Narihiko Maeda

Systematic study of insulator deposition effect (Si3N4, SiO2, AlN, and Al2O3) on electrical properties in AlGaN/GaN heterostructures

To systematically examine the effect of insulator deposition on the electrical properties in AlGaN/GaN heterostructures, the Si- and Al-based insulators (Si3N4, SiO2, AlN, and Al2O3) have been deposited on Al0.3Ga0.7N/GaN heterostructures. A significant increase in two-dimensional electron gas (2DEG) density (Ns) was observed for all the insulators with the order of Ns(Al2O3) > Ns(AlN) ~ Ns(SiO2) > Ns(Si3N4) > N0 (N0: Ns without insulators). This resulted in a decrease in sheet resistance (R) with the smallest order of R(Al2O3) < R(AlN) < R(Si3N4) < R0 ~ R(SiO2) (R0: R without insulators). This order is the same as that of Ns except for SiO2, where the 2DEG mobility largely degraded due to the diffusion of Si atoms into nitride layers. The increase in Ns was theoretically analyzed in terms of the change in the potential profile, and the following parameters were extracted: (i) the surface potential barrier (B), and (ii) the interface charge (NInt) between an insulator and AlGaN. B (eV) was estimated to be 1.7 (Si3N4), 2.2 (AlN), 2.7 (Al2O3), and 3.6 (SiO2), exhibiting a positive correlation between B and the bandgap of the insulator. NInt (1013 cm-2) was estimated to be ~0 (Si3N4), 0.1 (SiO2), 0.3 (AlN), and 0.5 (Al2O3); thus, the interface was found to be positively charged for AlN and Al2O3, whereas it was found to be almost neutral for Si3N4 and SiO2. Thus, the insulator deposition effect has been shown to be significant and to vary among insulators. The analysis shown here offers a guideline for understanding and designing the electrical properties in AlGaN/GaN heterostructures, where insulators are deposited as surface passivation and/or gate insulators.

Superior Pinch-Off Characteristics at 400°C in AlGaN/GaN Heterostructure Field Effect Transistors

AlGaN/GaN field effect transistors have been fabricated on SiC(0001) substrates, and the I–V characteristics in the devices have been examined from room temperature to 400°C. In addition to excellent current saturation characteristics, sufficient pinch-off characteristics have been obtained up to a temperature of 400°C for the first time, as the result of reduced crystal defects and reduced etching damage in the devices. The temperature dependence of the transconductance has been also examined. The degradation rate in the transconductance has been proved to be low above 300°C: the transconductance degraded by only 8% for a temperature increase from 350 to 400°C. Sufficient pinch-off characteristics and a relatively low degradation rate in the transconductance ensure the practical use of the devices at high temperatures.

High drain current density and reduced gate leakage current in channel-doped AlGaN∕GaN heterostructure field-effect transistors with Al2O3∕Si3N4 gate insulator

Channel-doped AlGaN∕GaN heterostructure field-effect transistors (HFETs) with metal-insulator-semiconductor (MIS) structures have been fabricated to obtain the high drain current density and reduced gate leakage current. A thin bilayer dielectric of Al2O3(4nm)∕Si3N4(1nm) was used as the gate insulator, to simultaneously take advantage of the high-quality interface between Si3N4 and AlGaN, and high resistivity and a high dielectric constant of Al2O3. A MIS HFET with a gate length of 1.5μm has exhibited a record high drain current density of 1.87A∕mm at a gate voltage (Vg) of +3V, which is ascribed to a high applicable Vg and a very high two-dimensional electron gas (2DEG) density of 2.6×1013cm−2 in the doped channel. The gate leakage current was reduced by two or three orders of magnitude, compared with that in normal HFETs without a gate insulator. The transconductance (gm) was 168mS∕mm, which is high in the category of the MIS structure. Channel-doped MIS HFETs fabricated have thus been proved to exhibit...

