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concept which has been described by Ladbroke in detail in [2]. In the circuit shown below, ... [2] Peter H Ladbroke,'MMIC Design:GaAs FETs and. HEMTs', ISBN ...

CHARACTERISATION OF METAMORPHIC HEMTS FOR LOW-NOISE WIDEBAND AMPLIFIERS Karine Enzel, Jan Grahn, Anders Mellberg, Herbert Zirath, Niklas Rorsman Chalmers University of Technology, Microwave Electronics Laboratory, SE-412 96 Göteborg, Sweden

Abstract - Metamorphic HEMTs developed by OMMIC have been DC- and RF-characterized. An extrinsic DC transconductance of 770 mS/mm was obtained for a 4x25µm MHEMT. Typical extrinsic unity current cut-off frequency, fT, was 140 GHz with a maximum oscillation frequency of 196 GHz. Broadband three-stage amplifiers utilizing these transistors were designed fabricated and characterized for various frequency bandwidths up to 90 GHz. For the 26-43 GHz bandwidth, the amplifier exhibited a gain of typically 25 dB and a return loss less than 5 dB. INTRODUCTION Low noise wideband amplifiers in the microwave and mm-wave region are essential for satellite communication systems and remote sensing. The natural device choice for a low-noise application is the pseudomorphic high-electron mobility transistor (PHEMT) due to its superior noise characteristics compared to other technologies. New commercial process generations based on the HEMT such as InP HEMT and metamorphic HEMT (MHEMT) are now emerging offering even lower noise performance and higher gain. The MHEMT process, originating from the GaAs PHEMT process, presents an attractive alternative compared to the high-cost InP HEMT technology. In this paper we present device measurement results for an MHEMT process denoted D01MH under development at the foundry OMMIC [1]. Furthermore, various wide-band amplifier designs between 26 to 90 GHz based on D01MH have been carried out and will be reported. DEVICE CHARACTERIZATION The gate length for the MHEMTs is 0.13 µm and the gate is of ‘mushroom’ type asymmetrically positioned between source and drain. A schematic cross-section is shown in Figure 1 where the thick graded buffer (~µm) makes it possible to use a relatively high In content in the active channel layer.

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InGaAs cap InAlAs Schottky InGaAs channel InAlAs buffer Graded buffer Semi-insulating GaAs substrate

and saturation velocity compared to the GaAs PHEMT material structure with higher ft and fmax, and lower noise figure as a result. Figure 2 shows measured current-voltage characteristics for a 4x25 µm transistor. The device pinches off around –1 V. The maximum saturation current at VDS=1.2 V and VGS=0 V is 550 mA/mm. In Figure 3, the DC transconductance and drain current are plotted as a function of gate voltage for VDS=1.2V. The transconductance peaks broadly around VGS=-0.3 V with a value of 770 mS/mm.

Figure 1: Cross section of an MHEMT Characterisation of Metamorphic HEMT for wideband amplifiers Karine Enzel, Jan Grahn, Anders Mellberg, Herbert Zirath, Niklas Rorsman

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Figure 2: I-V characteristic of a 4x25 um MHEMT

Figure 3: Transconductance and drain current of a 4x25 um MHEMT

Maximum available gain; dB(S(2,1)); dB(H(2,1))

The RF performance of the transistors was characterized by S-parameters measurement up to 50 GHz. From these measurements a model was extracted and used for estimating the figure of merits. The maximum stable gain, dB(S21), dB(H21) are plotted in Figure 4 as a function of frequency with VDS=1.5 V VGS=-0.4 and IDS=28 mA. A maximum oscillation frequency, fmax, of 196 GHz and a current gain transition frequency , fT, of 140 GHz were obtained. Figure 5 shows a picture of a 4x25 µm transistor. The noise figure was also measured on a 4x25 µm devices at Vds=1.5V and at 45% Idss. A minimum noise figure NFmin of 0.8-1.2 dB was obtained at 26 GHz.

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Figure 4: Maximum available gain, dB(S21) , dB(H21) as a function of frequency. VDS=1.5V, VGS=-0.4V, IDS=28mA

Figure 5: Picture of a 4x25-µm MHEMT

Characterisation of Metamorphic HEMT for wideband amplifiers Karine Enzel, Jan Grahn, Anders Mellberg, Herbert Zirath, Niklas Rorsman

MHEMT WIDEBAND AMPLIFIER DESIGNS A design methodology is at present under development for wideband amplifiers utilizing reflective matching in order to achieve simultaneously high gain and low noise. The basic idea is to use an interstage equalizer that compensate for the intrinsic 20 dB/decade gain roll-off of the transistor, a concept which has been described by Ladbroke in detail in [2]. In the circuit shown below, this is accomplished by loading the drain of each transistor by a shorted stub, this stub is also a part of the dcbias circuit. In addition, a series element which consists of a parallel RC-network is used in the interstage networks to improve the stability at lower frequencies as described by Ono et al [2]. The first conceptual designs were made demonstrating Q-band (33-50 GHz), V-band (50-75 GHz) and Fband (60-90 GHz) operation. The layout of a three stages Q, V, and W-band design is shown in Fig 6 below. Simulation shows that octave bandwidths should be feasible. The input and output reflection coefficients are typically below -10dB within the band.

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Picture and simulated gain of the Q, V, and W-band three-stage amplifier. The chip size is 3x1.5 mm.

Characterisation of Metamorphic HEMT for wideband amplifiers Karine Enzel, Jan Grahn, Anders Mellberg, Herbert Zirath, Niklas Rorsman

MEASURED RESULTS OF THE AMPLIFIERS

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REFERENCES

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[1] OMMIC homepage http://www.ommic.com [2] Peter H Ladbroke,’MMIC Design:GaAs FETs and HEMTs’, ISBN 0-89006-314-1, Artech House, 1989.

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[3] N Ono, K Onodera, K Arai, K Yamaguchi, Y Iseki,’K-band Monolithic GaAs HEMT Driver Amplifiers’, Proceedings of 2002 Asia Pacific Microwave Conference, pp. 1390-1392.

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The amplifiers were characterized ‘on wafer’ by measuring S-parameters using a 67 GHz PNA and a W-band (75-115 GHz) test-set, all from Agilent. All amplifiers show ultra wideband response with a gain of a few dB larger compared to the simulation. The gain characteristic shows however also more ripple compared to the simulation and the input and output reflection coefficients are generally higher compared to simulations. In addition, the upper cut-off frequency of the V and W-band amplifiers are 8-10 GHz lower compared to the simulations. We are at the moment investigating the reasons for the deviations. The noise-figure will be measured in the near future.

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Figure 7: Measured gain for the Q,V, and W-band three-stage amplifier ACKNOWLEDGMENTS This work was supported by the SSF’s (Swedish Foundation for Strategic Research) High Frequency Electronics program, part MMIC. Support for wideband amplifier development from the European Space Agency through ESTEC, project WBIFA, is highly appreciated. The interest from Thomas Lewin, Anders Sjölund, Petri Piironen, and Tapani Nähri is also acknowledged. Agilent is acknowledged for donation of the ADS software.

Characterisation of Metamorphic HEMT for wideband amplifiers Karine Enzel, Jan Grahn, Anders Mellberg, Herbert Zirath, Niklas Rorsman