IEEE ELECTRON DEVICE LETTERS, VOL. 35, NO. 3, MARCH 2014 321

0.2-μm InP/GaAsSb DHBT Power PerformanceWith 10 mW/μm2 and 25% PAE at 94 GHzMohammed Zaknoune, Etienne Okada, Estelle Mairiaux, Yannick Roelens, Damien Ducatteau,

Peter Frijlink, Marc Rocchi, and Hassan Maher

Abstract— We report a 94-GHz large-signal load-pull charac-terization of InP/GaAsSb double heterojunction bipolar tran-sistors. The investigated devices have an emitter area of0.20 × 9.5 µm2. Biased for highest power added efficiency (PAE),an output power of 6.62 mW/µm2 (11 dBm), a power gainof 5.2 dB, and a PAE of 27.7% have been obtained. Biasedfor highest output power, 10.26 mW/µm2 (12.8 dBm) has beenachieved without significant degradation of the PAE (25.2%) andthe power gain (4.5 dB).

Index Terms— DHBT, GaAsSb/InP, power, load pull, 94 GHz.

I. INTRODUCTION

THE demand in terms of power generation and poweramplification in the millimeter and sub-millimeter waves

range i.e. 100 GHz−1THz is becoming more and morecrucial. Many applications require sources as compact aspossible for generation or power amplification in particular fortransmission in free space [1], [2]. Among these applications[3], [4] it can be mentioned point-to-point gigabit wirelesscommunications, active imaging, collision avoidance radars,security, THz spectroscopy. For all of these systems solid stateelectronics is a promising way.

Solid state electronics dedicated for high power need activedevices, i.e. bipolar or field effect transistor, at least withvery high bandwidth and high breakdown voltage. To date,GaN high electron mobility transistor technology (HEMT)seems to be a strong candidate for high power applicationsbelow 100 GHz. GaN MMIC technology presents impressiveresults with high output power up to 5 W in W-Band [5]–[7].Regarding the silicon based technology, a very high outputpower of 10 dBm corresponding to 18.4 mW/μm2 havebeen demonstrated using a SiGe HBT (Heterojunction BipolarTransistor) [8]. Unfortunately, SiGe HBT is very limited bya low breakdown voltage BVceo. Typically for a 300 GHz-ft SiGe HBT, the BVceo is about 1.7 V [9]. From the BVceopoint of view, III-V based devices, in particular HBTs are

Manuscript received November 25, 2013; revised December 19, 2013;accepted December 28, 2013. Date of publication January 16, 2014; dateof current version February 20, 2014. This work was supported by the FrenchGovernment through ANR. The review of this letter was arranged by EditorM. Östling.

M. Zaknoune, E. Okada, E. Mairiaux, Y. Roelens, and D. Ducatteau arewith the Institut d’Electronique de Microélectronique et de Nanotechnologie,Villeneuve d’Ascq Cedex 59652, France (e-mail: zaknoune@iemn.univ-lille1.fr).

P. Frijlink, M. Rocchi, and H. Maher are with the Research andDevelopment, OMMIC, Limeil-Brévannes 94450, France.

Color versions of one or more of the figures in this letter are availableonline at http://ieeexplore.ieee.org.

Digital Object Identifier 10.1109/LED.2014.2298251

more apt for key applications which demand a high voltagesweep to drive critical components (e.g. Mach-Zehnder opticalmodulator used for high data rates fiber optic communicationsystems).

Thus, the way is open for III-V HBT technology whichcombines high breakdown voltage with high frequency gainand as a result high power-added-efficiency (P.A.E) [10]–[12].In this frame, the double heterojunction bipolar transistor(DHBT) on InP substrate is one of the most promisingcandidate. Indeed, beyond 100 GHz, it offers high frequencyoperation and high breakdown voltage. Whether for the typeI-InGaAs/InP DHBT or for the type II-GaAsSb/InP DHBT,the potential operating frequency is included between 100 GHzand 1 THz, while providing a breakdown voltage BVceo higherthan 4.5 V [13]–[15].

