High-Performance Microwave Gate-Recessed AlGaN/AlN/GaN MOS ...

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IEEE ELECTRON DEVICE LETTERS, VOL. 32, NO. 5, MAY 2011

High-Performance Microwave Gate-Recessed AlGaN/AlN/GaN MOS-HEMT With 73% Power-Added Efficiency Yue Hao, Senior Member, IEEE, Ling Yang, Xiaohua Ma, Member, IEEE, Jigang Ma, Menyi Cao, Caiyuan Pan, Chong Wang, and Jincheng Zhang

Abstract—Gate-recessed AlGaN/AlN/GaN metal–oxide– semiconductor heterostructure high-mobility transistors (MOS-HEMTs) on SiC substrate are fabricated. The device with a gate length of 0.6 μm and a gate periphery of 100 μm exhibits a maximum dc drain current density of 1.59 A/mm at VGS = 3 V with an extrinsic transconductance (gm ) of 374 mS/mm. An extrinsic current gain cutoff frequency (fT ) of 19 GHz and a maximum oscillation frequency (fmax ) of 50 GHz are deduced from S-parameter measurements. The output power density is 13 W/mm, and the associated power-added efficiency is 73% at 4-GHz frequency and 45-V drain bias. The power performance is comparable to state-of-the art AlGaN/GaN HEMTs, which demonstrates the great potential of gate-recessed MOS-HEMTs as a very promising alternative to GaN HEMTs. Index Terms—AlGaN/AlN/GaN metal–oxide–semiconductor heterostructure high-electron mobility transistor (MOS-HEMT), gate recessed, L-gate field plate, power amplifier. Fig. 1. Schematic cross section of a gate-recessed AlGaN/AlN/GaN MOS-HEMT.

I. I NTRODUCTION

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ALLIUM nitride high-electron mobility transistors (HEMTs) are great candidates in the field of high-power electronics [1]–[4]. Wong et al. [5] reported an output power density of 4.1 W/mm and an associated power-added efficiency (PAE) of 71% for N-face MIS-HEMTs at 4 GHz. Chu et al. [6] reported an output power density of 13.1 W/mm and an associated PAE of 72% for V-gate GaN HEMTs at 4 GHz. One critical issue limiting the RF power performance is the device degradation, primarily caused by high gate leakage current [7], [8]. Further improvement of III-nitride HFET performance results from high-quality SiO2 /AlGaN interface. This opened up a way to fabricate insulated-gate field-effect transistors [9], [10]. Low gate leakage currents allow for a larger voltage swing and better device reliability. Adivarahan reported a MOS-HEMT with an output power density of

18.6 W/mm and a PAE of 49.5% at a drain bias of 55 V at 2 GHz [8]. Those encouraging results have motivated us to develop a process for high-power gate-recessed AlGaN/AlN/GaN MOS-HEMT. In this letter, we demonstrate a 0.6-μm gate-recessed atomic-layer-deposited (ALD) Al2 O3 /AlGaN/AlN/GaN MOS-HEMTs on SiC substrates. The low-damage gate recess allowed for a reduction in the gate-to-channel distance to 16 nm. No current collapse was observed under 500-ns gatepulse measurements. A Class-B microwave performance with a maximum power density of 13 W/mm and an associated PAE of 73% (Vds = 45 V) at 4 GHz was demonstrated. It is also believed to be the best microwave performance ever reported at 4 GHz for recessed Al2 O3 /AlGaN/AlN/GaN MOS-HEMTs. The details of the device structure, processing, and microwave performance will be explained in the following sections.

Manuscript received January 28, 2011; revised February 11, 2011; accepted February 12, 2011. Date of current version April 27, 2011. This work was supported by the Major Program and State Key Program of National Natural Science Foundation of China under Grants 60736033 and 60890191. The review of this letter was arranged by Editor J. A. del Alamo. The authors are with the Key Laboratory of Wide Band-Gap Semiconductor Materials and Devices, School of Microelectronics, Xidian University, Xi’an 710071, China (e-mail: [email protected]; [email protected]; xhma@ xidian.edu.cn; [email protected]; [email protected]; 05041188@ 163.com; [email protected]; [email protected]). Color versions of one or more of the figures in this letter are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/LED.2011.2118736

II. G ROWTH E PITAXY AND D EVICE P ROCESSING Fig. 1 shows a schematic cross section of the gate-recessed AlGaN/AlN/GaN MOS-HEMT. The undoped AlGaN/AlN/ GaN heterostructure layers were grown at the University of Xidian on a 2-in semi-insulating 4H-SiC substrate by metal–organic chemical vapor deposition. The epitaxial layers contain a 100-nm AlN nucleation layer, a 1.6-μm unintentionally doped GaN buffer, a 1-nm AlN interlayer, and a 20-nm Al0.3 Ga0.7 N barrier. Hall measurements show a sheet

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HAO et al.: HIGH-PERFORMANCE MICROWAVE GATE-RECESSED AlGaN/AlN/GaN MOS-HEMT

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Fig. 2. (a) DC I–V curve of gate-recessed AlGaN/AlN/GaN MOS-HEMT with a high maximum output current of 1.59 A/mm. (b) Transfer and transconductance characteristics for nonrecessed and recessed MOS-HEMT.

