Solid-State Electronics 55 (2011) 19–24

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Solid-State Electronics

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RF power performance evaluation of surface channel diamond MESFETs

V. Camarchia a,b, F. Cappelluti b,⇑, G. Ghione b, M.C. Rossi c, P. Calvani c, G. Conte c, B. Pasciuto d, E. Limiti d,D. Dominijanni e, E. Giovine e

a Italian Institute of Technology, Center for Space Human Robotics, 10129 Torino, Italyb Department of Electronics, Politecnico di Torino, 10129 Torino, Italyc Department of Electronic Engineering, Università di Roma Tre, Roma, Italyd Department of Electronic Engineering, Università di Tor Vergata, 00195 Roma, Italye IFN-CNR, Roma, Italy

a r t i c l e i n f o a b s t r a c t

Article history:Received 15 April 2010Received in revised form 2 September 2010Accepted 4 September 2010Available online 29 September 2010

The review of this paper was arranged byProf. A. Zaslavsky

Keywords:Polycrystalline diamondField effect transistor (FET)RF powerLoad-pullHydrogen terminated

0038-1101/$ - see front matter � 2010 Elsevier Ltd. Adoi:10.1016/j.sse.2010.09.001

⇑ Corresponding author.E-mail address: federica.cappelluti@polito.it (F. Ca

We experimentally investigate the large-signal radio frequency performances of surface-channel p-typediamond MESFETs fabricated on hydrogenated polycrystalline diamond. The devices under examinationhave a coplanar layout with two gate fingers, total gate periphery of 100 lm; in DC they exhibit a holeaccumulation behavior with threshold voltage Vt � 0–0.5 V and maximum drain current density of120 mA/mm. The best small-signal radio frequency performances (maximum cutoff or transition fre-quency fT and oscillation frequency fmax) were obtained close to the threshold and were of the order of6 and 15 GHz, respectively. The power radio frequency response was characterized by driving the devicesin class A at an operating frequency of 2 GHz and identifying through the active load-pull technique theoptimum load for maximum power added efficiency. A power gain in linearity of 8 dB and an outputpower of approximately 0.2 W/mm with 22% power added efficiency were obtained on the optimum loadimpedance at a bias point VDS = �14 V, VGS = �1 V. To the best of our knowledge, these are the first largesignal measurements ever reported for surface MESFET on polycrystalline diamond, and show the poten-tial of such technology for the development of microwave power devices.

� 2010 Elsevier Ltd. All rights reserved.

1. Introduction

The outstanding electronic and thermal properties of diamond(high breakdown field, large carriers’ mobility and saturationvelocity, thermal conductivity about 15 times larger than GaNand 50 than GaAs) make it, at least in principle, an attractive mate-rial for high power radio frequency (RF) and microwave applica-tions both in the area of power amplifiers and of high-powersources [1]. Although synthetic diamond-based FETs have beenthe object of extensive study for many years (with the first devicesdemonstrated at the beginning of 1990s [2]), recent advances ingrowth techniques for single-crystal [3] and polycrystalline dia-mond have renewed the interest in the research on high-perfor-mance diamond-based devices. In particular, the growth ofpolycrystalline diamond films with 100–200 lm grain size on sev-eral kinds of large-area 4 in. substrates, has allowed to overcomethe area limitations (a few square millimeters) of single-crystalfilms, thus leading to the possible development of power devicelayouts. Concerning available techniques for the control of dia-mond conductivity (up to now p-type), during the last few years

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ppelluti).

two main approaches have emerged [4,5]. The first approach relieson extrinsic acceptor doping with boron, while the second exploitshydrogen (H) surface termination, that induces a quasi-2D holechannel a few nanometers below the surface [6]. Although the de-tailed underlying physical mechanism leading to the induced holechannel has not been conclusively established, see [7–14], theresulting charge control scenario is somewhat similar to the onefound in conventional AlGaAs/GaAs n-type HEMTs, an analogy thatcan be exploited in the development of compact large-signal de-vice models.

From the results available in the literature, the current develop-ment of FETs with delta-doped boron-doped channels [15] appearstoday to lag somewhat behind the one of H-terminated surface-channel FETs, based either on metal–insulator–semiconductor(MISFET) [16] or metal–semiconductor (MESFET) structures [17–20], which have demonstrated cutoff frequencies in the GHz range.Within the context of H-terminated MESFETs, much research effortis today focused on fully explaining the physical mechanismunderlaying H-termination induced p-type conductivity and onidentifying a proper surface treatment allowing for time-stableperformance [21]. H-terminated MESFETs with 0.1 lm gate lengthon polycrystalline diamond achieving 45 GHz cutoff frequencyhave been reported in [19].