In this letter, we present power characterization at 94 GHzof type-II InP/GaAsSb DHBTs. Devices with emitter-basejunction width of 0.20 μm and a device length of 9.5 μmhave been investigated. Large signal characterization has beenperformed in a load-pull system. When biased for highestoutput power, the devices achieved 12.8 dBm corresponding toa power density of 10.26 mW/μm2 with a P.A.E and a powergain of 25.2% and 4.5 dB respectively.

II. EPITAXIAL AND DEVICE FABRICATION

The structure was grown by MOCVD at OMMIC ona 3 inch diameter (100) semi-insulating InP:Fe substrate.The epitaxial structure of the DHBT consists of a 2500 Ån++-doped InP/In0.53Ga0.47As sub-collector, a 1300 Å InPcollector with a Si doping of 1 × 1017 cm−3, a 250 ÅGaAs0.51Sb0.49 base with a C-doping of 6 × 1019 cm−3,a 350 Å InP emitter with a Si doping of 6 × 1017 cm−3 anda 1200 Å n++InP/n++InGaAs emitter contact. The DHBTswere fabricated using a triple-mesa process described in ourprevious work [15].

III. POWER MEASUREMENT SETUP AND RESULTS

The InP/GaAsSb DHBT devices characterized in this letterhad an emitter-base junction area of 0.20 × 9.5 μm2, asdetermined by SEM. Typical common-emitter Ic(Vce) andGummel characteristics are shown in Fig. 1. The measuredDC current gain at Vcb = 0 V shows a peak value ofβ = IC/IB ∼10.5. The breakdown voltage BVceo, defined atJc = 10 μA/μm2, is 4.6 V for a 1300 Å InP collector layer.In our previous work, the small signal RF performancesobtained for a 0.55 × 3.5 μm2 device, determined by thede-embedding and the extrapolation of the |H21|2 and the

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322 IEEE ELECTRON DEVICE LETTERS, VOL. 35, NO. 3, MARCH 2014

Fig. 1. Common-emitter Ic(Vce) characteristics for a 0.20 × 9.5 μm2 DHBT.Inset: Corresponding Gummel plots at Vcb = 0 V.

Fig. 2. Block diagram of the power measurement setup in W-Band.

U were around 300 and 500 GHz for ft and fmax , respec-tively [15]. In the present study, for the 0.20 × 9.5 μm2

devices, RF performances ( ft and fmax) were around 300and 400 GHz, respectively. The lower fmax obtained forthe longest devices is due to the increased collector-basecapacitance Cbc which exceed the expected reduction of thebase resistance. Here, the total base resistance was principallylimited by its extrinsic part in relation to its high specific basecontact resistivity of 5 × 10−7 � · cm−2 [15].

The power measurement setup that has been used to carryout on wafer load-pull measurement in W-Band is shownin Fig. 2. This power setup has been developed in-houseand based on a high power network analyzer ZVA110 fromRohde&Schwarz. The latter is associated in input with afrequency extender in W-Band which has been developedby Radiometer Physics GmbH and in output with a veryhigh repeatable “slide screw” tuner which has been developedby Maury Microwave. This tuner has been characterized byS-measurements for all mechanical positions in order tocover the maximum load impedance in the Smith chart. Interms of tuner repeatability, minimum error vector magnitudeis close to −45 dB for all positions. A multiplier (×6)connected to the network analyzer synthesizer source providesthe incident wave. A power amplifier in W-Band has beenadded in order to increase the available power and could injecta maximum incident power level until 16 dBm at 94 GHz intothe DUT (device under test) in microwave probes environment.Each part of the power set-up has been characterized in orderto determine the errors terms for power calibration.

The interest of this setup is based on two points. Thefirst one is based on the measurement of the input reflected

Fig. 3. Power characteristics of 0.20 × 9.5 μm2 InP/GaAsSb DHBT.The base current is 2 mA and the voltage collector is 2 V.

