IEEE ELECTRON DEVICE LETTERS, VOL. 18, NO. 6, JUNE 1997 293 Microwave Performance of AlGaN/GaN Inverted MODFET’s O. Aktas, Z. F. Fan, A. Botchkarev, S. N. Mohammad, M. Roth, T. Jenkins, L. Kehias, and H. Morko¸ c Abstract— A continuous wave output power of 1.5 W/mm with a power added efficiency of 17.5% has been achieved at 4 GHz in inverted AlGaN/GaN MODFET’s (IMODFET’s) with 2 m gate lengths and 78 m gate widths. The current gain and available power gain cutoff frequencies were 6 and 11 GHz, respectively. We suggest that the input characteristics of GaN- based FET’s play an important role in the output power that can be obtained. In the present devices, high transconductance, 100 mS/mm, retained over a 5 V input swing is thought to alleviate the limitations imposed by the input characteristics. Moreover, the buried AlGaN buffer layer is suggested as having assisted in the reduction of the output conductance which aides the power gain. S EMICONDUCTOR GaN with its large peak electron ve- locity, large saturation velocity, and large bandgap is very suitable for microwave power devices [1], [2]. Consequently, there has been a flurry of activity on GaN-based MODFET’s [3]–[10]. These devices are receiving increased attention as the crystal growth techniques and processing methods advance to the point where the performance predicted by the material characteristics are beginning to appear feasible. The normal MODFET structures grown with reactive MBE and reported previously have exhibited transconductances of 220 mS/mm with low leakage currents [3], [4]. The same devices exhibited breakdown voltages exceeding 90 V for 1.5- m gate devices with 1- m gate to drain spacing which are far from optimum. The characteristics of MODFET’s under UV illumination and operation at 300 C have been reported [5]. The FET like devices in GaN grown on sapphire often suffer from negative differential output conductance which is often Manuscript received November 21, 1996; revised February 17, 1997. The work at the University of Illinois was funded by grants from AFOSR and ONR and monitored by G. L. Witt, C. E. C. Wood, Y. S. Park, and M. Yoder. H. Morko¸ c was funded by the Air Force Office of Scientific Research under a University Resident Research Program funded by the Air Force Office of Scientific Research. O. Aktas, Z. F. Fan, and A. Botchkarev are with the Materials Research Laboratory, Coordinated Science Laboratory, and Department of Electrical and Computer Engineering, University of Illinois, Urbana, IL 61801 USA. S. N. Mohammad was with the Materials Research Laboratory, Coordinated Science Laboratory, and Department of Electrical and Computer Engineering, University of Illinois, Urbana, IL 61801 USA. He is now with the School of Engineering, Howard University, Washington, DC 20059 USA. M. Roth is with the Wright Laboratory WL/POOD, Wright-Patterson AFB, OH 45433 USA. T. Jenkins and L. Kehias are with Wright Laboratory WL/AADD, Wright- Patterson AFB OH 45433-7322 USA. H. Morko¸ c was with the Materials Research Laboratory, Coordinated Science Laboratory, and Department of Electrical and Computer Engineering, University of Illinois, Urbana, IL 61801 USA. He is now with Wright Laboratory, Wright-Patterson AFB OH 45433 USA. Publisher Item Identifier S 0741-3106(97)04311-5. ascribed to poor thermal conductivity of sapphire substrates on which these structures are grown. As is the case with poor thermal conductivity, the negative differential output conduc- tance degrades microwave power gain and power performance [16]. The nonthermal component of the negative differential output conductance can be reduced by incorporating an energy barrier against carrier injection into buffer which is provided by the inverted MODFET (IMODFET) structure used in the present investigation [17]–[19]. The IMODFET structure employed in this investigation is as follows: sapphire substrate/1000 ˚ A AlN/1 m GaN/500 ˚ A AlGaN/30 ˚ A AlGaN:Si/30 ˚ A AlGaN/60 ˚ A GaN/40 ˚ A AlGaN. The mole fraction in AlGaN layers is 15% and the doping level is cm for the doped AlGaN layer. The investigated devices have gate lengths of about 2 m and two gate fingers with a total width of 78 m An electron Hall mobility of 350 cm Vs with a sheet carrier concentration of cm was measured at room temperature. At 77 K, the sheet carrier density was measured as cm with a mobility 1450 cm Vs The large conduction band offset between AlGaN/GaN [11], [12], possibly in conjunction with the strain induced piezoelectric effect [13]–[15], enables large carrier concentrations to be measured. Characteristically, inclusive of the published data as well, the high sheet carrier concentration is not supported by the current levels measured, a matter which is under active investigation. These results indicate that, unlike in the GaAs/AlGaAs case [20], the GaN/AlGaN interface with GaN grown on the AlGaN layer is of no worse quality than the normal order case, at least at the present time. The ohmic contacts consisted of Ti/Al/Ti/Au (200 ˚ A/200 ˚ A/400 ˚ A/100 ˚ A) which were annealed at 900 C for 30 s. The Schottky barriers were formed by Pt/Ti/Au (200 ˚ A/500 ˚ A/500 ˚ A). The 2- m devices fabricated on this layer exhibited a DC transconductance of 105 mS/mm peaking at a gate to source voltage of 2.5 V. The – characteristic of the devices is shown in Fig. 1. Small-signal -parameter measurements were performed at bias conditions used for the power measurements, i.e., 15 V, 2.5 V, and 20 mA for the drain voltage, gate voltage, and drain current, respectively. Short circuited current gain, maximum available power gain and the unilateral gain calculated from the small-signal -parameters are shown in Fig. 2. The unity current gain cutoff frequency and maximum frequency of oscillation were 6 GHz and 11 GHz, respectively, at both 15 and 30 V bias. The CW microwave power measurement results are presented in Fig. 3. The mea- surements were taken at 4 GHz with the input power swept 0741–3106/97$10.00  1997 IEEE