Materials Science and Engineering B59 (1999) 395 – 400 Microwave electronics device applications of AlGaN/GaN heterostructures Q. Chen a, *, J.W. Yang a,1 , M. Blasingame a , C. Faber a , A.T. Ping b , I. Adesida b a APA Optics, 2950 NE 84 th Lane, Blaine, MN 55449, USA b Microelectronics Laboratory, Uni6ersity of Illinois, Urbana, IL 61801, USA Abstract One thrust in the recent AlGaN/GaN based HFET development hinges on the use of SiC substrates for the growth of the AlGaN/GaN heterostructures. We have achieved G m and maximum drain current (I max ) as high as 222 mS mm -1 and 1.71 A mm -1 for HFETs grown on n-SiC. The HFETs on p-SiC have also shown G m and I max of 230 mS mm -1 and 1.43 A mm -1 . These devices exhibited cut-off frequency ( f t ) and frequency of oscillation ( f max ) of 55 and 56 GHz for HFETs on p-SiC, further demonstrating the applicability of AlGaN/GaN-based HFETs in high power microwave frequency range. The availability of high quality AlGaN/GaN heterostructure has also permitted the implementation of such new device concept as metal – insulator – semi- conductor FETs (MISFETs). Our MISFETs have shown low gate leakage in 96 V gate bias range with G m as high as 86 mS mm -1 . © 1999 Elsevier Science S.A. All rights reserved. Keywords: HFET; Devices; MISFETs 1. Introduction With a direct band gap, GaN and related compounds (InGaN and AlGaN) were recognized earlier for their applications in light emitters covering the ultraviolet (UV) and visible spectral range. An examination of the basic material properties and several composite mate- rial figures of merits of semiconductors, such as shown in Table 1, brings out the fact that the GaN-related compounds possess excellent property for high power (to some extent related to high temperature) electronic device applications. A straightforward electronic device application of GaN material is the fabrication of metal semiconductor field effect transistors (MESFETs) [2]. In such MES- FETs, the Schottky type gates are formed directly on the GaN channel. While a highly doped channel with a tight profile is desirable for high transconductance (G m ) of the MESFET, the formation of Schottky junction often requires a lightly doped channel. This compro- mise leads to thicker and lightly doped GaN channel. As a result, the MESFETs showed a low G m of 23 mS mm -1 which is to be compared to values of G m \200 mS mm -1 readily achievable in GaAs based MES- FETs. In addition, the low electron mobility in the bulk GaN channel (350 cm 2 V -1 s -1 typical) will give rise to a high series resistance, limiting DC and RF perfor- mance of the MESFETs. Electron mobility enhancement at AlGaN/GaN het- ero-interface was first reported in 1991 [3]. Heterostruc- ture field effect transistors (HFETs) were soon fabricated using such AlGaN/GaN heterostructures [4]. Despite the electron mobility enhancement, these early HFETs exhibited a DC G m of 28 mS mm -1 , similar to that of the MESFETs. It must be noted, however, that the excellent potential of these AlGaN/GaN-based HFETs for microwave electronics application was illus- trated by the early short gate (0.25 mm) HFETs which exhibited a cut-off frequency ( f t ) and maximum fre- quency of oscillation ( f max ) of 11 and 35 GHz, respec- tively [5]. Drastic improvement in DC and RF performance of the AlGaN/GaN-based HFETs came several years later after further advancement in material quality, het- erostructure design, and Ohmic contact formation. * Corresponding author. Tel.: +1-612-7844995; fax: +1-612- 7842038. 1 Currently with E and CE Department, University of South Caro- lina, Columbia, SC 29208, USA. 0921-5107/99/$ - see front matter © 1999 Elsevier Science S.A. All rights reserved. PII:S0921-5107(98)00356-0