Fig. 2. Scanning electron microscope (SEM) image of the device active area.

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While promising small-signal performances of both polycrystal-line and single-crystal diamond MESFETs have been reported in theliterature, only a few examples [17,18] of RF power measurementson single-crystal devices are available so far, possibly due to insta-bility problems related to a not yet optimized treatment of the H-terminated surface; record performances were reported, albeit at acomparatively low frequency (1 GHz), with a power density of2 W/mm [18].

In this paper, we present for the first time results on the large-signal RF characterization at 2 GHz of submicron H-terminatedMESFETs grown on polycrystalline diamond, showing the potentialof such a substrate for the development of diamond-based micro-wave power devices. Section 2 is devoted to the description of thedevice structure and realization. Section 3 presents the DC andsmall-signal device performances, while in Section 4 the activeload-pull power characterization is described. Conclusions are fi-nally reported in Section 5.

Fig. 3. Surface hole density as evaluated from C–V measurements.

2. Device structure and realization

The MESFET devices were fabricated on a 250 lm thick poly-crystalline 1.0 � 1.0 cm2 diamond substrate supplied by ElementSix, with surface roughness less than 50 nm and average grain sizeof about 80–100 lm. The smoothed film surface was treated in ahigh-temperature microwave hydrogen plasma by using an AstexPDS19 CVD system. The reaction chamber was initially heated at600 �C under vacuum; once the temperature was stabilized, a puri-fied hydrogen flow-rate of 500 sccm was established maintainingthe chamber pressure around 80 Torr. The hydrogen plasma wasthen ignited at 2.1 kW for 10 min. After the treatment, the chamberwas left to decrease naturally to room temperature under argonflow before the sample removal. As shown in Fig. 1, a butterflyshaped layout with optimized coplanar two-finger gate, able to re-duce the input–output impedance mismatch, has been designed inorder to ensure high-frequency device operation. The device activearea was defined by reactive ion etching (RIE) in oxygen and argonplasma, achieving electrical isolation among FET structures . Theisolation pattern was established in order to force the drain tosource current to flow underneath the gate, thereby avoiding par-asitic resistance effects. Deep submicron I-shaped gate electrode

Fig. 1. Schematic of the device structure.

with length LG down to 200 nm and widths in the range 50–200 lm have been realized by single-layer electron beam lithogra-phy. Gold and aluminum have been used for the realization of thedrain and source ohmic contacts, and the gate Schottky contact,respectively.

A self-aligned gate technology was employed to reduce thedrain to source effective channel length down to less than2.5 lm. A detail of the realized devices on polycrystalline diamondis reported in Fig. 1, while Fig. 2 shows a scanning electron micro-scope (SEM) image of the device active area. The channel, locatedabout 6 nm under the surface, is completely activated already atroom temperature, with surface carrier concentration around1013 cm�2, as evaluated from C–V measurements, see Fig. 3. Thechannel hole mobility was estimated both indirectly from the DCtranscharacteristics and from Hall-mobility measurements to beabout 95 cm2 V�1 s�1.

3. DC and RF small-signal characteristics

The MESFET devices (with two 0.2 lm length gate fingers andtotal periphery of 100 lm) were first characterized in DC, showinga maximum drain current of about 120 mA/mm (see Fig. 4). Thetranscharacteristics and gate bias dependence of the extrinsic

Fig. 4. Output characteristics of a 2 � 50 lm device with VDS from 0 V to �14 V andVGS from 0.9 V to �3 V (upper curve).

-4 -3 -2 -1 0 1 2 30

20

40

60

80

100

120

-4 -3 -2 -1 0 1 2 3-10

0

10

20

30

40

50

Fig. 5. Transcharacteristics (above) and transconductance gm (below) of the samedevice of Fig. 4.

Fig. 6. Scattering parameters measured at VDS = �14 V, VGS = �0.9 V (above) andVDS = �14 V, VGS = +0.3 V (below).

V. Camarchia et al. / Solid-State Electronics 55 (2011) 19–24 21

transconductance are reported in Fig. 5. The enhancement-likebehavior is characterized by a threshold voltage around 0.5 V andmaximum extrinsic transconductance around 40 mS/mm.

