© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Phys. Status Solidi A 206, No. 6, 1145–1148 (2009) / DOI 10.1002/pssa.200880957 p s sapplications and materials science

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Beyond the AlGaN/GaN HEMT: new concepts for high-speed transistors

Tomas Palacios*

Department of Electrical Engineering and Computer Science, Massachusetts Institute of Technology, Cambridge, MA 02139, USA

Received 20 September 2008, revised 27 December 2008, accepted 7 January 2009

Published online 27 March 2009

PACS 73.40.Kp, 73.61.Ey, 85.30.De, 85.30.Tv

* e-mail tpalacios@mit.edu, Phone: +1 617 324 2395, Fax: +1 617 258 7393, Web: mit.edu/tpalacios

© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

1 Introduction The performance of GaN-based high electron mobility transistors (HEMTs) has experienced an impressive improvement during the last few years, which has allowed their commercialization for cell phone base station applications and their experimental demonstration in radar electronics at X-band [1]. In spite of the large market at these frequencies, multiple applications require power amplification at even higher frequencies, Ka-band frequencies and above. For example, high speed wireless digital communications based on Local Multiport Distribu-tion Service (LMDS) which operate in the 26 GHz to 31 GHz range, future anti-collision car radar systems at 66–77 GHz, several military applications at 94 GHz and many sensing applications would benefit tremendously of signal sources above 100 GHz. Current gain cut-off frequencies ( fT) in excess of 160 GHz have been reported for transistors with gate lengths in the 60–90 nm range (Fig. 1) [2]. Power gain cut-off frequencies ( fmax) above 200 GHz have also been demonstrated. This high frequency performance has been

achieved while maintaining a breakdown voltage well above 30 V which has allowed more than 10 W/mm of output power at 40 GHz (Fig. 2) [3]. Although this power level is almost one order of magnitude higher than in any other semiconductor technology at these frequencies, the large signal gain of GaN-based mm-wave amplifiers (~5–7 dB) is still too low for practical applications. This paper analyzes some of the new approaches currently be-ing pursued by our group and others to increase the high frequency performance of mm- and sub-mm wave GaN transistors. 2 Scaling of GaN HEMTs The gate length (Lg) of GaN HEMTs has been reduced in the last few years below 60 nm. Different technologies have been developed to al-low these short dimensions including bilayer and trilayer e-beam lithography, Ge-spacer layers and dielectric gate sidewalls [2]. However, as it is shown in Fig. 3, the high frequency performance of these ultra-scaled GaN HEMTs has significantly deviated from the ideal 1/Lg behavior as

Solid state power amplifiers operating at mm- and sub-mm-

wave frequencies are key components of numerous commu-

nication, sensors and defense systems. Nitride semiconduc-

tors have the potential to provide unprecedented levels of

power at these frequencies. However, a multidimensional de-

sign and optimization is required to achieve acceptable levels

of power gain and efficiency at high frequencies. In this paper

we discuss some of the main challenges to increase the oper-

ating frequency of nitride transistors. New solutions are pro-

posed to increase the electron confinement, electron velocity

and access resistances of deeply scaled devices (Abstract fig-

ure) and to unleash their great potential.

40 nm

Scanning electron micrograph of the 40 nm gate in an

AlGaN/GaN high electron mobility optimized for mm-wave

applications.

1146 T. Palacios: Beyond the AlGaN/GaN HEMT

© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.pss-a.com

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Figure 1 (online colour at: www.pss-a.com) Small signal fre-

quency performance of an AlGaN/GaN HEMT with 90 nm gate

length.

the gate length shrinks. The origin of this performance degradation is the higher effect of parasitic resistances and capacitances (including the effective gate length) as the gate length decreases. Although in the last five years, the gate length of GaN HEMTs has been reduced by one order of magnitude, the reduction of the parasitic components has not followed the same pace. In Si transistor technology there are clear scaling rules that define how the access resistances and parasitic capaci-tances need to be reduced to get harmonious scaling as the gate length is reduced. In this paper we propose a similar approach to improve the performance of GaN transistors. Figure 4 shows the proposed roadmap for GaN transistors to achieve good scalability. To get an increase in frequency performance, the access resistances need to be reduced a 50% each time the gate length is reduced by half. In addition, the gate-to-drain capacitance needs to be re- duced by 20% and the output conductance kept constant. In the sections below, we will discuss different technolo-

Figure 2 RF output power density as a function of frequency in

mm-wave GaN HEMTs.

Figure 3 (online colour at: www.pss-a.com) Frequency perform-

ance in GaN HEMTs as a function of gate length. In spite of the

excellent performance achieved in these devices, the scaling be-

havior is far from being ideal (dashed line).

gies to fulfill the requirements of this roadmap and allow the fabrication of sub-mm wave GaN amplifiers. 3 Access resistances The large access resistances in AlGaN/GaN HEMTs increase the parasitic channel delay in these transistors, which accounts for more than 40% of the total delay in deep submicron devices. New approaches are required to reduce these resistances below 0.2 Ω mm between the source and the drain contacts. One of those approaches is based on the fabrication of N-face GaN HEMTs. N-face transistors have the potential of ultra low resist-ances as, in these devices, the ohmic contact is depos- ited on GaN instead of AlGaN. Also, as shown in Fig. 5, N-face transistors fabricated from Ga-face material through a wafer removal process have record low sheet re-sistances due to higher charge densities [4]. The reason for the higher charge densities in the N-face samples is a com-bination of different Fermi level pinning and strain relaxa-tion due to the wafer removal process. 4 Output conductance As the gate length is reduced

in GaN HEMTs to increase their operating frequency, it

Figure 4 (online colour at: www.pss-a.com) Proposed roadmap

for the harmonious reduction of gate length and parasitic ele-

ments in GaN HEMTs.

