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MESFETs on H­terminated Single Crystal Diamond

Paolo Calvani, Maria Cristina Rossi, Gennaro Conte, Stefano Carta, Ennio Giovine, Benedetto Pasciuto, Ernesto Limiti, Federica Cappelluti, Victor Ralchenko, A. Bolshakov and G. Sharonov

MRS Proceedings / Volume 1203 / 2009DOI: 10.1557/PROC­1203­J15­03

Link to this article: http://journals.cambridge.org/abstract_S1946427400015414

How to cite this article:Paolo Calvani, Maria Cristina Rossi, Gennaro Conte, Stefano Carta, Ennio Giovine, Benedetto Pasciuto, Ernesto Limiti, Federica Cappelluti, Victor Ralchenko, A. Bolshakov and G. Sharonov (2009). MESFETs on H­terminated Single Crystal Diamond. MRS Proceedings,1203, 1203­J15­03 doi:10.1557/PROC­1203­J15­03

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MESFETs on H-terminated Single Crystal Diamond

P. Calvani1, M.C. Rossi

1, G. Conte

1, S. Carta

1,2, E. Giovine

2, B. Pasciuto

3, E. Limiti

3, F.

Cappelluti4, V. Ralchenko

5, A. Bolshakov

5, G. Sharonov

6

1

Electronic Engineering Dept., Roma Tre University, Roma, Italy 2

IFN-CNR, Roma, Italy 3 Electronic Engineering Dept., Tor Vergata University, Roma, Italy

4 Department of Electronics, Politecnico di Torino, Torino, Italy

5 General Physics Institute, Russian Academy of Science, Moscow, Russia

6 Institute of Applied Physical Problems, Belarus State University, Minsk, Belarus

ABSTRACT

Epitaxial diamond films were deposited on polished single crystal Ib type HPHT

diamond plates of (100) orientation by microwave CVD. The epilayers were used for the

fabrication of surface channel MESFET structures having sub-micrometer gate length in the

range 200-800 nm. Realized devices show maximum drain current and trasconductance

values of about 190 mA/mm and 80 mS/mm, respectively, for MESFETs having 200 nm gate

length. RF performance evaluation gave cut off frequency of about 14 GHz and maximum

oscillation frequency of more than 26 GHz for the same device geometry.

INTRODUCTION

Current semiconducting materials do not offer high power RF (> 8 GHz) devices in

simple solid state device configurations, required for compact MMIC usually employed for

communication and radar applications. Among wide band gap materials, diamond has by far

the optimum material characteristics allowing for, at least in principle, the best power

amplification per unit gate length at microwave frequencies, to be employed in the fields of

electrical power management and wireless communications.

Owing to the encouraging high frequency and power performance, the technology of

hydrogenated diamond is currently receiving much interest for the fabrication of surface

channel metal semiconductor field effect transistors (MESFETs) [1-3]. In this structure,

hydrogen termination of diamond together with carrier transfer into surface acceptor states

promotes an upward surface band bending which gives rise to a space charge extending

several nanometers below the surface without the addiction of extrinsic doping impurities. Here

holes are confined perpendicularly to the surface from the electrostatic field arising from the

charge separation (positive charge in diamond and negative on the surface acceptors) whereas

are free to move in parallel to the surface [4], where they form a 2DHG closely following the

surface topography. In this way, surface hole densities up to 1013

cm-2

and mobility values

about 100-200 cm2/Vs cm

-2 are typically achieved [5].

In this context, two different strategies are currently pursued for surface channel device

realization, based on polycrystalline and on single diamond substrates. In the former case

large substrates required for electronic applications are already available, although achieved

results still appear to be affected by the polycrystalline sample structure. At variance, the

Mater. Res. Soc. Symp. Proc. Vol. 1203 © 2010 Materials Research Society 1203-J15-03

realization of diamond electronics on epitaxial layer appears to be limited to small areas (few

squared mm), although encouraging results both in terms of high frequency and power density

can be found in literature. In this paper we present results achieved for devices fabricated on

homoepitaxial diamond layers grown on HPHT diamond substrates by Microwave Plasma

Enhanced Chemical Vapor Deposition (MPCVD).

EXPERIMENTAL

Butterfly shaped Metal Semiconductor Field Effect Transistors (MESFETs) have been

fabricated on Single Crystal Diamonds. Homoepitaxial diamond layers of 0.5 and 1.0 µm

thickness have been grown by MPCVD in CH4 (2%)-H2 gas mixtures on (100) oriented HPHT

diamond plates polished to the surface roughness Ra ~4 nm. The deposition has been terminated

in a microwave hydrogen plasma (for 10 min) in order to induce typical band bending,

responsible for the generation of a p-type conductive surface channel with sheet resistivity of

~20 kΩ/. Ohmic drain and source contacts have been realized with gold evaporation on 1 µm

thick and 4.0 X 4.0 um2 area films while aluminum was used for self-aligned gate Schottky

contact. Electron Beam Lithography (EBL) has been used for the realization of gate length from

800 nm down to 200 nm. Adopted device scheme is sketched in figure 1.

Following a DC characterization performed by using a probe station equipped with a

HP4140B picoammeter, bias dependent S-parameter measurements of devices under test have

been performed in the 50MHz÷20GHz frequency range. S-parameters were measured by a

VNA calibrated with off-wafer SOLT procedure. The measurement set-up consists in: HP

8510C Vector Network Analyzer, Cascade RF1 Probe Station, Coplanar Probe GGB

Picoprobe (pitch=200µm), CS5 Calibration kit.