The measured DC output characteristics clearly show that in thelinear region (see Fig. 4 for �4 V 6VDS 6 0 V) the drain current isalmost independent from VGS, i.e. the output conductance for

VDS ? 0 is not controlled by the gate voltage, contrasting what hap-pens in conventional FETs. This phenomenon is related to the factthat the channel charge-control model exhibits, after a regionwhere the sheet hole density linearly depends on VGS, saturationof the channel mobile charge for large VGS absolute values. Withinthe framework of a device model based on the gradual channelapproximation, due to the interplay between the gate and drainpotential, the saturated charge-control model yields the typicalsaturated transcharacteristics shown in Fig. 5 (above). Such acharge-control model exhibiting saturation is formally similar tothe tanh control model proposed for AlGaAs/GaAs HEMTs by Rodinand Roblin [22]. Preliminary results indeed suggest that thisHEMT-like charge-control model can be effectively exploited, to-gether with multi-bias scattering parameter measurements, to de-velop large-signal models based on the Chalmers approach [23];results are here omitted for brevity and a full discussion will bepresented elsewhere.

Fig. 7. Above: dependence of the gate-to-source capacitance CGS and transconduc-tance gm as a function of VGS, VDS = �14 V. Below: dependence of CGS as a function ofVDS at VGS = �0.9 V and VGS = +0.3 V.

Fig. 8. Dependence of the cutoff frequency fT as a function of VGS, VDS = �14 V(above) and VDS, VGS = �0.9 V (below), as evaluated from the multi-bias small-signalequivalent circuit.

Fig. 9. Short-circuit current gain h21 and Maximum Available Gain (MAG) atVDS = �14 V, VGS = �0.9 V (solid line) and VDS = �14 V, VGS = +0.3 V (dashed line).

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The small-signal RF characterization was carried out in therange 0.1–20 GHz at different quiescent points. Two examples ofmeasured scattering parameters are reported in Fig. 6 at two differ-ent bias points: VDS = �14 V, VGS = �0.9 V close to the maximum gm

(Fig. 6 (above)) and VDS = �14 V, VGS = 0.3 V (Fig. 6 (below)) close tothe threshold. Multi-bias small-signal characterizations have beenexploited in order to develop a large-signal dynamic equivalent cir-cuit of the device, modeling the bias dependence of the small-sig-nal transconductance and reactive elements. Fig. 7 (above) showsthe extracted behavior of the gate-to-source capacitance CGS andsmall-signal intrinsic transconductance gm as a function of VGS, atVDS = �14 V. The gate-to-source capacitance increases as the device(i.e. the 2D hole channel) is turned on, as expected for MOSFETs orheterostructure-based FETs. The maximum intrinsic gm is esti-mated about 48 mS/mm, with an extracted parasitic series resis-tance Rs � 54 X. Then, at negative VGS (under the peak) the RF gm

falls off with a lower rate with respect to the DC transconductance,indicating a larger output power attainable under RF with respectto DC operation, as already reported e.g. in [24]. Such behavior(that has also been observed in conventional AlGaAs/GaAs HEMTs,and has been attributed to the freezing of ionized donor atoms inthe AlGaAs supply layer [25]) could be correlated to an equivalentacceptor doping model going beyond the transfer doping model,see e.g. [11–14]. Fig. 7 (below) reports the extracted behavior ofCGS as a function of VDS at VGS = �0.9 V (close to the maximumgm) and at VGS = +0.3 V (close to the threshold). On the other hand,the intrinsic transconductance turned out to be mildly dependenton VDS.

Fig. 8 reports the cutoff frequency fT calculated from the ex-tracted bias-dependent gm and CGS; both the intrinsic fT and theextrinsic one, accounting for the parasitic source resistance andgate pad capacitance �29 fF, are considered. The obtained biasdependence of gm and CGS suggests a dependence of the cutoff fre-quency (fT) on (VGS, VDS) in good agreement with the measured fre-quency response shown in Fig. 9. In fact, as already reported inliterature (see e.g. [17]), these devices usually show optimum fT

Fig. 10. Simplified scheme of the load/source-pull system with time-domain waveform capabilities of Politecnico di Torino.

Fig. 11. Load-pull map of the 2 � 50 lm device operating at 2 GHz, and biased inclass A (VDS = �14 V and VGS = �1 V). The marker shows the location of the optimumload for maximizing the PAE.

Fig. 12. Power sweep on the PAE optimum load (CL = 0.79, 4�). Power gain (dots),output power normalized to 1 mm total gate periphery (squares) and PAE(triangles) for VGS = �1 V and VDS = �14 V.

V. Camarchia et al. / Solid-State Electronics 55 (2011) 19–24 23

and fmax close to the threshold (see Fig. 9). According to our results,the observed dependence on fT with VDS can be mostly attributed tothe decrease of CGS with VDS. A slightly higher cutoff frequency wasobtained with the same process, by exploiting higher quality (andmore expensive) polycrystalline substrates, see [20].

