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Towards Terahertz MMIC Amplifiers: Present Status and TrendsLorene Samoska, Senior Member, IEEE

Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA, 91109, USA

Abstract - In this paper, we present an overview of highfrequency Monolithic Millimeter-wave Integrated Circuit(MMIC) amplifiers and discuss the state of the art for low noiseamplifiers and power amplifiers. We report on the challenges andinnovations required to achieve small-signal and power gainabove 300 GHz, and present a review of present technologystatus. The highest frequency MMIC amplifiers ever developedto date will be presented, starting at W-Band (75-110 GHz) andabove. Highlights include a MMIC low noise amplifier with gainup to 260 GHz, and a MMIC power amplifier operating up to 190GHz.Index Terms - MMICs, terahertz, amplifiers, HEMTs, InP.

I. INTRODUCTION

The growing field of terahertz technology relies on thedevelopment of high performance components from themillimeter-wave to the submillimeter-wave regime. Terahertzheterodyne receivers require mixers, multipliers andamplifiers with low noise figure and high output power. Thispaper addresses some of the issues involved in developingmonolithic millimeter-wave integrated circuit (MMIC)amplifiers for applications to terahertz receivers andtransmitters.While most solid state transistor cutoff frequencies are well

below the 1 THz range, the use of MMIC amplifiers isgrowing for terahertz heterodyne applications. In a terahertzreceiver, MMIC power amplifiers (PAs) are required forbuilding terahertz or sub-terahertz high power localoscillators. Frequently, MMIC power amplifiers are used asdrivers for chains of diode multipliers to increase outputpower and bandwidth for a local oscillator chain. MMIC lownoise amplifiers (LNAs) are increasingly being used asreceiver front-ends in the 0.1-0.3 THz range, as transistorcutoff frequencies improve. Traditionally, high electronmobility transistors (HEMTs) built in the GaAs or InPmaterials system have been the technology to beat in terms ofnoise performance, bandwidth and output power.Heterojunction bipolar transistors (HBTs) are growing inmaturity and applications, and are showing promise for bothpower and low noise applications as well. We will discuss thestate-of-the-art for the highest frequency MMIC poweramplifiers and low noise amplifiers to date, in all availabledevice technologies (HEMT, HIBT, InP, GaAs) and makesome projections about the future trends for solid statetransistor amplifiers beyond W-Band (75-110 GHz).

II. APPLICATIONS OFMMIC AMPLIFIERS BEYOND W-BAND

Historically, there have been few commercial applicationsabove W-Band for amplifier chips. The technology drivers forhigh frequency amplifiers have come from the space anddefense industries. Some of these applications includemillimeter-wave and submillimeter-wave receivers forastrophysics and earth remote sensing. The GaAs amplifierchips developed in [1] will drive the local-oscillator chains forterahertz mixers on the Herschel Space Observatory's HIghFrequency Instrument (HIFI). Herschel is a joint NASA/European Space Agency (ESA) mission to provide imagingand spectroscopy in the 400 GHz-1.9 THz region of thespectrum. [2]. The InP amplifier chips in [3] are being used toprovide local oscillator power in submillimeter-wave receiversin the Atacama Large Millimeter Array (ALMA). A similarchip in W-Band [4] is being considered as a local oscillatordriver on one of the receivers for the Conical MicrowaveImaging Sounder (CMIS), a weather satellite for the NPOESSnetwork. A novel application in the radar area is emerging forG-Band power amplifiers. G-Band Transmit/Receive (T/R)modules are being considered for future planetary entry,descent and landing applications, for highly accuratevelocimetry and altimetry measurements with a small antennasize. [5]. A medium power transmitter amplifier in G-Bandwould enable such an instrument for future planetary landingmissions. For most of these applications, having higher powerat a higher frequency would enable more measurements,science data, instruments, and possibly space missions.

