Tamotsu Hashizume

Mechanisms of current collapse and gate leakage currents in AlGaN'GaN heterostructure field effect transistors

In order to clarify the mechanisms of drain current collapse and gate leakage currents in the AlGaN/GaN heterostructure field effect transistor (HFET), detailed electrical properties of the ungated portion and Schottky-gated portion of the device were investigated separately, using a gateless HFET structure and an AlGaN Schottky diode structure. The gateless device was subjected to plasma treatments and surface passivation processes including our novel Al2O3-based surface passivation. dc I–V curves of gateless HFETs were highly nonlinear due to virtual gating by surface states. After drain stress, air-exposed, H2 plasma-treated and SiO2-deposited gateless HFETs showed an initial large-amplitude exponential current transient followed by a subsequent smaller, slow, and highly nonexponential response. The former was explained by emission from deep donors at Ec−0.37u2009eV, and the latter by emission from surface states. Capture transients with stress-dependent capture barriers were also observed. An x-ray photoe...

Surface passivation of GaN and GaN/AlGaN heterostructures by dielectric films and its application to insulated-gate heterostructure transistors

We have systematically investigated effects of plasma processing, formation of Si-based dielectrics, and formation of a thin Al2O3 film on the chemical and electronic properties of GaN and GaN/AlGaN heterostructure surfaces. The surface treatment in H2-plasma excited by electron-cyclotron-resonance (ECR) source, produced nitrogen-vacancy-related defect levels at GaN and AlGaN surfaces, while the ECR-N2-plasma treatment improved electronic properties of the surfaces. The deposition of a SiO2 film on GaN and AlGaN surfaces was found to induce high-density interface states, due to unexpected and uncontrollable oxidation reactions on the surfaces during the deposition process. In comparison, the SiNx/GaN passivation structure prepared by ECR-plasma assisted chemical vapor deposition showed good interface properties with the minimum Dit value of 1×1011u200acm−2u200aeV−1. However, excess leakage currents governed by Fowler–Nordheim tunneling were observed in the SiNx/Al0.3Ga0.7N structure, due to a relatively small con...

Minority carrier diffusion length in GaN: Dislocation density and doping concentration dependence

We investigated the minority carrier diffusion length in p- and n-GaN by performing electron-beam-induced current measurements of GaN p–n junction diodes. Minority electron diffusion length in p-GaN strongly depended on the Mg doping concentration for relatively low dislocation density below 108cm−2. It increased from 220to950nm with decreasing Mg doping concentration from 3×1019to4×1018cm−3. For relatively high dislocation density above 109cm−2, it was less than 300nm and independent of the Mg doping concentration. On the other hand, the minority hole diffusion length in n-GaN was shorter than 250nm and less affected by the dislocation density and Si doping concentration. We discuss the doping-concentration and dislocation-density dependence of minority carrier diffusion length.

Effects of interface states and temperature on the C-V behavior of metal/insulator/AlGaN/GaN heterostructure capacitors

The impact of states at the insulator/AlGaN interface on the capacitance-voltage (C-V) characteristics of a metal/insulator/AlGaN/GaN heterostructure (MISH) capacitor was examined using a numerical solver of a Poisson equation and taking into account the electron emission rate from the interface states. A parallel shift of the theoretical C-V curves, instead of the typical change in their slope, was found for a MISH device with a 25-nm-thick AlGaN layer when the SiNx/AlGaN interface state density Dit(E) was increased. We attribute this behavior to the position of the Fermi level at the SiNx/AlGaN interface below the AlGaN valence band maximum when the gate bias is near the threshold voltage and to the insensitivity of the deep interface traps to the gate voltage due to a low emission rate. A typical stretch out of the theoretical C-V curve was obtained only for a MISH structure with a very thin AlGaN layer at 300u2009°C. We analyzed the experimental C-V characteristics from a SiNx/Al2O3/AlGaN/GaN structure me...

Leakage mechanism in GaN and AlGaN Schottky interfaces

Based on detailed temperature-dependent current–voltage (I–V–T) measurements the mechanism of leakage currents through GaN and AlGaN Schottky interfaces is discussed. The experiments were compared to calculations based on thin surface barrier model in which the effects of surface defects were taken into account. Our simulation method reproduced the experimental I–V–T characteristics of the GaN and AlGaN Schottky diodes, and gave excellent fitting results to the reported Schottky I–V curves in GaN for both forward and reverse biases at different temperatures. The present results indicate that the barrier thinning caused by unintentional surface-defect donors enhances the tunneling transport processes, leading to large leakage currents through GaN and AlGaN Schottky interfaces.