Fig. 4. Power characteristics of 0.20 × 9.5 μm2 InP/GaAsSb DHBT.The base current is 2.6 mA and the voltage collector is 2.8 V.

coefficient of the DUT loaded by the tuner during large signalmeasurement, thanks to the high power W-Band frequencyextenders. As a result, this setup allows an accurate mea-surement of the device injected power leading to a confidentdetermination of the P.A.E. The second one is based onthe achievable reflection coefficients. In order to reduce thelosses, coplanar probes have been specifically designed andthe output probe is directly connected by a very short link tothe tuner. On the whole, both the probe and wave guide allowa maximum reflection coefficient of 0.7 at 94 GHz.

In this letter the device was biased at a forced base currentcondition. First of all, the bias point was extracted from theIce(Vce) to operate in class A under 2 V of Vce. At the biaspoint, HBT small signal S22 measured at 94 GHz is used todetermine load-pull impedance. Then, in order to apply theload at the HBT output which allows a maximum outputpower, the tuner has to present the conjugated impedanceobtained from the S22*. The load impedance was finallyoptimized for reaching the maximal output power by littlevariation of the load impedance in the Smith chart aroundthe pre-established value. The output power was measuredwith absorbed power being swept at constant Ib and Vcebias.

Fig. 3 shows a typical result of the power characterization ofa 0.20 × 9.5 μm2 device biased at Vce = 2 V and Ib = 2 mA.The obtained collector current density is about 12 mA/μm2.The load impedance was set at (27.1 + j 18.2) � forhigh power gain and optimal power-added-efficiency. A linearpower gain of 9.2 dB and l-dB compressed output power of

ZAKNOUNE et al.: 0.2-μm InP/GaAsSb DHBT POWER PERFORMANCE 323

Fig. 5. Cartography of the a) Output Power, b) Power Gain and the c) Power Added Efficiency of 0.20 × 9.5 μm2 InP/GaAsSb DHBT biased at Vce = 2 Vand Ib = 2 mA.

8.4 dBm were demonstrated. The maximum output power was11 dBm corresponding to a power density of 6.62 mW/μm2

(1.32 W/mm). The corresponding P.A.E and power gain were27.7 % and 5.2 dB respectively.

In order to obtain the maximum output power, the devicehas subsequently been biased at higher collector currentand higher collector voltage. The base current was set atIb = 2.6 mA and the Vce at 2.8 V. The corresponding collectorcurrent density is about 16 mA/μm2. Fig. 4 shows the powercharacteristics at this bias point and under the same loadimpedance ((27.1 + j 18.2) �).

In this case, typical results show a linear power gainof 8 dB and l-dB compressed output power of 11.5 dBm.The maximum output power was 12.8 dBm correspondingto a power density of 10.26 mW/μm2 (2.05 W/mm). Thecorresponding P.A.E and power gain were 25.2% and 4.5 dB,respectively. At the same base current, devices have beencharacterized up to 3 V, however, with no more output power.

Compared to HEMT technology and excluding HBT-MMICstudies, fewer results have been published in power atW-band on individual transistor. T. Jensen et al. have reporteda power density of 3.7 mW/μm2 at 96 GHz on an InP/InGaAsDHBT [16]. The previous result of InP/GaAsSb DHBT [14]exhibited an output power of 10.5 dBm corresponding to apower density of 1.65 mW/μm2 at 40 GHz.

In Fig. 5, a load map has been generated in order to definethe optimum load for the three parameters such as outputpower, power gain and power added efficiency. The device wasbiased at Vce = 2 V and Ib = 2 mA and the absorbed powerhas been set at 3-dB compression of output power. As we cansee, a high output power level can be obtained in a large area ofthe Smith chart and the two other parameters are relatively wellmatched around the optimum load impedance, pre-defined fora maximum output power. From our experience, this is not thecase for the HEMT, where each area can be strongly separatedin the Smith chart. Thus, for the HBT, this behavior allows theoptimization of the device not only for one parameter or onekind of precise application, but for all parameters together.

IV. CONCLUSION

The large-signal performance of InP/GaAsSb DHBTshas been reported at 94 GHz. The obtained maximumoutput power was 12.8 dBm corresponding to a power densityof more than 10 mW/μm2 with a P.A.E of more than 25%.

This letter demonstrates also the remarkable ability of theInP/GaAsSb DHBT, which allows simultaneous optimizationof the output power, power gain and power-added-efficiency.

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