III. MMIC POWER AMPLIFIERS AT W-BAND AND ABOVE

MMIC power amplifiers in the 100 GHz range are typicallyClass A amplifiers with power-added efficiencies of the 2-20% depending on the bandwidth of the amplifier. In general,the wider the bandwidth, the lower the overall gain per stageand the lower the efficiency. Most W-Band (or highfrequency) PAs are made with HEMTs, although a few HBTresults can be found in the literature. While the gate length ofa HEMT determines the cutoff frequency, it is the gate widthwhich determines the maximum output power that can beachieved with the device. A wider gate-width, larger peripherydevice will lead to more overall output power. To date, thehighest power MMIC chips utilize gate peripheries of 1.28mm. These large chips have been limited to W-Band operationdue to the very low impedance matching circuits required toproduce adequate gain in the chips. In Ref [1] and [6], both

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GaAs and InP HEMT technology are used with 1.28 mm gateperipheries in the output stage of the chips, leading to thehighest output power at 94 GHz to date (200-427 mW).

Cutoff frequencies in GaAs are lower than in InP due to thelower electron mobility in GaAs HEMTs, and GaAs HEMTshave been limited to W-Band use. Beyond 120 GHz, the onlyavailably technology capable of producing output powergreater than a few mW is currently InP. InP amplifier chipsbeyond W-Band typically run highly compressed in order toachieve output power levels in the tens of milliwatts range.At frequencies above 100 GHz, power-combining HEMT

devices in large combiner subcircuits becomes problematic.Eight-way device combiners are still possible in W-Band, butbecome unfeasible in G-Band due to the large relative size ofthe chip. While scaling of transmission lines is possible toreduce chip size, several other components in the MMIC (theHEMTs themselves and through-substrate vias) have fixedgeometrical sizes which cannot be scaled effectively withoutcompromising HEMT performance. The largest power-combining network reported above W-Band utilizes a 4-waycombiner.

In Fig. 1, we show the measured MMIC amplifier data formaximum reported output power versus frequency for the bestMMIC power amplifiers to date. In the 70-110 GHz range,large periphery InP and GaAs HEMT amplifiers dominate forhighest output power, with 200-400 mW possible in GaAs [1]and up to 427 mW reported for InP [6]. Metamorphic HEMTs(InP HEMTs grown on GaAs substrates) also report outputpower levels in this range [7,8] Smaller periphery InP mediumpower amplifiers for wide-band applications covering the fullwaveguide bands of WR1O (75-110 GHz) [4] and WR8 (90-140 GHz)[3,4] typically power-combine 2 or 4 devices toachieve output power levels in the 20-50 mW range. Severaldouble heterostructure bipolar transistor (DHBT) results inInP have also been reported in W-Band [9,10] under 100 mW.In G-Band (140-220 GHz), results include a medium powerInP HEMT MMIC with 15-20 mW from 140-170 GHz [11], aDHBT amplifier with 8 mW at 176 GHz [12], and a recent InPHEMT MMIC with 20 mW from 175-190 GHz [13].

There are several factors limiting high power MMICamplifiers at G-Band and above. These include the cutofffrequency of the larger periphery transistors, the relativelylower breakdown voltage of InP H1IEMT devices as comparedto GaAs, and the difficulty of increasing power-combining on-chip due to very low-impedance matching required, finite viasize, and metal-to-metal spacing lithography constraints.

In Fig. 2, we have plotted many of the results from Fig. 1 asoutput power versus output periphery for the HJEMT devices.HBTs are not included in this graph. Larger periphery devices( > 0.5 mm) have traditionally been restricted to W-Bandoperation, while smaller peripheries enable G-Band MMICPAs. As expected, the smaller periphery devices at higherfrequency can achieve output power on the order of 25 mW.

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Fig. 1. State of the art for MMIC power amplifiers to date:Maximum output power vs. frequency for various MMICtechnologies.

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IV. MMIC Low NOISE AMPLIFIERS (LNAs)

MMIC LNAs have been in wide use even for G-Bandfrequencies for several years. The main device technology forhigh performance LNAs is the InP HEMT. The LNA HEMTsare very similar to those used in power amplifiers, but havemuch shorter gate widths (typically 20-50 tm peripheries).This results in low current operation near the peaktransconductance of the device, hence is suited for low noise.A relative newcomer to the low-noise arena, antimonideHEMTs, also known as Sb-based or "antimonide-basedcompound semiconductors" (ABCS), have become interestingdue to the very low turn-on voltage required for low noiseoperation, which makes them particularly appealing for array

and imaging applications.In Fig. 3, we plot noise figure vs. frequency for a number of

room temperature LNAs reported in the literature. The solidline in Fig. 3 is a best fit to measured data as computed by S.Weinreb [14]. Highlights include a 5.5 dB noise figureachieved at 183 GHz [15] in InP and 3.9 dB noise figure at 94GHz in ABCS technology [16]. While the ABCS results are

promising, InP is still the technology to beat.20 __ _ _ _ _ _

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Fig. 3. Noise figures vs. frequency for low noise amplifier MMICsusing InP and ABCS technology. Black line indicates best-fitmeasured data [14].