Phys. Status Solidi A 206, No. 6 (2009) 1147

www.pss-a.com © 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Original

Paper

Figure 5 (online colour at: www.pss-a.com) Transport character-

istics of Ga-face AlGaN/GaN HEMTs and of N-face devices fab-

ricated through wafer removal process.

is important to minimize short channel effects and, espe-cially, maintain a high output resistance. Our group is pur-suing two different approaches to assure a high output re-sistance: gate recesses and InGaN backbarriers. When the gate length is reduced, the electrostatic integ-rity of the gate needs to be preserved by keeping the gate-length to channel distance ratio constant. Figure 6 shows the high frequency performance of a GaN HEMT for dif-ferent gate recesses. As the gate to channel distance de-creases (and the ratio improves), the frequency perform-ance increases both in terms of fT and fmax. An important drawback of reducing the gate-to-channel distance is the important increase in gate leakage current. To overcome this problem, we are developing high-k gate dielectric layers based on Al2O3, HfO2 and Ga2O3 deposited by plasma oxidation and atomic layer deposition. A reduction of more than four orders of magni-tude in the gate leakage current with respect to conven-

Figure 6 Frequency performance as a function of the gate-to-

channel distance in an AlGaN/GaN HEMT with a gate length of

230 nm.

Figure 7 (online colour at: www.pss-a.com) Structure of GaN

HEMT with an InGaN backbarrier layer to increase the confine-

ment of channel electrons.

tional Schottky contacts is possible by using these dielec-trics [5]. Double heterojunction structures are also useful to in-crease the electrostatic control of the channel electrons by the gate by increasing the spatial confinement of the channel electrons. Our group is using InGaN backbarriers structures such as the ones shown in Fig. 7. In this structure, the po-larization difference between InGaN and GaN induces an electric field inside the InGaN layer that pushes the conduc-tion band in the GaN buffer up as shown in Fig. 7. This con-duction band discontinuity is an effective backbarrier for the channel electrons and significantly improves the output re-sistance of submicron transistors by 50%, which increases the operating frequency by 20% as shown in Fig. 8. 5 Electron velocity The electron velocity in nitride semiconductors is among the highest in any material sys-tem. In GaN channels, for example, a maximum peak elec-tron velocity of 2.4 × 107 cm/s has been predicted by Monte Carlo simulations [6]. However, this very high elec-tron velocity has not been exploited in actual devices due to the large access resistances and effective gate length.

40 45 50 55 60 65 70 75 8080

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InGaN back-barrier HEMT (050208FB) Standard HEMT (050210FC)

Figure 8 (online colour at: www.pss-a.com) Frequency perform-

ance of InGaN channel devices compared to standard transistors.

The different fT values are the result of different gate lengths.

1148 T. Palacios: Beyond the AlGaN/GaN HEMT

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Figure 9 (online colour at: www.pss-a.com) Electron velocity as

a function of electric field in different semiconductors.

These parasitic elements reduce the relative importance of the intrinsic delay in the total frequency performance. In addition, the large electric field in the channel of these de-vices makes the saturation electron velocity dominate the electron velocity instead of the peak velocity. Different approaches are being pursued to increase the effective electron velocity in nitride HEMTs. For example, the use of an InGaN channel would allow to potentially reach electron velocities as high as 6 × 107 cm/s (Fig. 9). Several groups are currently growing AlGaN/InGaN/GaN transistors although the performance is still low due to the difficult growth of high quality InGaN materials. Our group is also working on engineering the electric field in the channel of nitride transistors to optimize the

Figure 10 (online colour at: www.pss-a.com) Longitudinal elec-

tric field and average electron velocity in the channel of GaN

HEMTs in a standard transistor structure (a) and in a transistor

with drain recess region (b).

electron velocity. In standard transistors under normal op-erating conditions, the peak electric field in the channel is 260 kV/cm which is much higher than the electric field re-quired for maximum electron velocity. This high electric field reduces the effective electron velocity to a value close to the saturation velocity, ~1.5 × 107 cm/s. By performing a lateral recess in the drain access region, it is possible to reduce the peak electric field in the channel by increasing the field in the access region (Fig. 10). The reduction in the peak electric field enables an increase in the effective elec-tron velocity that can be as high as 50% [7]. 6 Conclusion Nitride semiconductors are, arguably, the best materials for ultra high frequency electronics. In spite of the impressive results already demonstrated in these devices, we are just starting to unveil the ultimate po-tential of this semiconductor family. A harmonious scaling is however necessary to reduce parasitic delays caused by the large access resistances and capacitances. This multi-dimensional scaling, in combination with the high electron velocities predicted in InGaN channels, the design flexibil-ity allowed by polarization and the large breakdown volt-ages typical of nitrides will enable the demonstration of ni-tride transistors operating in the 400 GHz region in the near future.

Acknowledgments The author would like to thank

DARPA (Dr. M. Rosker) and the ONR MURI program (moni-

tored by Dr. P. Maki and Dr. H. Dietrich) for their support. The

author would also like to thank his students, J. W. Chung, Z. Xu,

O. Saadat and B. Lu, for their contributions to the work reported

in this paper.

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