carrier density of 1.4 × 1013 cm−2 and an electron mobility of 1500 cm2 /V • s. The sheet resistance is approximately 290 Ω/ at room temperature. Device processing started with Ti/Al/Ni/Au source and drain electrodes. These were annealed at 870 ◦ C for 30 s in a rapid thermal anneal. Device isolation was achieved by reactive ion etching using Cl2 plasma. SiN passivation was achieved with plasma-enhanced chemical vapor deposition. The gate recess was achieved by a sequence of plasma etching steps. First, the gate footprint (Lg = 0.6 μm) was opened by etching SiN using CF4 plasma. Then, a low-power Cl2 plasma etching was performed to reduce the gate-to-channel distance to 16 nm. The Cl2 recipe used a pressure of 10 mtorr, a flow rate of 15 sccm, and an RF power of 50 W. The etch rate is 12 nm/min. The low RF power leads to low surface plasma damage. After the gate recess, no degradation of the sourceto-drain current was observed, as shown in the inset of Fig. 1. The AlGaN surface is not perfectly hydroxide terminated after chemical cleaning. Thus, an initial phase of the ALD process is necessary to provide additional -OH groups. (Ten H2 O pulses were introduced to form an oxide layer.) Then, a 5-nm Al2 O3 film was deposited at 300 ◦ C using an ALD system, with TMA and H2 O as precursors. After gate photolithography, a Ni/Au/Ni electrode was evaporated. The drain side of the gate electrode overlaps the SiN and the Al2 O3 film by a length of 0.7 μm (LFP = 0.7 μm). All devices in this letter have a gate width of 2 × 50 μm, a gate–source spacing of 0.7 μm, and a gate–drain spacing of 2.8 μm. The pads are a coplanar waveguide layout.

III. DC AND M ICROWAVE R ESULTS Fig. 2 shows the dc characteristics of a typical gate-recessed AlGaN/AlN/GaN MOS-HEMT with a gate length of 0.6 μm. The saturation current at VGS = 3 V was 1.59 A/mm. A maximum extrinsic transconductance of 374 mS/mm was obtained with these transistors [Fig. 2(b)]. A clear shift of pinchoff voltage from −3.9 to −3.4 V is observed as a result of recessing [Fig. 2(b)]. This can be attributed to the decrease in gate–channel separation. An increase in maximum transconductance from 356 to 374 mS/mm is also observed.

Fig. 3. (a) DC and pulsed I–V curves of gate-recessed AlGaN/AlN/GaN MOS-HEMT under a 500-ns gate and drain pulse. (b) Small-signal measurement of gate-recessed AlGaN/AlN/GaN MOS-HEMT.

The transistors showed no current dispersion in pulsed I−V measurement, as shown in Fig. 3(a). This demonstrates a high quality of the Al2 O3 /AlGaN interface. The quiescent bias is (VDS0 , VGS0 ) = (45 V, −6 V). Under the quiescent bias, there is essentially no drain current flowing through the channel. A 500-ns pulse was applied to the gate and drain, as shown in the inset of Fig. 3(a). Small-signal microwave measurements were also performed on this transistor using coplanar probes. S-parameters are measured in the frequency range from 10 MHz to 40 GHz. The value of the unity current gain cutoff frequency (fT ) is determined by the extrapolation of the current gain |H21 |. The maximum oscillation frequency (fmax ) is determined from the Mason’s gain [Fig. 3(b)]. The extrinsic fT and fmax values are, respectively, 19 and 50 GHz at VDS = 20 V and VGS = −2 V. Continuous wave RF power measurement was performed at 4 GHz using a Maury load-pull system. Devices with a gate length of 0.6 μm were used. The drain bias was 45 V. The gate was biased at pinchoff voltage for the Class-B operation. Both input and output matching were tuned for optimum PAE. A saturated power density (Psat ) of 13 W/mm was achieved with an associated PAE of 73% [Fig. 4(a)]. The load match was ΓL = 0.872 + j0.079, corresponding to the maximum |ΓL | available in our tuning range (max |ΓL | ≈ 0.88). Source matching point was ΓS = 0.756 + j0.433, which is still within the available tuning range of the system (max |ΓS | ≈ 0.88). This is one of the highest reported PAEs at this bias and power level [7], [8]. The excellent power performance is attributed

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IEEE ELECTRON DEVICE LETTERS, VOL. 32, NO. 5, MAY 2011

IV. C ONCLUSION The gate-recessed AlGaN/AlN/GaN MOS-HEMT has been designed, fabricated, and measured. Devices with a high output current density and transconductance have been achieved, owing to a low-damage gate-recess process and a highquality Al2 O3 film. The epitaxial passivation approach, in conjunction with gate-recessed MOS structure, successfully eliminated the dc–RF dispersion. State-of-the-art power performance has been demonstrated, suggesting that the recessed Al2 O3 /AlGaN/AlN/GaN MOS-HEMTs are very promising for C-band power applications. R EFERENCES

Fig. 4. (a) Power sweep curves of gate-recessed AlGaN/AlN/GaN MOS-HEMT. (b) 4-GHz power density and PAE as a function of the drain bias.

to the high-quality AlGaN/AlN/GaN heterojunction and the advanced MOS gate-recessed structure. Fig. 4(b) shows the power density and PAE as a function of the drain bias. A slight increase in PAE was observed over the bias range. The high output power and its continuous increase with drain bias indicate minimal dispersion in these devices. The three-terminal OFF-state breakdown voltage of the devices (Lgd = 2.8 μm) is about 82 V. (Vbr is defined at a drain current of 1 mA/mm.) This limits the stable power operation to below 45-V drain bias. No significant RF power degradation was observed during a 10-h electric stress at a power level of 13 W/mm and an associated PAE of 73%. The excellent microwave performance of gate-recessed Al2 O3 /AlGaN/AlN/GaN MOS-HEMTs made a solid foundation for the development of large-periphery power-microwave devices.

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