Al0.3Ga0.7N/Al0.05Ga0.95N/GaN composite-channel HEMTs with enhanced linearity (2004)

We report an Al0.3Ga0.7N/Al0.05Ga0.95N/GaN compositechannel HEMT with enhanced linearity. Through channel engineering, i.e. inserting a 6-nm thick AlGaN layer with 5% Al composition in the channel region, a composite-channel HEMT was demonstrated. Transconductance and cutoff frequencies of a 1 μm x 100 μm HEMT are kept near their peak values throughout the lowand high-current operating levels, a desirable feature for linear power amplifiers. Driven by W-CDMA signals with a center frequency of 2 GHz, an adjacent channel leakage ratio (ACLR) of less than –45 dBc was achieved with a power added efficiency (PAE) of 45% and a power density of 3.4 W/mm for composite-channel HEMTs (CC-HEMTs) grown on sapphire substrates, without using any linearization techniques. Introduction Wide bandgap AlGaN/GaN HEMTs, with their high power handling capability at high frequencies, are emerging as the promising candidates for RF and microwave power amplifiers used in next generation wireless base stations (1)(4). The 3G wireless communication systems, such as WCDMA/UMTS, impose stringent requirement for the linearity of the power amplifiers due to the large dynamic range in the variable envelop of the modulation signals. As a result, recently, there have been intensive activities in characterizing the linearity of conventional AlGaN/GaN HEMT (3), (5)-(7) and metal-oxide-semiconductor (MOS) heterojunction fieldeffect transistors (8). It has been shown (3) that conventional AlGaN/GaN HEMTs require advanced linearization techniques such as digital predistortion to satisfy the adjacent channel leakage power ratio (ACLR) requirement for WCDMA standard. To reduce the burden of the linearization techniques, it is necessary to optimize the HEMT epilayer structure for improved linearity. One undesirable feature with regard to the linearity is the significant reduction of transconductance (Gm) and gain at high current levels (4). Some groups are investigating ways of improving the linearity by using field-plate (9) and modifying the access resistance (10). However, there have been limited activities in engineering the channel region of the III-nitride HEMTs for improved linearity. In this paper, we propose and demonstrate a novel compositechannel HEMT (CC-HEMT) through channel engineering. The CC-HEMT exhibits nearly-flat transconductance and cutoff frequencies at both low and high current operation levels --the desirable feature for achieving high linearity. The CC-HEMTs also exhibit superior RF linearity, which is characterized through intermodulation measurement and ACLR measurement. Device Structure and Fabrication The composite channel HEMT structure, with the schematic cross section shown in Fig. 1, was grown on (0001) sapphire substrates in an Aixtron AIX 2000 HT system. After initial desorption at 1200 °C, a GaN nucleation layer was grown at 550 °C, followed by a 2.5-μm-thick unintentionally doped GaN buffer layer. Then the major channel layer, a 6-nm-thick AlGaN layer with 5% Al composition, was grown followed by the AlGaN barrier layer with 30% Al composition. The barrier layer consists of a 3-nm undoped spacer, a 10-nm carrier supplier layer doped at 3.5x10 cm, and a 2-nm undoped cap layer. Fig. 1. Cross-section of the CC-HEMT. Between the Al0.3Ga0.7N barrier and the GaN is a 6-nm-thick undoped AlxGa1-xN layer with low Al composition (x=0.05). For the device fabrication, device mesa was formed using Cl2/He plasma dry etching in an STS inductively coupledplasma reactive ion etching (ICP-RIE) system. It is followed by the source/drain ohmic contacts formation by a rapid thermal annealing of e-beam evaporated Ti/Al/Ni/Au at 850°C for 30 seconds. Using on-wafer transfer length method (TLM) patterns, the ohmic contact resistance was typically measured to be 1.0 ohm-mm. Gate electrodes with 1 μm 2 nm i-Al0.3Ga0.7N 3 nm i-Al0.3Ga0.7N Ni/Au Ti/Al/Ni/Au SiN