1/f Noise and trap density in n-channel strained-Si/SiGe modulation doped field effect transistors Kristel Fobelets a, * , Sergey L. Rumyantsev b,c , Thomas Hackbarth d , Michael S. Shur b a Department of Electrical and Electronic Engineering, Imperial College London, Exhibition Road, London SW7 2BT, UK b Department of Electrical, Computer, and Systems Engineering, CII 9017, Rensselaer Polytechnic Institute, Troy, NY 12180-3590, USA c Ioffe Institute of Russian Academy of Sciences, 194021 St-Petersburg, Russia d Daimler-Chrysler AG, Research Center Ulm, Wilhelm-Runge-St. 11, 89081 Ulm, Germany article info Article history: Received 13 October 2008 Received in revised form 11 February 2009 Accepted 25 March 2009 Available online 25 April 2009 The review of this paper was arranged by Prof. S. Cristoloveanu Keywords: Electrical noise Strained-Si MODFET abstract The low frequency (1/f) noise characteristics of Schottky-gated strained-Si n-channel modulation doped field effect transistors have been investigated as a function of Ge concentration for different virtual sub- strates. The gate voltage dependence of the 1/f noise agrees well with the McWhorter carrier number fluctuations model. The trap density (extracted using a Ge dependent potential barrier height and tunnel- ling constant) is low in devices on thick virtual substrates (N t = (2–6)  10 16 cm 3 eV 1 ), and does not degrade with the increase of the Ge concentration from 30% to 40%. This trap density is the same for thin Helax virtual substrates (He + ions implanted thin substrate) but increases two orders of magnitude for thin low-temperature grown substrates. Ó 2009 Elsevier Ltd. All rights reserved. 1. Introduction High electron mobility transistors (HEMT) can deliver high speed and power performance [1]. These structures are tradition- ally fabricated in III–V materials. A similar approach can be taken for more commonly available IV–IV elements such as Si in combi- nation with SiGe. Schottky-gated n-channel modulation doped Si/ SiGe transistors – MODFETs – have a strained-Si (s-Si) channel sur- rounded by relaxed SiGe layers [2]. One of these layers is the vir- tual substrate (VS), which is a relaxed Si 1x Ge x layer on top of a traditional Si substrate. The Ge concentration, x, in the VS is graded to allow relaxation of the layer, while preventing dislocation prop- agation in the growth direction. The strain in the s-Si channel causes a conduction band offset between the s-Si and surrounding SiGe, forming a quantum well channel. The strain also causes an in- crease in carrier mobility due to the reduction of the effective mass and intervalley scattering. Similarly, s-Si channels can be used for the fabrication of MOSFETs with strain in the channel underneath the gate oxide giving increased speed performance [3]. Electrical noise in FETs gives an indication of the material quality and is important for RF applications. The low frequency noise and trap density in s-Si MOSFETs is mainly determined by the oxide quality on these devices, similar to Si MOSFETs. In s-Si MOSFETs, the trap density can increase with increasing Ge concentration in the vir- tual substrate (0–40% Ge) on which the strained channel is defined [4]. This degradation is attributed to Ge diffusion during non-opti- mised high-temperature fabrication steps that reduces the oxide quality. In Ref. [5] a MOS-gated Si/SiGe MODFET with deposited oxides was investigated. The noise in these devices is mainly deter- mined by the oxide–semiconductor interface quality. Since SiGe is a poor thermal conductor and the traditional VS is 3–5 lm thick, Si/SiGe MODFETs suffer from self-heating at high drain bias. In order to control the self-heating effect and reduce the fabrication costs, thin substrates are used. Thin SiGe VSs have been shown to reduce the self-heating effect [6]. Ref. [6] presented RF noise in this material and some 1/f noise results, the latter, how- ever, without further analysis. In Ref. [7] the low frequency noise was studied on s-Si n- and p-type MOSFETs on thin and thick sub- strates. The thin substrate was grown via a low-temperature growth process. The Ge concentration was different for both sub- strate types and therefore noise dependence on Ge concentration and substrate growth technique was convoluted. The thick sub- strate showed unexpectedly higher noise levels than the thin sub- strate. The extracted trap density in [7] was the lowest reported for s-Si n-channel MOS-gated FETs. In this work, we investigate Schottky-gated Si/SiGe MODFETs using two Ge concentrations, 30% and 40%. This allows investiga- tion of the trap density as a function of Ge concentration only, thus 0038-1101/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.sse.2009.03.022 * Corresponding author. Tel.: +44 (0)2075946236; fax: +44 (0)2075946308. E-mail address: k.fobelets@ic.ac.uk (K. Fobelets). Solid-State Electronics 53 (2009) 626–629 Contents lists available at ScienceDirect Solid-State Electronics journal homepage: www.elsevier.com/locate/sse