IEEE ELECTRON DEVICE LETTERS, VOL. 32, NO. 5, MAY 2011 635 N-Polar GaN MIS-HEMTs With a 12.1-W/mm Continuous-Wave Output Power Density at 4 GHz on Sapphire Substrate Seshadri Kolluri, Student Member, IEEE, Stacia Keller, Steven P. DenBaars, Fellow, IEEE, and Umesh K. Mishra, Fellow, IEEE Abstract—This letter presents a high-performance N-polar AlGaN/GaN metal–insulator–semiconductor high-electron- mobility transistor grown by metal–organic chemical vapor deposition on sapphire substrate. The devices were passivated with Si x N y deposited by plasma-enhanced CVD and consisted of a gate structure recessed through the Si x N y passivation, with integrated slant field plates, to prevent dc to RF dispersion and improve the breakdown voltage. Devices with a drawn gate length of 0.7 μm and a gate drain spacing of 0.8 μm showed a breakdown voltage of 170 V, corresponding to a three-terminal leakage current of 1 mA/mm. Due to the high breakdown voltage of the devices, continuous-wave RF power measurements at 4 GHz could be measured at a drain bias of 50 V, yielding an output power density of 12.1 W/mm. To the best of our knowledge, this is the highest power density reported so far for an N-polar device and also matches the highest power density reported for a Ga-polar HEMT on the sapphire substrate. Index Terms—GaN, high-electron-mobility transistor (HEMT), metal–organic chemical vapor deposition (MOCVD), N-polar. I. I NTRODUCTION H IGH-ELECTRON mobility transistors (HEMTs) based on Ga-polar AlGaN/GaN system have demonstrated great potential for microwave power amplification ranging from L-band to W-band. N-polar AlGaN/GaN HEMTs have wit- nessed a significant interest recently [1], [2], as a potential al- ternative to Ga-polar HEMTs for high-frequency applications, motivated by advantages such as a lower contact resistance [3]. With progress in the growth and the processing of the N-polar devices, excellent small signal performance [4] and RF power performance with a power-added efficiency (PAE) comparable to Ga-polar devices have been achieved by both molecular beam epitaxy (MBE) [5], [6] and metal–organic chemical vapor deposition (MOCVD) [7], [8]. Wong et al. [6] have previously reported N-polar HEMTs with the highest output power density of 8.1 W/mm at 4 GHz. However, this power density is significantly less than the highest reported Manuscript received January 30, 2011; revised February 16, 2011; accepted February 16, 2011. Date of current version April 27, 2011. This work was supported in part by the Office of Naval Research MINE project (monitored by Dr. P. Maki and Dr. H. Dietrich). The review of this letter was arranged by Editor G. Meneghesso. The authors are with the Department of Electrical and Computer Engineering, University of California, Santa Barbara, CA 93106 USA (e-mail: seshadri@ece.ucsb.edu; stacia@ece.ucsb.edu; denbaars@engineering. ucsb.edu; mishra@ece.ucsb.edu). 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.2011.2119462 Fig. 1. (a) Schematic cross section and (b) simulated band diagram of the device. values for Ga-polar devices on both SiC [9] and sapphire [10] substrates because of the lower breakdown voltage in N-polar devices grown by MBE. Recent N-polar devices grown by MOCVD with an Al 0.6 Ga 0.4 N cap and in situ high-temperature CVD-deposited Si x N y gate insulator have shown a breakdown voltage in excess of 150 V corresponding to a three-terminal leakage current of 1 mA/mm [7]. However, in [7], the devices failed beyond a drain bias of 30 V on the 4-GHz load-pull system, due to measurement limitations. In this letter, with further improvements in the device design and measurement, such as a reduced source to drain spacing (to reduce parasitic resistances), a recessed gate structure with integrated slant field plates (for better dispersion control), and an increased output bias voltage, we achieved a record RF output power density for an N-polar HEMT. II. DEVICE DESIGN AND FABRICATION The epitaxial structure and a schematic cross section of the fabricated device is shown in Fig. 1(a). A simulated band diagram of the device structure under the gate, obtained using a 1-D Schrödinger–Poisson solver [11], is shown in Fig. 1(b). A 2.4-eV conduction band offset between GaN and Si x N y [12] and a barrier height of 2.5 eV at the gate-metal/Si x N y interface were assumed to match the charge observed from Hall measurements. The structure was grown by MOCVD using trimethylgallium, trimethylaluminum, and ammonia as precursors on (0001) sapphire with a misorientation angle of 4 ◦ toward the sapphire a-plane. Growth on a misoriented substrate was essential to achieve smooth high-quality N-polar films [13]. High-temperature nitridation of the sapphire substrate, with an NH 3 precursor, was used prior to GaN deposition, 0741-3106/$26.00 © 2011 IEEE