Technology for Advanced Microwave Power FET Structures


Work Area: Alternative Advanced Semiconductor Materials, Devices and Process Steps

Keywords microwave heterostructure FET technology, GaAs- and InP-based MBE technology, low-temperature growth

Start Date: 1 November 1992 / Duration: 36 months / Status: running

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Abstract A new type of heterostructure MISFET or GaAs using a thin GaAs layer grown at extremely low temperature (LT- GaAs) is proposed for power application in the microwave and mm-wave regime. Work includes MBE growth, heterostructure materials analysis, MISFET processing, device modelling and RF-power assessment. Record breakdown voltages have been obtained for high current density devices.


At present the limitations in terms of power density, frequency of operation and efficiency determine the capabilities of microwaves to realise low-cost integrated circuits, especially in the millimetre-wave range. The purpose of this work is to improve the high frequency power-handling capability of III-V FETs beyond the present limits by incorporating epitaxial heterostructure layers grown at extremely low temperatures, especially LT-GaAs.

Approach and Methods

III-FETs have shown the highest cut-off frequencies and lowest noise figures of all 3-terminal devices. However, their power handling capability is still severely below their intrinsic limit, mainly due to surface-induced breakdown phenomena. Furthermore, for sub-micron gate lengths the conventional technology implies a tightly controlled recess etching process. The proposed structure is an essentially planar GaAs-MISFET structure containing a novel MBE-grown LT-GaAs layer configuration in the gate barrier layer system to improve the gate breakdown characteristics. Thus, two important limitations of the microwave power FET technology will be addressed.

The focus of the program will be on the growth of LT-material containing a large number of recombination centres, its incorporation into advanced high mobility channel GaAs-HFET structures and the demonstration of improved microwave power performance.

For this cross-disciplinary task the consortium brings together complementary skills in the area of heterostructure materials growth and microwave device fabrication (Ulm, Lille), the complex evaluation of novel materials structures (Cardiff), and the detailed evaluation and modelling of microwave FETs (Lille) with know-how gained from past ESPRIT activities.

Progress and Results

Growth conditions for LT-GaAs layers and AlAs diffusion barriers have been investigated. The materials and electronic properties have been analysed. The structures have then been incorporated into first GaAs-channel FET-structures and 1 micron gate length MISFETs have been fabricated showing a record DC-IV power product. Latest results are 23.9 W/mm IV-product indicating a class A RF-power handling capability of 2.7 W/mm. A two-terminal gate-drain breakdown voltage of 50V has been obtained and a source-drain three terminal breakdown voltage of 34 V for a device with 720 mA/mm maximum output current. These results lie clearly outside the limits of the commonly accepted lateral spreading model for power MESFETs. It is suggested that this is due to the specific properties (like a controled leakage current and high carrier recombination rate) of the LT-GaAs layer. The charge control by the leaky MIS-diode has been analysed (ref.[4] and [5]) and presently the stability of these first devices is assessed.


Through the program, insight will be gained into the fundamental power limits of high speed III-V HFETs. Furthermore, the novel structure has the potential of reduced processing complexity compared to conventional MMIC technologies. For this purpose it is planned to exploit the fabrication of such structures on 3" diameter substrates (PICOGICA, Siemens). The structure has the potential to provide a new generic element for next generation microwave power FETs and MMICs.

Latest Publications


Universität Ulm - D
Albert-Einstein-Allee 5
Postfach 4066
D - 89081 ULM


Université des Sciences and Techniques de Lille - F
University of Wales, Cardiff - UK


Prof. E. Kohn
tel +49/731 502 4210
fax +49/731 54113

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TAMPFETS - 6849, August 1994

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html version of synopsis by Nick Cook