to conventional silicon n-MOSFETs [1]. These are enhance- ment-mode (EM) devices and have surface channels (SC). But there is little found in the literature on their depletion-mode (DM) buried-channel (BC) counterparts, despite the fact that they too can share the convenience of a standard Si MOS process, and despite the advances in n-type strained-Si–SiGe modulation-doped field effect transistors (MODFETs), where cutoff frequencies in the range of 90 GHz have been reported Manuscript received October 28, 2003. This work was supported by EPSRC under Grant GR/N65851/01. The review of this paper was arranged by Editor C. McAndrew. The authors are with the Department of Electrical and Electronic Engineering, Imperial College London, London SW7 2AZ, U.K. (e-mail: k.michelakis@im- perial.ac.uk). Digital Object Identifier 10.1109/TED.2004.832727 [2]. In BC MOSFET, the semiconductor heterostructure con- tains multiple layers, with the carriers not always confined in the intended strained-Si channel, and the most relevant approach is to evaluate the free carrier density and the drift mobility profiles with the depth and/or with the vertical effec- tive field and the applied gate bias. In our case though, we feel that the most suitable figures of merit are the average effective drift mobility and the apparent sheet electron density, as these parameters reflect the operation of the device as being a whole entity. A number of techniques for mobility and carrier density profiling have been developed and matured through the years, mainly on GaAs FETs, because of the multiple layers and the varying doping profiles involved in these devices. A review on such techniques and also a discussion on the apparent nature of the carrier concentration, along with the relevance of the Debye length, can be found in [3]. In this paper, we adapted the technique by Pucel and Krumm [5], which is used in evaluating mobility profiles in Schottky-gate MESFETs, by taking into ac- count the existence in our devices of the gate-oxide capacitance and by introducing an effective junction capacitance , which follows the ideal Schottkys depletion approximation. By applying this method on fabricated long-gate transistors we were able to obtain the low-field average drift mobility profile and the apparent sheet electron density profile versus the applied gate-to-source voltage and the vertical effective electric field. The method is detailed in Section II together with the assumptions that are used. In Section III, after a description of the device fabrication procedure, the experimental setup is explained and the results are analyzed and discussed. The conclusions are finally summarized in Section IV. II. ASSUMPTIONS AND METHOD Our method is based on the fundamental assumption that in Si–SiGe BC DM n-MOSFETs, of which an example het- erostructure can be seen in Fig. 1, the measured total gate capac- itance can be expressed as a series combination of the oxide capacitance , which is independent of the applied vertical bias and an effective junction capacitance , which is bias de- pendent [4] (1) is calculated at the highly positive gate bias limit, where all charge is attracted to the gate oxide interface, where be- 0018-9383/04$20.00 © 2004 IEEE