Analysis and control of excess leakage currents in nitride-based Schottky diodes based on thin surface barrier model

Using a rigorous computer simulation program for current transport through a Schottky barrier with an arbitrary potential profile, the leakage current mechanism in GaN and AlGaN Schottky diodes was investigated on the basis of the thin surface barrier (TSB) model recently proposed by the authors’ group. Computer simulation assuming various possible defect density distributions was carried out to reproduce the measured temperature dependent current voltage (I–V)-temperature characteristics of the GaN and AlGaN Schottky diodes which showed excessive reverse leakage. By assuming exponentially decaying distributions from surface for defect donors with energy depth of 0.25 eV for GaN and 0.37 eV for Al0.15Ga0.85N, I–V curves measured by our group as well as reported in the literatures were almost completely reproduced both in forward and reverse direction over a wide temperature range. The defect donors are proposed to be N vacancies or their related complexes that are formed during metal deposition. The resul...

Mechanism of current leakage through metal/n-GaN interfaces

Abstract Detailed current–voltage–temperature (I–V–T) measurements were performed on the Schottky diodes fabricated on MOVPE-grown n-GaN layers. A large deviation from the thermionic emission (TE) transport was observed in the reverse I–V curves with a large excess leakage. From the calculation based on the thermionic-field emission (TFE) model, it was found that the tunneling plays an important role in the carrier transport across the GaN Schottky barrier even for doping densities as low as 1×1017xa0cm−3. A novel barrier-modified TFE model based on presence of near-surface fixed charges or surface states is proposed to explain the observed large reverse leakage currents.

Mechanism of surface conduction in the vicinity of Schottky gates on AlGaN∕GaN heterostructures

Lateral surface leakage current (Is) on an AlGaN∕GaN heterostructure was systematically investigated by using a two-parallel gate structure with a gap distance (LGG) of 200nm–5μm. The surface current Is systematically increased as LGG decreased. A simple resistive layer conduction that should show 1∕LGG dependence failed to account for the drastic increase in Is when LGG was reduced to less than 1μm. However, no dependence on LGG was seen in vertical current that flows in the Schottky interface. The Is showed a clear temperature dependence proportional to exp(−T−1∕3), indicating two-dimensional variable-range hopping through high-density surface electronic states in AlGaN. A pronounced reduction in surface current of almost four orders of magnitude was observed in a sample with SiNx passivation.

Drain Current Stability and Controllability of Threshold Voltage and Subthreshold Current in a Multi-Mesa-Channel AlGaN/GaN High Electron Mobility Transistor

The surrounding-field effect in a multi-mesa-channel (MMC) with an AlGaN/GaN structure, in which a periodic trench structure is fabricated directly under a gate electrode, was successfully observed. This effect resulted in a shallower threshold voltage, a smaller subthreshold slope, and a higher current drivability of a high electron mobility transistor (HEMT) than those of a standard planar-type HEMT. In addition, the MMC HEMT showed a low knee voltage, even with a wide spacing between the gate and drain electrodes. Excellent current stability in the saturation region of the MMC HEMT, probably due to the effective radiation of heat from both mesa sides of the channel, was also observed. Both planar and MMC HEMTs showed similar breakdown voltages under off-state operation, indicating no significant degradation in the breakdown characteristics of AlGaN/GaN HEMTs with a periodic trench structure in the gate region.

Large reduction of leakage currents in AlGaN Schottky diodes by a surface control process and its mechanism

Leakage currents in AlGaN Schottky diodes were investigated systematically by using a rigorous computer simulation based on the thin surface barrier model taking account of unintentionally doped surface donors. The leakage currents in AlGaN Schottky diodes have stronger bias dependence and smaller temperature dependences as compared with those of GaN diodes. It was shown that these features were associated with shallow oxygen donors located near the AlGaN surface. Then, an attempt was made to remove oxygen and suppress leakage currents by a surface control process using an ultrathin Al layer and subsequent annealing. An in situ x-ray photoelectron spectroscopy analysis indicated the formation of Al2O3 layer during the surface control process, suggesting efficient gettering of oxygen from the surface. C-V analysis directly indicated the reduction of shallow donors by the surface control process. A remarkable reduction of reverse leakage currents of four to five orders of magnitude took place in large area ...