212 IEEE ELECTRON DEVICE LETTERS, VOL. 33, NO. 2, FEBRUARY 2012 Low-Leakage-Current AlN/GaN MOSHFETs Using Al 2 O 3 for Increased 2DEG Tongde Huang, Xueliang Zhu, Ka Ming Wong, and Kei May Lau, Fellow, IEEE Abstract—Metal-oxide-semiconductor heterostructure field ef- fect transistors (MOSHFETs) were fabricated with an AlN/GaN heterostructure grown on Si substrates. A 7-nm Al 2 O 3 serv- ing as both gate dielectric under the gate electrode and passi- vation layer in the access region was used. It was found that the Al 2 O 3 was superior to SiN x in increasing the 2-D elec- tron gas (2DEG) density and thereby reducing the access re- sistance. In addition, the OFF-state leakage current (I off ) in these AlN/GaN MOSHFETs was reduced by four orders of mag- nitude to 7.6 × 10 −5 mA/mm as a result of the Al 2 O 3 gate dielectric, compared to that of AlN/GaN HFETs. Meanwhile, the subthreshold slope was improved to a nearly ideal value of 62 mV/dec because of the extremely low I off . The MOSHFETs with 1-μm gate length exhibited good DC characteristics. A maxi- mum drain current of 745 mA/mm and a peak extrinsic transcon- ductance of 280 mS/mm were achieved. Index Terms—AlN, Al 2 O 3 passivation, GaN, heterostructure field effect transistors (HFETs). I. I NTRODUCTION T HIN AlN barrier offers the greatest scalable potential among different Al x Ga 1−X N alloy compositions because of its relatively large band gap and strong polarization effects. This can help improve the aspect ratio of HFETs and thus reduce the limitation of short channel effects on high-frequency operation. Despite such merits, it is difficult to grow high- quality AlN layers due to the existence of large tensile strain between the lattice-mismatched GaN and AlN (about 2.4%). Recently, outstanding performance of AlN/GaN HFETs grown by molecular beam epitaxy has been successfully demonstrated [1]–[6], including a peak transconductance of 700 mS/mm [3], a unity current gain cutoff frequency f T of 220 GHz, and a unity power gain frequency f max of 400 GHz [5]. However, limited progress has been made on AlN/GaN transistors grown by metalorganic chemical vapor deposition (MOCVD), despite MOCVD is a more commonly used technique for GaN-based devices. The previously reported AlN/GaN HFETs could not completely pinch-off due to low quality of the AlN barrier layers [7]–[10]. Recently, AlN/GaN HFETs have been grown by MOCVD on Si substrates with in situ grown SiN x cap Manuscript received October 26, 2011; revised November 11, 2011; ac- cepted November 12, 2011. Date of publication December 27, 2011; date of current version January 27, 2012. This work was supported in part by a Grant (CA07/08.EG02) from the Research Grants Council and a Grant (ITS/523/09) from the Innovation and Technology Commission (ITC) of Hong Kong Special Administrative Government (HKSAR). The review of this letter was arranged by Editor J. A. del Alamo. The authors are with the Department of Electronic and Computer Engi- neering, Hong Kong University of Science and Technology, Kowloon, Hong Kong (e-mail: huangtongde@ust.hk; eexlzhu@ust.hk; parcow@gmail.com; eekmlau@ust.hk). Color versions of one or more of the figures in this letter are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/LED.2011.2176909 Fig. 1. Cross-sectional schematic of the AlN/GaN MOSHFETs (a), and AFM image of AlN surface (b). layer for strain relaxation and increase of the 2-D electron gas (2DEG) density [11]–[13]. The SiN x in the source/drain region must be selectively etched for ohmic contact formation. The reported I off was still larger than 10 −3 mA/mm. For high- performance RF power amplifiers, lower leakage current is more desirable, to avoid additional noise generation during transistor operation. In this paper, an AlN/GaN heterostructure was grown on Si substrates by MOCVD for HFETs fabrication. We found that Al 2 O 3 deposited by atomic layer deposition (ALD) is a more effective passivation dielectric than SiN x , with additional ben- efit of increasing the 2-DEG density in AlN/GaN HFETs. ALD technology has been widely used in III-nitride MOSHFETs for the gate dielectric [1], [14]–[16]. However, limited work was focused on the passivation effect of high-kAl 2 O 3 . In this paper, we report AlN/GaN MOSHFETs fabricated with Al 2 O 3 serving simultaneously as gate dielectric and passivation layer showing salient features such as low OFF-state current and re- duced access resistance. The fabricated MOSHFETs exhibited an extremely low I off of 7.6 ×10 −5 mA/mm at V gs = −5 V. II. MATERIAL GROWTH AND DEVICE FABRICATION The AlN/GaN heterostructure was grown on a Si (111) substrate by MOCVD. Fig. 1(a) shows the cross-sectional schematic of the GaN (1 nm)/AlN (3 nm)/GaN MOSHFETs with a 7-nm Al 2 O 3 as gate dielectric and passivation. The epi- layer thicknesses were estimated based on calibrated growth rates. The epitaxial structure consisted of, from bottom to top, a 40-nm AlN nucleation layer, an 800-nm GaN buffer layer, AlN(6 nm)/Al 0.19 Ga 0.81 N(28 nm) super-lattice interlayer with a total thickness of 300 nm for strain relaxation, a 1000-nm GaN layer, and a 3-nm AlN barrier layer. Finally, a 1-nm GaN protective layer was grown as cap. Fig. 1(b) is an atomic force microcopy (AFM) image of the as-grown sample surface, which is smooth with a clear atomic step structure. Dark de- fects, representing the intersection of threading dislocation, are not in high density. The root mean square roughness measured across the 5 μm × 5 μm scan was 0.6 nm. Room-temperature (RT) Hall measurements showed a 2-DEG density of 0741-3106/$26.00 © 2011 IEEE