Suppression of current collapse in insulated gate AlGaN/GaN heterostructure field-effect transistors using ultrathin Al2O3 dielectric

Abstract

We investigated effects of electronic states at free surfaces of AlGaN/GaN heterostructure field-effect transistors ͑HFETs͒ on the inner current transport at the heterointerfaces. The analysis on transient currents for the air-exposed and H 2 -plasma-treated devices showed that N-vacancy-related near-surface traps play an important role in current collapse in AlGaN/GaN HFETs. An Al 2 O 3 -based surface passivation scheme including an N 2 -plasma surface treatment was proposed and applied to an insulated-gate HFET. A large conduction-band offset of 2. Although significant progress has been achieved in the GaN-based high-power/high-frequency electronic devices, surface-related problems still need an immediate solution. In particular, so-called current collapse effects not only degrade microwave output performance but also impede reliable operation of the GaN-based power devices. The current collapse phenomena have often been observed in Schottky-gate AlGaN/GaN heterostructure field-effect transistors ͑SG HFETs͒ under both gate stress 1-6 and drain stress. 2,5,7 Some models based on the electron trapping by surface states have been proposed. 6,7 However, the mechanism for the current collapse is not clarified as yet.In this letter, we report on passivation effects of the Al 2 O 3 -based insulated-gate ͑IG͒ structure on current collapse in AlGaN/GaN HFETs. Although SiN x films have been used to mitigate the surface problems on the GaN-based devices, 4 -9 it can be argued that its band gap (E G ϳ5 eV) and dielectric constant ͑⑀ϳ7͒ are not suitable for utilization as a gate barrier to Al x Ga 1Ϫx N ͑e.g., E G ϳ4.1 eV and ⑀ϳ9 for xϭ0.3͒. In this aspect, Al 2 O 3 has an advantage over SiN x for the application to IG-type AlGaN/GaN HFETs, due to its larger band gap and higher dielectric constant. For understanding the collapse mechanism and optimizing passivation process, the dynamic response of surface states was investigated in terms of transient currents in gateless HFETs.The AlGaN/GaN heterostructures used in the present study are schematically shown in Fig. 1. The structure consists of undoped GaN, undoped Al x Ga 1Ϫx N, Si-doped Al x Ga 1Ϫx N, and undoped Al x Ga 1Ϫx N layers, all grown by metalorganic vapor phase epitaxy on c-plane sapphire or 6H-SiC substrate. The Al content x ranged from 0.28 to 0.30. Typical values of the electron concentration and mobility of the samples at room temperature were 1.1ϫ10 13 cm Ϫ2 and 900 cm 2 /V s, respectively. As an ohmic contact, a Ti/Al/ Ti/Au layered structure was formed on the surface of GaN/ AlGaN, followed by annealing in N 2 ambient at 800°C for 2 min.A gateless HFET structure with a drain-source spacing of 4 m, shown in Fig. 1͑a͒, was prepared to investigate the correlation between the surface electronic states and inner current transport. The surface of the gateless device was subjected to H 2 plasma and N 2 plasma excited using an electron cyclotron resonance ͑ECR͒ source with a microwave power of 50 W. The process temperature and time were 280°C and 1 min, respectiv...

Characterization of interface states in Al2O3/AlGaN/GaN structures for improved performance of high-electron-mobility transistors

“…GaN-based MOS-HEMTs using an Al 2 O 3 as the insulator have been demonstrated by many groups. 3,4,[6][7][8][9] The typical values of bandgap and permittivity reported for the Al 2 O 3 film are 7-9 eV and 8-10, respectively. [6][7][8][9][10] The Al 2 O 3 film prepared by atomic layer deposition (ALD), in particular, showed relatively low electronic state densities in the Al 2 O 3 /n-GaN system.…”

“…3,4,[6][7][8][9] The typical values of bandgap and permittivity reported for the Al 2 O 3 film are 7-9 eV and 8-10, respectively. [6][7][8][9][10] The Al 2 O 3 film prepared by atomic layer deposition (ALD), in particular, showed relatively low electronic state densities in the Al 2 O 3 /n-GaN system. 9,[11][12][13] However, in the MOS-HEMT structures, the insulator/semiconductor interface is usually formed on the AlGaN/GaN heterostructures, resulting in the existence of two interfaces under the gate electrode.…”

Characterization of capture cross sections of interface states in dielectric/III-nitride heterojunction structures

“…[1][2][3][4][5][6] It was demonstrated by many groups that GaN-HEMTs with an insulated gate (IG) structure, apart from being necessary for achieving normally off operation, exhibit important advantages over Schottkygate-based GaN-HEMTs such as lower gate leakage current, higher breakdown voltage, better thermal stability of the gate and less current collapse. [6][7][8][9][10] In order to obtain a good IG structure, suitable insulating materials such as Al 2 O 3 , SiO 2 , and SiN ensuring a large band offset and high permittivity are widely applied. [11][12][13] Furthermore, the chosen insulator material should provide a high quality, stable insulator/semiconductor interface with a low density of interface electronic states.…”

Non-localized trapping effects in AlGaN/GaN heterojunction field-effect transistors subjected to on-state bias stress

“…Furthermore, very recently, Tajima and Hashizume 5 proposed a third model, in which current collapse can also be induced by surface trapping at the source side of the gate edge, where an electric field of as high as 5 MV/cm exists during the application of off-stress. Owing to the success of using surface passivation to suppress the current collapse, [6][7][8][9] the virtual gate mechanism has become widely accepted as the main mechanism for current collapse. 10 Following this understanding, regarding the reliability of AlGaN/ GaN HFETs, degradation at the gate edge has been recognized as the only degradation mechanism regardless of whether onstress or off-stress is applied.…”