A report is made on the DC, RF and large-signal pulsed characteristics of unpassivated AlGaN=InGaN=GaN high electron-mobility transistors (HEMTs) grown by molecular beam epitaxy on sapphire substrates. The devices with a 0.5 mm gate-length exhibited relatively flat transconductance (g m ) with a maximum drain current of 880 mA= mm, a peak g m of 156 mS=mm, an f T of 17.3 GHz, and an f MAX of 28.7 GHz. In addition to promising DC and RF results, pulsed I-V measurements reveal that there is

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... current collapse in the AlGaN=InGaN HEMTs. These results indicate that the output power of InGaN channel HEMTs should not be limited by surface-staterelated current collapse. Introduction: Heterostructures of AlGaN and GaN have attracted widespread attention for their high power and high frequency capability even in harsh environments. Recently, AlGaN=GaN HEMTs on semi-insulating SiC substrates have been reported with high poweradded efficiency and high power at K-band [1]. Further improvement in microwave power applications can be achieved with the use of an InGaN channel layer in the context of examples established by InAlAs=InGaAs=InP-and AlGaAs=InGaAs=GaAs-based materials systems [2, 3] . InGaN-based quantum wells have been widely used to improve the efficiency of blue-green light emitting diodes and laser diodes [4] . The placement of an InGaN channel between GaN and=or AlGaN layers produces a double heterostructure, which has enhanced carrier confinement properties compared with the triangular potential well that exists in a single-heterostructure. Furthermore, the channel experiences lattice strain from both sides, which is expected to lead to enhanced sheet carrier density and higher mobility [5] . Previously reported InGaN-channel HEMTs were grown by metal organic chemical vapour deposition (MOCVD) [3, 5, 6] . In this Letter, we report on having studied the potential of AlGaN=InGaN=GaN HEMTs grown by molecular beam epitaxy (MBE). DC and RF characteristics of devices resulted in high current density of 880 mA=mm with linear transfer characteristics and high unity current-gain cutoff frequency ( f T ) of 17.3 GHz for a 0.5 mm gate-length device. In addition to these promising results, the pulsed I-V measurements show that current collapse caused by surface states can be avoided by using an InGaN channel in GaN HEMTs. Fabrication: The epitaxial structure that was used in this study was grown by RF plasma-assisted MBE on a [0001] sapphire substrate. The layer structure consists of an AlN nucleation layer, 2 mm GaN buffer layer, 5 nm InGaN channel, 18 nm Al 0.25 Ga 0.75 N layer, and 2 nm GaN capping layer. During the growth, the InGaN channel was calibrated for 10% InN mole fraction. All layers were unintentionally doped. Hall measurements were used to determine the sheet carrier concentration of 1.3 Â 10 13 =cm 2 and an electron mobility of 710 cm 2 = V Á s at room temperature. The low mobility may be attributed to roughness at the AlGaN=InGaN interface. Mesa isolation was achieved by inductively-coupled plasma reactive-ion etching (ICP-RIE) with a plasma containing chlorine and argon. The multilayer ohmic contact consisting of Ti=Al=Mo=Al was defined by the photoresist lift-off process and then rapid-thermal-annealed [7] . A multilayer resist was used to form Ni=Au mushroom-shaped gates. The devices used in this study had gate lengths (L G ) of both 0.5 and 1 mm with source-drain spacing (L SD ) of both 3 and 4 mm. The width of all devices was 100 mm. Results: The DC transfer characteristics are plotted in Fig. 1 for several different device dimensions when biased at V DS ¼ 8 V. A 0.5 mm gate-length AlGaN=InGaN HEMT with a 3 mm source-drain spacing showed maximum drain current of 880 mA=mm and complete pinch-off at V GS ¼ À4.5 V. Small-signal transconductance, g M , for the device was 156 mS=mm at V GS ¼ À1.5 V. The devices show less g M roll-over than typical AlGaN=GaN HEMTs fabricated using the same process [8], which results from the enhanced carrier confinement of the InGaN channel. Fig. 1 also shows the measured f T against V GS for a 0.5 mm gate-length device with 3 mm source-drain spacing; these results indicate that the RF transconductance also maintains relatively flat characteristics similar to the DC transconductance, g M . Fig. 1 DC transfer characteristics of AlGaN=InGaN HEMTs with different gate lengths (L G ) and source-drain spacings (L SD ) All devices biased at V DS ¼ 8 V. Measured unity-gain cutoff frequency ( f T ) of 0.5 mm gate-length AlGaN=InGaN HEMT with L SD ¼ 3 mm shown with squares Fig. 2 Measured unity-gain cutoff frequency (f T ) and DC drain current (I DS ) against drain bias (V DS ) at several different gate voltages (V GS ) for device with gate-length 0.5 mm and source-drain spacing 3 mm Fig. 3 Measured pulsed I-V (dashed line) and DC I-V (solid line) for device with gate-length 0.5 mm and source-drain spacing 4 mm Pulsed I-V measurements taken at DC bias conditions of V DS ¼ 20 V and V DS ¼ À5 V. Gate bias swept from þ2 to À5 V in steps of À1 V. Pulse width 200 ns, pulse repetition 1 kHz Fig. 2 shows the measured f T and drain current when V DS is varied from 0 to 20 V with V GS set at À2.5, À2 and À1 V. The f T is approximately 15 GHz when V GS above À2 V and is fairly independent of V DS as long as the device remains in saturation. The device is observed to maintain high f T at high drain bias voltage, as required for high power amplifiers. Good carrier confinement in the InGaN channel as indicated by the bias-dependent results shown by Figs. 1 and 2 should prevent RF dispersion or current collapse that may occur owing to carrier spill-over into the buffer or barrier layer [9] . Pulsed I-V measurements of the devices were taken on the ACCENT DIVA dynamic I-V analyser [10]. This system employs dual pulsing; both the gate and the drain of a HEMT are pulsed on top of the DC bias conditions. Fig. 3 shows the comparison of the measured pulsed I-V with the DC I-V curves. It is observed that the pulsed I-V response tracks the DC I-V response for lower bias conditions (up to a few volts), above which the DC current is reduced relative to pulsed current because of self-heating. The similarity of the two curves is in contrast to the pulsed I-V characteristics of typical unpassivated AlGaN=GaN devices, in which surface traps are seen to severely limit the drain current under pulsed operation [11] . Conclusions: We have fabricated AlGaN=InGaN=GaN HEMTs grown by MBE on sapphire substrates. Devices with gate-lengths of 0.5 and 1.0 mm show relatively flat transconductance when compared with typical unpassivated AlGaN=GaN HEMTs. The pulsed I-V measurements show little dispersion, indicating that the devices have few active surface traps. These results suggest that the addition of InGaN to the channel improves the electron transport characteristics, by suppressing current collapse that is related to the surface states.