M. Zeuner

Alternatives to thick MBE-grown relaxed SiGe buffers

Abstract We have investigated several growth concepts for strain relieved SiGe buffers as basis for high frequency transistors. Modulation doped quantum wells (MODQWs) were realized by molecular beam epitaxy (MBE) on top of thick graded buffers prepared by MBE, ultra-high vacuum chemical vapor deposition (UHVCVD) and low-energy plasma-enhanced CVD (LEPECVD). Additionally, thin buffers including a specific layer grown at low temperature (LT) were realized entirely by MBE. The overgrown thick CVD samples show comparable transport properties and thermal stabilities to those on thick graded MBE buffers. Mobilities of up to 90 000 cm 2 /V s have been measured at 30 K. Thin LT-MBE structures show slightly worse properties but are superior to conventional constant composition buffers.

SiGe-based FETs: buffer issues and device results

Abstract SiGe quantum well structures gain increasing interest in the Si technology. The preparation of a Si channel or a Ge-rich or even a pure Ge channel with a respective two-dimensional carrier gas opens the attractive possibility to fabricate high performance n - or p -type field effect transistors. For both device types, a virtual substrate surface is required which is created by a strain relieved buffer layer grown on a Si standard wafer. The paper reviews various approaches of SiGe buffers including special attempts to reduce the thickness and to improve the quality. N - and p -type modulation-doped field-effect transistors are presented which show comparably good device characteristics and cut-off frequencies in the range of 100–120 GHz.

Low temperature analysis of 0.25 /spl mu/m T-gate strained Si/Si/sub 0.55/Ge/sub 0.45/ n-MODFET's

A low temperature dc and HF investigation of 0.25 /spl mu/m T-gate Si/Si/sub 0.55/Ge/sub 0.45/ n-MODFETs is presented. Outstanding maximum oscillation frequencies f/sub max/ range from 100-120 GHz at 300 K up to 195 GHz at 50 K. These high-frequency characteristics are the first reported at low temperature on Si/SiGe n-MODFETs and are also the highest room temperature data reported so far; physical modeling is used to explain the main trends observed when cooling down the n-MODFET. Many experimental data are presented. The dependence on temperature and biases of the important small-signal equivalent circuit parameters is investigated to analyze the device high-frequency performances and the minimum noise figure of the intrinsic device is determined.

High-frequency SiGe-n-MODFET for microwave applications

n-type SiGe modulation-doped hetero field-effect transistors (MODFETs) with a 0.25-μm Schottky-gate on a Si/sub 0.55/Ge/sub 0.45/ buffer are presented. The layer structure was designed to enable elevated sheet carrier densities of n/sub s/=7.0×10/sup 12/ cm/sup -2/ at moderate electron mobilities of 1050 cm2/Vs. Reducing the thickness of the cap layers enhances the control of the gate on the 2DEG and leads to a high transconductance of 320 mS/mm. Targeting analog applications, we focused on large current densities around 400 mA/mm. Due to advanced RF-characteristics the 100-GHz hurdle of fmax was passed for the first time with fmax(U)=120 GHz and fT was determined at 42 GHz.

n- and p-Type SiGe HFETs and circuits

n- and p-Type SiGe HFETs exhibit advanced performance especially favourable for RF-applications. Due to strained channels high carrier mobilities at room temperature (2700 and 1870 cm2/V s) and large sheet carrier densities (ns=6.4×1012 cm×2 and ps=2.1×1012 cm×2) have been achieved. For the n-MODFET (LG>=150 nm) tensile strained Si channels embedded in SiGe layers lead to a maximal gme of 476 mS/mm and to cut-off frequencies of ft=43 GHz and fmax=92 GHz. The best results for p-type HFETs were attained for a pure Ge channel MODFET with ft=32 GHz and fmax=85 GHz. Analog and digital circuit realizations for the n-MODFET resulted in a transimpedance amplifier yielding a Z21 of 56 dB Ω at a bandwidth of 1.8 GHz and an inverter with a gate delay of 25 ps.

Sub-100 nm Gate Technologies for Si/SiGe-Buried-Channel RF Devices

A novel fabrication process of sub-100 nm self-aligned T-gates for heterostructure field-effect transistors (HFETs) using optical contact lithography is presented. A 500-nm-wide polyimide fin is used as an implantation mask, shrunk by dry etching and subsequently replaced by a gate metal. A low-resistive gate head to form a T-shape is independently defined by wet chemical etching. Using this method, Si/SiGe modulation-doped field-effect transistors (MODFETs) have been prepared, having a gate length of 90 nm. The self-alignment enables the realization of very small source/gate and gate/drain spacings of 200 nm. This yields, together with an optimized salicide (self-aligned silicide) ohmic contact, a much lower access resistance compared to conventional gates defined by e-beam lithography. A record transit frequency fT of 90 GHz and a very high transconductance of 570 mS/mm have been achieved for MODFETs.

Comparison of lateral and vertical Si-MOSFETs with ultra short channels

Abstract The fabrication of a vertical MOSFET is compatible with standard CMOS technology for lateral MOSFETs. Process modules like gate oxidation, polysilicon gate contact, oxide spacer, contact implantation, salizidation, isolation and metallization were used for the integration of lateral and vertical hetero- and homo-MOS devices. For the vertical device the epitaxy of the drain–channel–source layer stack and the mesa etching are new processes. For a 100 nm vertical n-MOSFET ( N A =1×10 18 cm−3) with a 5 nm thick thermal oxide we obtained: g m =375 mS/mm ( U DS =2 V), U T =0.3 V, S=80 mV/dec and I Dmin =1×10 −10 A/μm. For comparison, with a 0.5 pm lateral n-MOSFET we achieved: g m =340 mS/mm, U T =0.34 V, S=66 mV/dec and I Dmin =1×10 −10 A/μm. In addition, the intrinsic RF performance has been simulated to complete the comparison of the lateral and vertical n-MOSFET. It is shown, that the high source–drain capacitance CDS of the vertical MOSFET reduces the transit frequency.

90 GHz f/sub T/ SiGe HFET with fully optical self-aligned sub 100 nm gate

Advanced material properties and sophisticated layer structures of SiGe hetero field-effect transistors are the basis for expected and partly demonstrated elevated RF and noise performance of these devices. However, a lack of lateral device optimization, due to lithography restrains and non self-aligned technology processes often limits the electrical performance of these transistors. Self-aligned technology concepts for hetero field-effect transistors often fail because of the incompatibility between metal gates and high temperature process steps. The new fully optical, self-aligned integration concept presented here uses a replacement-gate structure to overcome this problem.

High performance 0.25 /spl mu/m T-gate SiGe-n-MODFET

The authors present an n-type hetero-transistor MODFET with improved DC- and RF-characteristics. The layer sequence of the SiGe T-gate n-MODFET is grown by solid source molecular beam epitaxy (MBE) on a high resistivity p-type [100] Si substrate.

32 GHz and 40 GHz bandwidth distributed amplifier MMICs based on N-channel SiGe MODFETs

Two distributed amplifiers using n-type SiGe MODFETs as active devices and a coplanar waveguide topology on high resistivity silicon substrate were fabricated. Each of the amplifiers consist of six identical stages, one using a cascode transistor configuration and the other a common source transistor configuration. The distributed amplifier in cascode configuration has an increased bandwidth of 40 GHz and a gain of 4 dB with a ripple of /spl plusmn/0.4 dB. The other amplifier has a bandwidth of 32 GHz and a gain of 5.5 dB with a ripple of /spl plusmn/0.8 dB.