Stable AlGaN/GaN high electron mobility
transistors with tungsten nitride gate
metallisation
Y. Lu, E.Y. Chang, J.-C. Huang, T. Chang and
C.-T. Lee
An AlGaN/GaN high electron mobility transistor (HEMT) with tung-sten nitride (WNx) Schottky gate fabricated on a sapphire substrate is
presented. Gate forward current stress was chosen to evaluate the stability of the Schottky gate. After stress, this WNxHEMT remains
stable, while the conventional Ni/Au HEMT shows performance degradation and failure. The maximum output power density from this device is 5 W/mm at 2 GHz. A combination of these findings indi-cates the robust performance of this WNxmaterial and its potential as a
Schottky gate for AlGaN/GaN HEMTs.
Introduction: AlGaN/GaN high electron mobility transistors (HEMTs) with high breakdown voltage and high channel carrier concentration are capable of delivering high power at radio frequencies[1 – 3]. Biasing the device at a high voltage or high channel current is a requirement for high-power operations, and inevitably causes large electric fields and high temperatures on the device itself. Previous studies have investigated degradation mechanisms related to those two factors[4 – 7]. While elec-tric fields could introduce defects due to the piezoelecelec-tric nature of the GaN material system, the effects of the current and self-heating extend beyond the epitaxy layer because the temperature increases in the channel might lead to reaction of the metal contacts with the underlying semiconductors.
Another source of degradation in AlGaN/GaN HEMTs when used for RF power applications is the forward conduction current of the gate. This phenomenon occurs when the device is pushed into saturation under a large RF input signal. The current can cause changes in the characteristics of Schottky contacts and devices. Researchers do not know the exact mechanism of degradation, but the high current-induced temperature that damages the contacts may be a contributing factor [4, 6]. To reduce the forward conduction current, one study used a MISFET structure that inserts a thin insulator under the gate[8]. In this Letter, we propose a different approach to minimise possible reactions between the gate electrode and AlGaN layer using a thermally stable tungsten nitride thin film as the gate material. Based on our previous study on nitrogen-rich tungsten nitride Schottky diodes that demonstrate a high Schottky barrier height and good thermal stability
[9], this Letter presents a WNxgated HEMT device with no sign of
degradation from gate forward current stress and which delivers excel-lent power performance.
Experiment: The AlGaN/GaN heterostructures in this study consist of a 3.5 mm-thick undoped GaN buffer on a sapphire substrate, followed by a 29 nm layer of Al0.26Ga0.74N. The mobility and sheet carrier
concentrations are 900 cm2/V s and 1.0 1013cm22, respectively.
The device was isolated by mesa etching using a Cl2/Ar gas mixture
in an induced coupled plasma (ICP) etcher. Ti/Al/Ni/Au metal layers were evaporated and subsequently annealed in an N2atmosphere
at 8008C to form ohmic contacts. The wafer was then split in half and two different Schottky gate schemes of WNx/Au and Ni/Au metal
stacks were deposited on separate pieces. The nitrogen-rich WNx/Au
metal stacks were deposited as described in [9]. The resulting WNx
layer was 100 nm thick. A 300 nm Au film was then deposited on top of the WNxfilm by electron beam evaporation to reduce the gate
resist-ance. AlGaN/GaN HEMTs with Ni/Au gate metal were also fabricated for comparison using electron beam evaporation. These two types of devices were passivated using 100 nm plasma-enhanced chemical vapour deposition (PECVD) silicon nitride. The pad connections of these devices were formed by electrochemical-plated 2 mm-thick gold with air bridges. The devices had a gate length of 1 mm, gate width of 100 mm (2 50 mm) and a source-to-drain distance of 5 mm.
The DC characteristics of the AlGaN/GaN HEMTs were measured using an Agilent E5270B. The gate current stress was applied by increasing the gate voltage by 0.5 V every 30 min, which effectively forward-biased the gate electrode. The maximum gate current density used was 1 A/mm, which was limited by the equipment capability. Between the increase of the gate voltage, the gate leakage current IGoff
was extracted to monitor the degradation at the conditions (IGat Vds¼
0.1 V, Vgs¼ 25 V) as described in[6]. Microwave power was measured
on-wafer using a Focus load-pull system at 2 GHz.
Results: The DC gate current stress test was carried out by forward-biasing the gate contact. This method creates both electrical and thermal stress in the vicinity of the gate.Fig. 1compares the results of two different gate metal schemes. This Figure shows that the Ni/Au contacts are not stable at stress currents above 0.9 A/mm, and the gate leakage current exhibits a drastic increase. This degradation during gate forward stress is consistent with previous studies[4, 6]. The Ni/ Au HEMTs failed after 9 h as a result of this stress current. As for the WNx/Au HEMTs, the device remained stable and exhibited no
appre-ciable change in the gate leakage current after 24 h of the stress test.
Fig. 2compares the device characteristics before and after the stress, and there is no observable device degradation. These WNx/Au
HEMTs have a saturation current of 585 mA/mm, maximum transcon-ductance of 180 mS/mm, and an off-state breakdown of more than 100 V after stress. 0 4 8 12 16 20 24 0 0.2 0.4 0.6 0.8 1.0 0 2 4 6 8 10 WNx HEMT Ni/Au HEMT IG st re ss , A/ m m time, h IGof f , m A /m m
Fig. 1 Dependence of gate stress current and monitored gate leakage current on stress time for Ni/Au HEMTs and WNx/Au HEMTs
–8 –6 –4 –2 0 2 0 200 400 600 800 0 50 100 150 200 before stress after stress Id , mA/mm Vgs, V V ds = 5 V transconductance, mS/mm
Fig. 2 Comparison of DC characteristics of WNx/Au HEMTs before and
after gate forward current stress
–10 –5 0 5 10 15 20 0 5 10 15 20 25 30 35 0 10 20 30 40 50 60 Pout, dBm gain, dB Pou t , dB m; g a in , dB Pin, dBm PAE, %
Fig. 3 Power performance of 2 50 mm WNx-gated AlGaN/GaN HEMT at
2 GHz, biased at drain voltage of 40 V
Maximum output power density 5 W/mm, power added efficiency (PAE) 43%
The load-pull measurement results of the WNx-gated AlGaN/GaN
HEMTs are shown in Fig. 3. These devices were biased under class
AB conditions with a drain voltage of 40 V and a quiescent current density of 100 mA/mm. Their maximum output power density is 5 W/mm and the power added efficiency (PAE) is 43 %. These results indicate the feasibility of using a WNx contact metallisation
scheme in AlGaN/GaN HEMTs for high power microwave applications.
Conclusions: This Letter presents an AlGaN/GaN HEMT with nitro-gen-rich tungsten nitride gate metallisation. Forward gate current stress was carried out on the device for 24 h, with no apparent change in gate leakage current after stress. This device has a maximum output power density of 5 W/mm and a PAE of 43% at 2 GHz. Results imply that the WNx/Au Schottky gate contact on AlGaN/GaN
hetero-structures is stable and can serve as an alternative scheme for the gate metallisation of AlGaN/GaN HEMTs.
Acknowledgments: The authors acknowledge assistance from the National Science Council, and the Ministry of Economic Affairs, Taiwan, under the contracts 97-2221-E-009-156-MY2 and 97-EC-17-A-05-S1-020.
#The Institution of Engineering and Technology 2009 10 June 2009
doi: 10.1049/el.2009.1643
C.-Y. Lu, E.Y. Chang, J.-C. Huang, C.-T. Chang and C.-T. Lee (National Chiao Tung University, Department of Material Science and Engineering, 1001 University Road, Hsinchu 300, Taiwan)
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