IEEE ELECTRON DEVICE LETTERS, VOL. 23, NO. 11, NOVEMBER 2002 637 Microwave Noise Performance of AlGaN–GaN HEMTs With Small DC Power Dissipation J. S. Moon, M. Micovic, A. Kurdoghlian, P. Janke, P. Hashimoto, W.-S. Wong, L. McCray, and C. Nguyen Abstract—We report low microwave noise performance of discrete AlGaN–GaN HEMTs at dc power dissipation comparable to that of GaAs-based low-noise FETs. At 1-V source–drain (SD) bias and dc power dissipation of 97 mW/mm, minimum noise figures (NF ) of 0.75 dB at 10 GHz and 1.5 dB at 20 GHz were achieved, respectively. A device breakdown voltage of 40 V was observed. Both the low microwave noise performance at small dc power level and high breakdown voltage was obtained with a shorter SD spacing of 1.5 m in 0.15- m gate length GaN HEMTs. By comparison, NF with 2 m SD spacing was 0.2 dB greater at 10 GHz. Index Terms—Gallium compounds, microwave devices, noise measurement, semiconductor device noise. I. INTRODUCTION I T IS well known that InP- and GaAs-based HEMTs offer excellent microwave noise performance with a very low dc power consumption [1]. However, they generally suffer from low breakdown voltages (typically less than 5 V). In contrast, wide bandgap GaN can support high breakdown fields, 8 times that of GaAs, and GaN is expected to have a much higher degree of survivability under an intense pulse of microwave illumination. Thus, there has been growing interest in the development of robust low noise amplifiers (LNAs) based on III-nitride HEMT devices, besides its microwave power am- plifier applications [2]–[5]. Previously, a minimum noise figure (NF ) of 1.06 dB at 10 GHz was reported for GaN HEMTs with a 0.25- m gate length [6]. A lower NF of 0.6 dB was subsequently achieved by reducing the gate length to 0.15 m [7]. Recently, by further reducing the gate length to 0.12 m, NF of 0.5 dB was reported at 8 GHz. Devices were biased at V and mA/mm [8]. The reported noise performance was, however, obtained at a high level of dc power dissipation and at a source–drain (SD) bias greater than 5 V. Operating GaN HEMTs at a lower SD bias below V re- sulted in suppression of gain and higher NF . Thus, the noise performance of GaN HEMTs at low SD bias (e.g., 1–2 V) and low dc power levels comparable to that of GaAs-based FETs has not been reported. The primary difficulty to date in achieving a low noise figure (NF) at low SD bias is due to high knee voltage often observed in GaN HEMTs. Manuscript received April 29, 2002; revised June 10, 2002. The review of this letter was arranged by Editor D. Ueda. J. S. Moon, M. Micovic, A. Kurdoghlian, P. Janke, P. Hashimoto, W.-S. Wong, and L. McCray are with the HRL Laboratories LLC, Malibu, CA 90265 USA (e-mail: jsmooon@hrl.com). C. Nguyen is with Global Communication Semiconductors, Inc., Torrance, CA 90505 USA. Digital Object Identifier 10.1109/LED.2002.803766 Reducing knee voltage or parasitic excess resistance has been addressed by fabricating self-aligned AlGaN–GaN HEMTs [9]. Extrinsic transconductance was improved by 50% due to the reduced source resistance. The device breakdown was, however, reduced to 8 12 V. We report here, for the first time, microwave (4–26 GHz) noise performance of discrete GaN HEMTs at dc power dis- sipation levels comparable to those of GaAs-based FETs, but with high device breakdown voltage. We measured an NF of 0.75 dB and associated gain (AG) of 11 dB at 10 GHz with 1-V SD bias and a drain current level of 97 mA/mm. This very low microwave noise and low dc power performance was a result of reduced SD spacing of 1.5 m in 0.15 m gate length GaN HEMTs. Both and were improved by 15% at SD bias of 1 3 V, compared to those of 2 m SD spacing devices. Un- like the self-aligned gate GaN HEMTs [9], no change in the de- vice breakdown voltage (40 V in this case) was observed. II. DEVICE FABRICATION Devices in this letter were Al Ga N/GaN HEMTs grown by RF-assisted nitrogen plasma molecular beam epitaxy (MBE) on 2-in semi-insulating (0001) 4H-SiC substrates. Ohmic contact was formed by Ti/Al evaporation. A transmis- sion-line-method (TLM) measurement showed ohmic contact resistance of 0.7 mm and sheet resistance of 350 . Ni/Au mushroom-shaped gates with a length of 0.15 m were fabricated using electron beam lithography. Devices had gate widths ranging from 50 m to 100 m. SD spacing ( ) for these devices also varied between 1.5 m and 2 m. For a device with of 1.5 m, gate-to-source and gate-to-drain separations were 0.65 and 0.85 m, respectively. III. DEVICE DC AND MICROWAVE NOISE PERFORMANCE On-wafer common source dc current–voltage ( – ) and transconductance characteristics of a 0.1-mm (2 50 m) GaN HEMT are shown in Fig. 1 for m and 1.5 m. These devices have drain current densities of 1.6 A/mm at V with a pinchoff voltage of 7 V. As shown in Fig. 1(a), on-resistance of the device with m is smaller than that of the device with m; on-re- sistances are 1.85 -mm, and 2.2 -mm for devices with and m, respectively. Hence, the knee voltage was smaller for the device with m. Fig. 1(b) shows the measured extrinsic transconductance (gm) from both devices at and V. Both devices exhibit peak gm, ranging from 220 to 280 mS/mm, consistent with the same gate-to-source spacing for the two devices. The gate-to-drain 0741-3106/02$17.00 © 2002 IEEE