This article has been accepted for inclusion in a future issue of this journal. Content is final as presented, with the exception of pagination. IEEE TRANSACTIONS ON ELECTRON DEVICES 1 DC and RF Performance of Gate-Last AlN/GaN MOSHEMTs on Si With Regrown Source/Drain Tongde Huang, Student Member, IEEE, Zhao Jun Liu, Member, IEEE, Xueliang Zhu, Jun Ma, Xing Lu, and Kei May Lau, Fellow, IEEE Abstract— This paper presents the fabrication and char- acteristics of self-aligned gate-last AlN/GaN metal–oxide– semiconductor high electron mobility transistors (MOSHEMTs) featuring regrown source/drain for low ON-state resistance ( R ON ). Previously, we demonstrated conventional enhancement-mode AlN/GaN MOSHEMTs on Si substrate with excellent DC per- formance but limited RF characteristics by large parasitic gate-to-source/drain overlap capacitance. In this paper, the self- aligned gate-last process was developed to minimize the parasitic capacitance. SiN x sidewall and supporting layer were inserted to separate the gate head and source/drain. In the gate-last devices, f T has been improved to be ∼40 GHz with a channel length ( L g ) of 210 nm. Delay time analysis showed that drain delay was relatively small compared with gate transit and parasitic charging time because of the self-aligned structure. Index Terms—AlN/GaN, atomic-layer-deposited (ALD) Al 2 O 3 , enhancement-mode (E-mode), gate-last, metal–oxide–semi- conductor high electron mobility transistor (MOSHEMT), self-aligned. I. I NTRODUCTION C OMBINED high frequency and high breakdown char- acteristics offer GaN high-electron mobility tran- sistors (HEMTs) practical advantages in applications of RF/millimeter-wave power amplifier and power switching. After demonstration of the first AlGaN barrier HEMT two decades ago [1], several emerging barrier designs, including AlN [2]–[5], lattice matched InAlN [6]–[8], and quaternary InAlGaN [9], [10], have been implemented to improve the transistor performance and reliability, leading to state-of-the- art results in terms of f T / f max and output power density [9], [11]. Scaled gate lengths down to < 100 nm have been employed for accelerated device speed (v e / L g ), whereas the barrier thickness (t d ) must be maintained to provide high enough aspect ratio ( L g / t d ) for mitigation of short chan- nel effects. AlN barriers, having the highest spontaneous polarization among all the III-nitride materials, can lead to maximized 2-D electron gas (2DEG) density even at thick- nesses < 10 nm [12]. Ultrathin AlN barriers allow good Manuscript received April 1, 2013; revised July 22, 2013; accepted July 22, 2013. This work was supported in part by the Research Grants Council theme- based research scheme of the Hong Kong Special Administrative Government under Grant T23-612/12-R “Cost-effective and eco-friendly LED system-on- a-chip.” The review of this paper was arranged by Editor R. Quay. The authors are with the Department of Electronic and Computer Engi- neering, Hong Kong University of Science and Technology, Hong Kong (e-mail: huangtongde@ust.hk; eezhj@ust.hk; eexlzhu@ust.hk; jmaaa@ust.hk; eexlu@ust.hk; eekmlau@ust.hk). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TED.2013.2274656 gate control capability over channel carriers, which results in high frequency performance up to W-band (75–110 GHz). Both Ga- and N-polar AlN barrier HEMTs grown on SiC substrate by molecular beam epitaxy have been demonstrated. Ga-polar based AlN HEMTs achieved a record f T / f max of 342/518 GHz with L g = 20 nm by advanced T-gate fabri- cation technology [13]. The N-polar AlN HEMTs fabricated with self-aligned and selectively etching process also achieved a f T / f max of 115/35 GHz for L g = 120 nm [14]. The f max was limited by the thin gate metal with high resistivity. Although metal organic chemical vapor deposition (MOCVD) is widely used for growth of III-nitride, it is difficult to grow high quality thin AlN layers with excellent AlN/GaN interfaces required by devices. There are limited reports concerning AlN/GaN HEMTs grown on Si (111) substrates by MOCVD. Until recently, AlN/GaN HEMTs grown by MOCVD on Si substrates have been reported with in situ SiN x cap layer for strain relaxation [12], [15], [16]. The SiN x in the gate region must be selectively etched before gate metal deposition. In this paper, a GaN cap was used as a protective layer for the AlN barrier underneath. All the AlN/GaN heterostructure was grown by MOCVD on high resistive Si substrate. The cor- responding heterostructure is similar to that described in [17]. Section II compares two AlN/GaN metal-oxide-semiconductor high electron mobility transistors (MOSHEMTs) architectures fabricated with former process [17] and this gate-last process. In Section III, the thermal stability of different ohmic metals is discussed. Cr/Au instead of previously reported Ti-based metal system [18] has been used because of better thermal stability. Section IV describes the self-aligned gate-last process developed to minimize the parasitic capacitance. Then, DC and RF performances of the fabricated devices by former and gate-last process are discussed in Section V. Increased value of f T · L g confirmed the improved RF performance by the self-aligned process. The delay time analysis was conducted in Section VI. This is the first time that self-aligned gate-last process is implemented in GaN MOSHEMT. The gate-first process has been previously used in [14]. II. DESIGN OF GATE-LAST DEVICES In [17], enhancement-mode (E-mode) AlN/GaN MOSHEMTs were fabricated with 6-nm Al 2 O 3 . A record transconductance (G m ) of 509 mS/mm and drain current ( I d ) of 860 mA/mm was demonstrated for E-mode devices on Si. Despite of excellent DC performance, the fabricated 0018-9383 © 2013 IEEE