We have summarized the state-of-the-art for MMIC LNAsin Table 1, where we list the frequency, amplifier description,gain, noise figure (if measured) and gain per stage for the datapoints in Fig. 3. Also included is the gate-length of the HEMTdevices. As with power amplifiers, the gate length determinesthe cutoff frequency and the maximum available/stable gain.

While the gain per stage is an important figure of merit, itshould be noted that the gain-per-stage for a given chip willdepend strongly on the bandwidth of the design. In addition tothe low noise figures reported from Fig. 3, some recent resultsare showing impressive frequency performance. MetamorphicHEMT amplifiers developed at Fraunhofer show gain beyond220 GHz, with impressive gain-per-stage results[29, 30]. Alsoincluded is the highest reported MMIC amplifier to date, with10 dB gain at 235 GHz from a 3-stage design, and gain up to260 GHz [31]. Future work will involve measuring the noisefigures of these MMICs beyond G-Band.

V. FUTURE TRENDS

Projections for MMIC HEMT amplifiers with gate lengthsshorter than 0.07itm indicate that over 6 dB gain per stage ispredicted at 300 GHz, for a gate length of 35 nm [19]. Themove to shorter gate lengths, compact dry-etched thru-substrate vias, and substrate thickness of 1 mil (25 rim) willgreatly improve device cutoff frequencies and gain per stage,enabling submillimeter-wave amplifiers for the first time.

ACKNOWLEDGEMENT

The author wishes to acknowledge the assistance of RichardLai of NGST, David Chow of HRL Laboratories, LLC, andTodd Gaier and Sander Weinreb of JPL for supporting data.This work was carried out at the Jet Propulsion Laboratory,California Institute of Technology, under a contract with theNational Aeronautics and Space Administration.

REFERENCES

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TABLE I: SUMMARY OF LNA MMIC AMPLIFIER DATAFrequency Technology LNA Description Noise Gain Gain per Stage Reference

[GHz] Figure77-105 0.1tm InP, 1999 4 stage (NGST/JPL) 3.0 dB 20 dB 5 dB [17]67-100 0.1tm InP, 2005 4 stage (HRL) 2.5 dB 26 dB 6.5 dB [18]91-97 0.1iim InP 2000 1-stage (NGST) 2.2 dB 8 dB 8 dB [19]70-105 InP MHEMT,2005 2 Cascode Stage (Fraunhofer) 2.5 dB 20 dB 5 dB [20]94 Sb-based, 2005 5-stage (RWSC) 3.9 dB 20 dB 4 dB [16]94 Sb-based, 2005 3-stage (NGST) 5.4 dB 11 dB 3.7 dB [21]

85-119 0.1tm InP, 2000 4-stage (HRL) 3.7 dB 20 dB 5 dB [22]155 0.1tm InP, 1997 3-stage (NGST) 5.1 dB 10 dB 3 dB [23]

150-215 0.1tm InP, 1999 6-stage (JPL/NGST) 8.1 dB 15-27 dB 3-4 dB [24]150-205 0.1tm InP, 1999 8-stage (HRL/JPL) N/A 17 dB 2 dB [25]

160-200 0.08itm InP, 2001 3-stage (NGST) 5.5 dB 15 dB 5 dB [15]180-205 0.1tm InP, 2001 3-stage (CSIRO/NGST) 12 ± 4 dB 15 dB 4 dB [26]150-215 0.07itm InP, 2005 3-stage (NGST) N/A 12 dB 4 dB [27]175 InP HBT, 2003 1-stage (UCSB) N/A 6 dB 6 dB [28]220 InP MHEMT, 2004 4-stage (Fraunhofer) N/A 20 dB 5 dB [29]220 InP MHEMT, 2005 1 Cascode Stage (Fraunhofer) N/A 10 dB 5 dB [30]235 0.07iim InP, 2005 3 stage (JPL/NGST) N/A 10 dB 3.5 dB [31]

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