Fabrication of high performance AlGaN/GaN FinFET by utilizing anisotropic wet etching in TMAH solution Dong-Hyeok Son 1 , Young-woo Jo 1 , Ryun-Hwi Kim 1 , Chan Heo 1 , Jae Hwa Seo 1 , Jin Su Kim 1 , In Man Kang 1 , Sorin Cristoloveanu 2 , and Jung-Hee Lee 1 1 School of Electronics Engineering, Kyungpook National University, Daegu, Korea 2 Institute of Microelectronics, Electromagnetism and Photonics, Minatec, Grenoble Polytechnic Institute Grenoble, France E-mail: jlee@ee.knu.ac.kr Abstract— AlGaN/GaN-based fin-shaped field-effect transistors (FinFETs) with very steep sidewall and various fin- widths (Wfin) have been fabricated by utilizing electron-beam lithography and additional anisotropic sidewall wet etch in tetramethyl ammonium hydroxide (TMAH) solution. The device with Wfin of 180 nm exhibits normally-on performance with threshold voltage of - 3.5 V and extremely broad transconductance (gm) characteristic ranging from - 2 to ~ 3 V at VD = 5 V which is essential for high linearity device performance. This broad gm characteristic is because the current from the side-wall MOS channel becomes comparable to that from the two-dimensional electron gas (2DEG) channel and hence significantly contributes to the total device current. On the other hand, devices with smaller Wfin = 50 and 100 nm exhibit normally-off performance with positive threshold voltage of 2.0 and 0.6 V, respectively, and less broad gm characteristics because the current from the side-wall MOS channel dominates the total device current. Keywords— AlGaN/GaN; FinFET; ; TMAH wet etch; 2DEG channel; sidewall MOS channel I. INTRODUCTION GaN-based high-electron-mobility transistors (HEMTs) are considered to be a promising candidate for next generation power switch and amplifier due to superior properties of III- nitride materials such as wide band-gap energy, high breakdown field, and high electron saturation velocity, which results in high output current and high breakdown voltage [1]- [3]. High two-dimensional electron gas (2DEG) density, formed at the AlGaN/GaN heterointerface, enables HEMTs to have high current and high mobility for high power operation in RF amplifiers. In RF amplifiers, the linearity performance is also important to minimize the distortion between input and output, but conventional AlGaN/GaN HEMT is unsuitable for wireless communication systems due to its narrow transconductance (gm) characteristic even though its other advantages. In power switching systems, normally-off operation with positive threshold voltage and low leakage current is generally required for simplifying the design of driving circuits, reducing the power loss, and eliminating the switching errors. However, AlGaN/GaN HEMTs have normally-off performance due to its large 2DEG and some efforts have been reported to achieve and develop a GaN- based normally off device: recessed-gate structure [1], plasma treatment based on fluoride (F) [2], and P-GaN gate structure [3]. On the other hand, not only recessed–gate structure but also F-plasma treatment requires deliberate control of plasma to minimize the side effect. P-GaN gate needs high doping concentration of Mg ions which is challenging to achieve. Recently, Tri-gate or fin-shaped structure GaN-based FETs (FinFETs) have been investigated and exhibiting superior performances such as extremely low off state leakage current, low on-resistance and high on/off ratio [4-6]. In addition, it is also reported that the threshold voltage of FinFET is varied by the fin width and normally-off operation can be achieved at small width of fin [4]. Furthermore, FinFET combined with AlGaN/GaN heterostructure can offer the broad gm characteristics due to their excellent gate controllability and possible combination of 2DEG channel of AlGaN/GaN interface and inversion channel of MOSFET located at sidewall of fin. In this work, AlGaN/GaN FinFETs were fabricated and characterized according to the fin width (Wfin). The Wfin is easily distributed from 250 nm to 50 nm by controlling the process time of wet etch which features slow and constant etch rate. The fabricated devices show extremely low off-state current and low SS in comparison with Metal-insulator- semiconductor (MIS) HEMT which was fabricated with FinFETs on the same wafer. II. GROWTH AND DEVICE FABRICATION The device structure of the fabricated AlGaN/GaN FinFETs investigated is shown in Fig. 1. The epitaxial layers consist of 2-μm-thick highly resistive GaN layer, 60 nm-thick undoped GaN layer, and 15 nm-thick Al0.26Ga0.74N layer, which were