30-nm-gate AlGaN/GaN MIS-HFETs with 180 GHz fT

Abstract

AlGaN/GaN heterostructure field-effect transistors (HFETs) are excellent candidates for high power and high frequency applications operating in the millimeter-wave frequency range. Therefore, it is important to clarify how fast GaN HFETs can operate, because this will be key information to judge the frequency limit of GaN HFET applications. We recently reported AlGaN/GaN HFETs with a current-gain cutoff frequency (fT) of 163 GHz, which is a record for GaN transistors [1]. The high fT was achieved with our novel approach using high-Al-composition and extremely thin AlGaN barriers, SiN gate-insulating and passivation layers deposited by catalytic chemical vapor deposition (Cat-CVD), and 60-nm-long T-gates, which makes it possible to maintain a high aspect ratio for sub-0.1 -pim gates and suppress the short-channel effect. To enhance high-frequency characteristics, for the present work, we further decreased the gate length (LG) and fabricated 30-nm-gate AlGaN/GaN HFETs. The decrease in LG successfully resulted in the enhancement offT, which reached a new record of 180 GHz. Figure 1 shows a schematic cross section of the SiN(2 nm)/Alo.4Gao.6N(8 nm)/AlN(1.3 nm)/GaN(1500 nm) MIS-HFET. The structure was grown on a sapphire substrate by plasma-assisted molecular-beam epitaxy (PAMBE), and all of the epitaxial layers were nominally undoped. The patterning processes except for the gate finger were done by photolithography with a contact aligner. Device isolation was performed by mesa dry etching with C12/BCl3/Ar mixture gas. Source and drain ohmic contacts were formed by rapid thermal annealing of Ti/Al/Ni/Au at 820°C. The specific contact resistance was 3 x 10-6 Qcm2. Cat-CVD at 300°C was used to deposit a 2-nm-thick SiN film on the device. The SiN film worked not only as a passivation film but also as a gate-insulating layer. 30-nm-long T-shaped gates were defined by electron-beam (EB) lithography with a triple-layer resist, and gate metal with Ti/Pt/Au was deposited and lifted off. Finally, contact pad metal for probing with Ti/Au was deposited and lifted off. The source-drain spacing was 2 pim, and the gate width was 50x2 pim. Four-point van der Pauw Hall patterns were fabricated on the same wafer during the device processing. After SiN deposition, a mobility of 827 cm2/Vs, an electron density (Ns) of 2.07x1013 cm-2, and a sheet resistance (Rsh) of 364 Q/square were obtained for the HFET structure from the Hall effect measurement. The large Ns and low Rsh in spite of the thin AlGaN barrier were attributed to the Cat-CVD SiN passivation [2]. Figure 2 shows DC current-voltage (I-V) curves for the 30-nm-gate HFETs. The devices had a good pinch-off characteristic. The maximum drain current density (IDs) reached 1.49 A/mm for a gate bias of +1 V. Figure 3 shows the transfer characteristics for a drain bias of 2 V. The peak extrinsic transconductance (gm) was 402 mS/mm. Figure 4 shows the gate leakage current characteristics. The two-terminal reverse breakdown voltage, which was defined by a gate-drain leakage current density of 1 mA/mm, was about -18 V. Figures 5 and 6 show the current gain (H21), maximum stable gain (MSG), and unilateral gain (Ug) as a function of frequency. ThefT extracted from an extrapolation of H21 with -20 dB/decade was 180 GHz. The maximum oscillation frequency (fmax) for MSG and Ug were 185 and 189 GHz, which were obtained by extrapolations ofMSG with -20 dB/decade at 50 GHz and of Ug from 20 to 50 GHz with -20 dB/decade, respectively. The pad parasitic capacitances were de-embedded with the conventional Y-parameter subtraction method. To our knowledge, thef1 is a new record for GaN transistors. In conclusion, we fabricated 30-nm-long T-gate Al04Gao6N(8 nm)/GaN MIS-HFETs with a Cat-CVD SiN gate-insulating and passivation layer. The devices exhibited excellent DC and RF device characteristics; especially an excellentfT of 180 GHz. This work was partially supported by "The research and development project for expansion of radio spectrum resources" of the Ministry of Internal Affairs and Commnunications, Japan.