586 IEEE TRANSACTIONS ON ELECTRONIC DEVICES, VOL. 48, NO. 3, MARCH 2001 Very-High Power Density AlGaN/GaN HEMTs Yi-Feng Wu, Member, IEEE, David Kapolnek, James P. Ibbetson, Primit Parikh, Member, IEEE, Bernd P. Keller, and Umesh K. Mishra, Fellow, IEEE Abstract—Research work focusing on the enhancement of large-signal current–voltage (I–V) capabilities has resulted in significant performance improvement for AlGaN/GaN HEMT’s. 100–150 m wide devices grown on SiC substrates demonstrated a record power density of 9.8 W/mm at 8 GHz, which is about ten times higher than GaAs-based FETs; similar devices grown on sapphire substrates showed 6.5 W/mm, which was thermally limited. 2-mm-wide devices flip-chip mounted on to AlN sub- strates produced 9.2–9.8 W output power at 8 GHz with 44–47% PAE. A flip-chip amplifier IC using a 4-mm device generated 14 W at 8 GHz, representing the highest CW power obtained from GaN-based integrated circuits to date. Index Terms—AlGaN, FET, flip-chip, GaN, HEMT, microwave power. I. INTRODUCTION W IDE-BANDGAP AlGaN/GaN high electron mobility transistors (HEMTs) have great promise in power generation at high frequencies, due to the high breakdown field and excellent electron transport properties. These devices are typically grown on either SiC or sapphire substrates. The former provides an excellent thermal conductivity of 3.5 to 4.5 W/cm C, while the latter is available at a lower cost and in larger wafer sizes. For a specific device technology, output power density is a major performance benchmark. This is usually evaluated using small devices with minimum thermal complications. For practical applications, however, the device periphery needs to be scaled up to obtain high total output power. Earlier demonstrations include devices grown on sapphire substrates with power density of 4.6 W/mm and a total power of 7.6 W [1], as well as devices on SiC with power density of 6.9 W/mm and a total power of 9.1 W [2]. In this paper, we present further improvement of the AlGaN/GaN HEMT technology focusing on enhancement of large-signal current–voltage (I–V) capabilities under rf operations. II. DEVICE TECHNOLOGY The devices in our laboratories were grown by metal organic chemical vapor deposition on either semi-insulating SiC sub- strates or sapphire substrates. The epi-layers consisted of a semi- insulating GaN buffer and a modulation-doped AlGaN layer Manuscript received May 15, 2000; revised September 25, 2000. This work was primarily supported by the Office of Naval Research. The review of this paper was arranged by Editor U. K. Mishra. F.-Y. Wu, J. P. Ibbetson, P. Parikh, and B. P. Keller are with Cree Lighting Company, Goleta, CA 93117 USA. D. Kapolnek is with Ericsson Datacom Inc., Goleta, CA 93117 USA. U. K. Mishra is with Cree Lighting Company, Goleta, CA 93117 USA, and also with the Department of Electrical and Computer Engineering, University of California, Santa Barbara, CA 93106 USA. Publisher Item Identifier S 0018-9383(01)01534-9. Fig. 1. I–V characteristics under dc and ac gate drives for (a) an AlGaN/GaN HEMT with severe trapping effect or dispersion, and (b) an AlGaN/GaN HEMT with minimum trapping effect. to supply charge for the two-dimensional (2-D) gas as well as to offer a Schottky-gate barrier. The basic device fabrication process included ohmic contacts by Ti/Al/Ni/Au [3], mesa iso- lation by Cl reactive ion etching and gate formation by Ni/Au. Important issues in device development are addressed below. A. Maximizing Channel Current and Breakdown Voltages The output power density of a field-effect-transistor (FET) is directly related to its current density and breakdown voltage. For an AlGaN/GaN HEMT, the maximum channel current is set by the conduction band discontinuity between the AlGaN donor/barrier layer and the GaN channel layer. Device optimiza- tion has to do with increasing this discontinuity by increasing 0018–9383/01$10.00 © 2001 IEEE