/A Novel Manufacturing Process of AlGaN/GaN HEMT for X-Band High-Power Application on Si (111) Substrate Cong Wang, Ram Krishna Maharjan, Sung-Jin Cho, and Nam-Young KimProceedings of APMC 2012: MA130224

In this paper, successful operation at 10 GHz of 0.5 m gamma gate AlGaN/GaN high electron mobility transistor (HEMT) is demonstrated on Si (111) substrate.

Various material and processing approaches regarding double surface passivation and post-gate annealing processes are evaluated in terms of device performances.

In order to achieve better immunity to current collapse effects, we conducted experiments that investigate the relationship between the HEMTs electrical characteristics and different passivation films (SiNx or SiO2) using plasma-enhanced chemical vapor deposition (PECVD).*Abstract

Double Surface Passivation ProcessThe typical I-V output and transfer characteristics of these three AlGaN/GaN HEMTs are shown in Fig. 1 (a) and (b), respectively. Samples A and B exhibit better saturation and pitch-off characteristics than the unpassivated HEMT.They yielded a maximum saturation current density (IDS max) of 643 and 540 mA/mm, respectively, showing 40% and 17% increases relative to that found for the unpassivated HEMT.*Fig. 1. (a) I-V characteristics with varying gate biases, and (b) the transfer characteristics of the SiNX/SiNX-passsivated HEMT and SiO2/SiO2-passivated HEMT compared to the unpassivated HEMT.

*The I-V output and transfer characteristics of the two type AlGaN/GaN HEMTs are shown in Fig. 2 (a) and (b). An increase in the IDS max (16%) from 643 mA/mm to 750 mA/mm and the gm max (10%) from 200 mS/mm to 220 mS/mm can be observed after the post-gate N2 RTA treatment.

enhancement of these DC characteristics is primarily cause by the RTA process, which reduces the electrically active states at the Schottky metal/AlGaN interfaces. Fig. 2. (a) I-V characteristics with varying gate biases, (b) transfer characteristics

*After the N2 annealing treatment, the gate-drain breakdown voltage of the HEMTs increased from 113 V to 141 V. increase in the breakdown voltage.

This increase of breakdown voltage is due to the decrease of the gate leakage current. An increase of the forward gate threshold voltage to 2 V has been verified in Sample D. The focused ion beam (FIB) photographs of the cross-sectional gamma gate with double surface passviation structure are shown in Fig. 2 (d) and (e).(c) the gate-drain breakdown voltage of the SiNX/SiNX-passsivated HEMT using post-gate N2 RTA treatment compared to the as-fabricated HEMT, (d) the cross-sectional FIB image of the proposed HEMT, and (e) the enlarged gamma gate with the proposed double passivation structure.

Fig. 3 (a) shows an optical micrograph of a fabricated 200 m twofinger freestanding HEMT with a gate length of 0.5 m, a drain-gate space of 2.5 m, and a gate-source space of 1 m.

Fig. 3 (b) shows the short circuit current gain (|h21|) and the maximum stable gain (MSG) for the device with 2 (200 0.5) m2 gate periphery. The fT and fMAX are determined by the |h21| and the MSG, respectively. The extrinsic fT and fMAX are 24.6 GHz and 45.4 GHz respectively at VDS = 10 V and VGS = -3 V.*Results And Discussions

Large signal measurements have been performed using a load-pull system at 10 GHz. The device was biased at a drain bias of 30 V.

The measured input power (Pin) against the output power (Pout) responses of the device is illustrated in Fig. 3 (c), which illustrates an output power density of 5.8 W/mm, a peak PAE of 51% at 21 dBm Pin with a linear gain of 14.4 dB and a power gain of about 13.68 dB.

ConclusionThis paper presents the state-of-the-art results obtained by 0.5 m gamma-gate AlGaN/GaN HEMTs fabricated on Si (111) substrates.

In order to meet the performance improvement requirements needed for high power applications, these optimized solutions which enhance the characteristics and reduce the cost of HEMTs on Si substrates are proposed.

Excellent performances have been achieved for 0.5 m gate-length HEMTs. The maximum output power density of 5.8 W/mm is achieved, with an associated power gain of 13.68 dB and a maximum PAE of 51%. Due to these results, this manufacturing process is shown to be an optimal solution for fabricating HEMTs in X-band high-power applications.*

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