RESULTS AND DISCUSSION

Figure 2 shows the DC-output characteristics of a surface-channel-MESFET with 200

nm gate length Lg and 25 µm gate width W, in the gate bias voltage range -2.5-1 V. A

maximum drain to source current of 180 mA/mm has been obtained. As shown, short gate

length devices exhibit a non complete channel closure, even when the gate junction is slightly

forward biased. Such a behavior is tentatively related to short channel effects, although the

existence of a residual conductive channel uncontrolled by the gate electrode cannot be ruled

out.

Figure 1. Device scheme cross section.

DC input characteristics of the same device are reported in figure 3. At variance with the

usually found square law dependence of IDS on VGS, indeed detected for devices with 400 and

800 gate length, an almost linear trend, usually observed in short channel devices, is detected

for Lg=200 nm, when the device is operating in the saturation region, with VDS ≥ 4 V. For

VDS < 4 V a linear trend is still observed, although exhibiting a smaller slope and holding in a

different VGS interval. Furthermore, a small but not negligible drain to source current is

detected for low VGS and related to sub-threshold conduction processes. The onset of short

channel effects can be also evidenced from the equally spaced output current-voltage

characteristics and is further corroborated by a threshold voltage VT shifting from 0 to 0.5 V,

as detected when the gate length is changed from 800 to 200 nm (not shown).

Since a negative gate to source voltage is able to modulate the channel current, a quasi-

enhancement behavior has been attributed to the diamond FET. Field effect holes mobility

extracted from input characteristics appears field dependent at low VDS, approaching a

constant value of about 150 cm2/Vs for VDS ≥ 4 V, when a constant slope of the trans-

characteristics is found. Such a field effect mobility value is also confirmed by Hall

measurements performed on un-gated hall bar structures. The corresponding trans-

conductance is reported in figure 4. The maximum value reached for VDS=-10.0 V was more

than 80 mS/mm. As usually reported in hetero-structure devices, a transconductance peak

slightly shifting toward high VGS is found for increasing VDS.

The measured frequency dependence of the current gain |H21|2 and Maximum Available Gain

(MAG) obtained at VDS=-10 V and VGS=-0.3 V are reported in figure 5 for the same device

with Lg=0.2 µm and W=25 µm. Device show a fMAX of more than 26 GHz with a gain of 22

dB at 1 GHz while the achieved threshold frequency fT was about 14 GHz.

-200

-160

-120

-80

-40

0-20.0-16.0-12.0-8.0-4.00.0

I DS (mA/mm)

VDS (V)

∆∆∆∆VGS=0.25 V

VGS=-1.75 V

VGS=0.25 V

Figure 2. Current voltage output characteristics for a MESFET with 200 nm gate length.

-200

-160

-120

-80

-40

0

-2.0-1.5-1.0-0.50.00.51.0

I DS (mA/mm)

VGS (V)

VDS=-5.0 V

VDS=-1.0 V

∆∆∆∆VDS=1.0 V

Figure 3. Current-voltage input characteristics for a MESFET with 200 nm gate length. Continuous

lines are only guidelines.

0

20

40

60

80

100

-1.5-1.0-0.50.00.51.0

Transconductance (mS/mm)

VGS (V)

VDS=-5.0 V

VDS=-1.0 V

∆∆∆∆VDS=-0.5 V

Figure 4. Typical MESFET transconductance with MESFET with 200 nm gate length. Continuous

lines are only guidelines.

-10

0

10

20

30

40

0,1 1 10 100

Gain (dB)

Frequency (GHz)

MAG

Current Gain

Vgs=-0.3V; V

ds=-10V

IDS=-1.5 mA

Lg=0.2 um; W

g=100 um

Current Gain

@1GHz = 22.3 dB

fT=13.9 GHz

fmax=26.1 GHz

Figure 5. Current gain and Maximum Available Gain (MAG) for a MESFET with 200 nm gate length and gate

width W=25 µm.

CONCLUSIONS

Epitaxial diamond films have been grown by MPCVD on polished single crystal Ib

type HPHT diamond plates of (100) orientation. Such epilayers have been employed for the

fabrication of surface channel MESFET structures having sub-micrometer gate length (200-

800 nm). A detailed DC and RF characterization has been carried out on realized devices,

giving maximum IDS and trasconductance values of about 190 mA/mm and 80 mS/mm,

respectively, for MESFETs having 200 nm gate length. RF performance evaluation have been

carried out through S parameters analysis, leading to fT ~14 GHz and fmax 26 ~GHz for the

same device geometry. Such values, although not yet optimized, confirmed the achieved high

quality of epitaxial diamond film and hydrogen termination.

REFERENCES

1.H. Taniuchi, H. Umezawa, T. Arima, M. Tachiki, H. Kawarada, IEEE Electron Dev. Lett.

22, 390 (2001).

2.M. Kasu, K. Ueda, H. Ye, Y. Yamauchi, S. Sasaki, T. Makimoto, Diamond Relat. Mater.

15, 783 (2006).

3.H. Ye, M. Kasu, K. Ueda, H. Ye, Y. Yamauchi, N. Maeda, S. Sasaki, T. Makimoto,

Diamond Relat. Mater. 15, 787 (2006).

4.F. Maier, J. Ristein, L. Ley, “Origin of the surface conductivity in diamond“, Phys. Rev. B

85, 3472 (2000). 5.D. Takeuci, M. Riedel, J. Ristein, L. Ley, Phys. Rev. B 68, 4130 (2003).