4. RF power performance

The evaluation of RF power performance was carried out at2 GHz, under standard class A operation (VDS = �14 V, VGS = �1 V),employing a nonlinear test bench based on the active load-pulltechnique [26–28]. A simplified scheme of the set-up is shown inFig. 10.

The RF waveforms are measured in real-time at the DUT refer-ence planes by means of a Vector Network Analyzer (VNA). Theoutput loading conditions are changed at the fundamental fre-quency by means of the active loop, see Fig. 10. The active tech-nique allows for reflection coefficients up to, or even above unity.This is a fundamental feature in the characterization of diamond-based devices of small periphery, since their optimum loads typi-cally exhibit a reflection coefficient with magnitude close to unity,that cannot be synthesized, due to losses, with conventional pas-sive tuners [29]. A classical VNA calibration between port 1 and2 (e.g. TRL, LRM) is completed with absolute power at port 3 [28].

According to the load-pull approach, the optimum load can beinvestigated for several performances, typically the maximum

power, the maximum power added efficiency (PAE), the minimumdistortion (under a two-tone test). In the present work we concen-trated on the optimum PAE goal, taking into account that thepower can be also increased by pushing the drain bias towards lar-ger absolute values than 14 V; this, however, was not done to avoiddevice damage. An example of PAE load pull map is shown inFig. 11; we found an optimal load for maximum PAE of CL = 0.79,phase 4�. As expected the optimum load has a small inductive reac-tive part compensating the output capacitance; due to the compar-atively low frequency, however, the resistive part is dominant.Fig. 12 shows the results of a power sweep for a 100 lm device,with the device terminated on the optimum load. The maximumoutput power density is 0.2 W/mm with 22% PAE, and linear powergain of 8 dB. Assuming a maximum drain current around 11 mA fora 100 lm device, the maximum output power value is compatiblewith a peak RF voltage around 9 V (consistent with the given biasand with a knee or saturation drain voltage of �5 V), whichamounts to a maximum instantaneous drain voltage of �23 V.The theoretical value of the drain efficiency obtained with suchvalues is around 30%, which further decreases to the PAE due to fi-nite gain. Larger efficiencies could be reached by increasing thedrain bias, but, as already mentioned, this could ultimately leadto irreversible device damage.

During the large-signal load-pull characterization the tested de-vices underwent several RF power sweeps (20–30) without anyappreciable DC or RF performance degradation. DC degradation fol-lowing RF test was reported e.g. in [30] for devices on single-crystal

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diamond due to surface problems which were then claimed to besolved in [31]. In [32], Kasu and coworkers observed a drain cur-rent degradation induced by increase of gate leakage duringpulsed-DC tests. However, for the present device, we did not ob-serve any increase of gate current (which remained always lowerthan about 1 lA up to the maximum applied DC gate bias of�4 V) either under DC and RF measurements. Extensive reliabilitytests in single-crystal or polycrystalline diamond devices have notbeen reported yet as far as we know and were anyhow beyond thescope of the present work, also considering that the contact tech-nology exploited does not allow repeated measurements on thesame device for probe scratching (for adhesion problems) and, asa consequence, any significant aging test.

From the reported RF power characterization, the extrapolatedoutput power density at 1 GHz is about 0.8 W/mm; such value,though still much lower than the expected material limit, pointsout the potential of this technology on polycrystalline diamondwhen compared to the record performance of 2 W/mm on single-crystal diamond [18] at 1 GHz.

5. Conclusions

We have experimentally investigated the RF small-signal andpower performances of surface channel diamond MESFETs fabri-cated on hydrogenated polycrystalline diamond. The realized de-vices exhibit a threshold voltage between 0 V and 0.5 V,maximum transconductance and DC drain current around 40 mS/mm and 120 mA/mm, respectively. An optimum fT = 6 GHz andfmax = 14.8 GHz were detected from the small-signal RF character-istics. Large-signal power measurements provided a RF outputpower of 0.2 W/mm at the fundamental frequency of 2 GHz with8 dB gain and 22% PAE at a bias point VDS = �14 V, VGS = �1 V;the load for maximum PAE was obtained through active load-pullmeasurements. The power density extrapolated at 1 GHz, albeitlower than the one obtained from single-crystal diamond devices,suggests the potentiality of polycrystalline diamond MESFETs forpower RF applications and is expected to have space for improve-ment through technology optimization.

Acknowledgements

The work was partly supported by the COFIN2007 projectM3ICGAN. Helpful discussions with Fabrizio Bonani and MarcoPirola (Dipartimento di Elettronica, Politecnico di Torino) are grate-fully acknowledged.

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