Current Collapse in AlGaN/GaN HEMTs Investigated by Electrical and Optical Characterizations

Mizutani, Takashi; Ohno, Yutaka; Akita, M.; Kishimoto, Shigeru; Maezawa, K.

doi:10.1002/1521-396x(200212)194:2<447::aid-pssa447>3.0.co;2-7

Cited by 38 publications

(35 citation statements)

References 9 publications

(10 reference statements)

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“…Thus, they revealed that enhancement of the tunneling transport processes by the barrier thinning due to the processing-induced V N -related defect donor is the dominant mechanism associated with large leakage currents through GaN and AlGaN Schottky interfaces. 9 The current collapse effects have often been observed AlGaN/GaN HFETs under quiescent gate stress 10 or pulsemode gate stress. 11,12 The collapse is also induced by drain stress.…”

Section: Introductionmentioning

confidence: 99%

Lateral tunneling injection and peripheral dynamic charging in nanometer-scale Schottky gates on AlGaN/GaN hetrosturucture transistors

Kotani¹,

Kasai²,

Hashizume³

et al. 2005

Journal of Vacuum Science &Amp; Technology B: Microelectronics and Nanometer Structures Processing, Measurement, and Phenomena

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The gate leakage and gate control characteristics of AlGaN/GaN heterostructure field effect transistors ͑HFETs͒ were systematically investigated in an attempt to clarify possible effects of surface states. The experiments were compared to rigorous computer simulations. We observed large amounts of leakage currents in the Schottky diodes fabricated on the AlGaN epitaxial layers. By the calculation based on a thin surface barrier model in which the effects of surface defect donor were taken into account, this large leakage was well explained by enhancement of tunneling transport processes due to the barrier thinning associated with ionization of surface-defect donor. On the other hand, the analysis on the current-voltage characteristics for the nanometer-scale Schottky contacts on AlGaN/GaN HFETs, indicated additional lateral leakage components. The comparison of the gate control characteristics between experiment and calculation clearly showed that the effective lateral expansion of gate length significantly impeded the g m enhancement by the reduction of geometrical gate length. This can be explained by the lateral electron tunneling process at the AlGaN surface stimulated by the pronounced gate leakage currents. Due to frequent tunneling transfer at the gate periphery, surface state occupancy near the gate becomes governed by the metal Fermi level, causing the dynamic surface state charging effects. This resulted in effective widening of the gate length, leading to degradation of gate control performance in AlGaN/GaN HFETs.

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Section: Introductionmentioning

confidence: 99%

Lateral tunneling injection and peripheral dynamic charging in nanometer-scale Schottky gates on AlGaN/GaN hetrosturucture transistors

Kotani¹,

Kasai²,

Hashizume³

et al. 2005

Journal of Vacuum Science &Amp; Technology B: Microelectronics and Nanometer Structures Processing, Measurement, and Phenomena

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“…However, some crucial problems such as large leakage current in Schottky contacts [1][2][3][4] and drain current collapse [5,6] that are related to the AlGaN surface are not completely solved. Degradation of output power under large bias and high frequency conditions due to these is particularly serious for enhancement high-power/high-frequency performances of AlGaN/GaN HFETs [7].…”

Section: Introductionmentioning

confidence: 99%

“…Although these problems impede full-scale practical exploitation of nitride-based transistors, they are not fully solved nor understood except that these problems are due to filling and emptying of surface deep traps [5,6]. However, how such filling and emptying of states takes place have not been clarified yet.…”

Section: Introductionmentioning

confidence: 99%

Tunneling Injection of Electrons at Nanometer-Scale Schottky Gate Edge of AlGaN/GaN Heterostructure Transistors and Its Computer Simulation

Kotani

Kasai

Hasegawa

et al. 2005

e-J. Surf. Sci. Nanotechnol.

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Gate leakage currents in AlGaN/GaN HFETs were investigated by comparing experiments with computer simulations based on the thin surface barrier (TSB) model involving unintentional surface donors. Leakage currents in large area Schottky diodes were explained by the TSB model involving nitrogen vacancy related deep donors and oxygen shallow donors. On the other hand, in AlGaN/GaN HFETs with nanometer scale Schottky gates, gate leakage currents include an additional leakage component due to lateral electron injection through tunneling at the gate edge where the barrier thinning is mainly controlled by oxygen donors. By combining vertical and lateral tunneling components, experiments could be reproduced on computer. Lateral components may be responsible for current collapse.

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“…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][2][3][4][5][6] and drain stress. 2,5,7 Some models based on the electron trapping by surface states have been proposed.…”

mentioning

confidence: 99%

“…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.…”

mentioning

confidence: 99%

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

Hashizume¹,

Ootomo²,

Hasegawa³

2003

Applied Physics Letters

249

147

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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...

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Current Collapse in AlGaN/GaN HEMTs Investigated by Electrical and Optical Characterizations

Cited by 38 publications

References 9 publications

Lateral tunneling injection and peripheral dynamic charging in nanometer-scale Schottky gates on AlGaN/GaN hetrosturucture transistors

Lateral tunneling injection and peripheral dynamic charging in nanometer-scale Schottky gates on AlGaN/GaN hetrosturucture transistors

Tunneling Injection of Electrons at Nanometer-Scale Schottky Gate Edge of AlGaN/GaN Heterostructure Transistors and Its Computer Simulation

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

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