High Temperature Characteristics of Insulated-Gate AlGaN/GaN Heterostructure Field-Effect Transistors with Ultrathin Al2O3/Si3N4 Bilayer

The device performance of AlGaN/GaN-based metal–insulator–semiconductor heterostructure field-effect transistors (MIS-HFETs) with an ultrathin (1 nm/0.5 nm) Al2O3/Si3N4 bilayer has been investigated at elevated temperatures up to 200°C. The devices exhibited excellent transconductance characteristics with high maximum transconductances and ultralow gate current leakages under reverse gate bias conduction at both room and high temperatures due to the employment of an ultrathin bilayer with large dielectric constants and the large conduction band offset between Al2O3 and nitrides. The excellent characteristics observed at high temperatures might indicate the very high interfacial quality between nitrides and bilayer insulator. The results in this report demonstrate that Al2O3/Si3N4 bilayer insulator is a superior candidate for nitride-based MIS-HFET devices operating at high temperatures.

Mechanism of superior suppression effect on gate current leakage in ultrathin Al2O3/Si3N4 bilayer-based AlGaN/GaN insulated gate heterostructure field-effect transistors

On the basis of the thin barrier surface (TSB) model, the mechanism of gate current leakage under reverse gate–source bias in nitride-based heterostructure field effect transistors (HFETs) and metal–insulator–semiconductor (MIS) HFETs with an ultrathin (1 nm/0.5 nm) Al2O3/Si3N4 bilayer has been investigated. The simulations show that the electron tunneling through the Schottky barrier is the dominant mechanism for gate current in conventional HFETs due to the high density of donor like defects on the surface. An Al2O3/Si3N4 bilayer insulator can substantially reduce the donor like surface defect density and then significantly suppress the gate current leakage in nitrides-base MIS-HFET devices.

Comparison of AlGaN/GaN Insulated Gate Heterostructure Field-Effect Transistors with Ultrathin Al2O3/Si3N4 Bilayer and Si3N4 Single Layer

Device performances have been compared between two types of AlGaN/GaN metal-insulator-semiconductor heterostructure field effect transistors (MIS-HFETs) with Al2O3/Si3N4 bilayers and a Si3N4 single layer. Al2O3/Si3N4 bilayer-based MIS-HFETs have much lower gate current leakage than Si3N4-based MIS devices by more than 3 orders of magnitude under reverse gate biases. An ultralow gate leakage of 1×10-11 A/mm at -15 V has been achieved in the Al2O3/Si3N4 bilayer-based MIS devices though higher maximum drain-source current has been obtained in the Si3N4-based MIS devices. A maximum transconductance of more than 180 mS/mm with ultra-low gate leakage has been achieved in the ultrathin Al2O3/Si3N4 bilayer-based MIS-HFET device with a gate length of 1.5 µm, which is much higher than that of less than 130 mS/mm in the Si3N4-based MIS devices. The reduction in the transconductance of Al2O3/Si3N4 bilayer-based devices was much smaller than that in the Si3N4-based MIS devices due to the employment of ultrathin bilayers with a large dielectric constant.This work demonstrates that an Al2O3/Si3N4 bilayer insulator is a superior candidate for nitride-based MIS-HFET devices.

Al2O3/Si3N4 insulated gate channel-doped AlGaN/GaN heterostructure field-effect transistors with regrown ohmic structure: Low gate leakage current with high transconductance

An advanced structure of AlGaN/GaN heterostructure field-effect transistors (HFETs) has been proposed and fabricated, which is characterized by the following structural features: (i) a metal-insulator-semiconductor (MIS) structure using an Al2O3/Si3N4 bilayer gate insulator to reduce the gate leakage current, (ii) a thin AlGaN barrier with a doped channel to simultaneously obtain the high transconductance and high drain current, and (iii) a regrown ohmic structure to reduce the contact resistance. The fabricated devices have been proved to exhibit attractive characteristics such as low gate leakage current, low contact resistance, high drain current, and high transconductance. An HFET with a gate length of 0.1 µm has exhibited a gate leakage current density of below 10-4 A/mm even at a gate voltage of +3 V. It has also exhibited a low contact resistance of 0.3 Ωmm, a high maximum drain current density of 1.23 A/mm, and a high transconductance of 280 mS/mm, which is the highest transconductance ever reported in the category of MIS-HFETs. The cutoff frequency and maximum oscillation frequency, measured with the pad capacitances included, were 52 and 75 GHz, respectively. The proposed structure has thus been proved to be effective in further improving the device performance in GaN-based HFETs.

Enhanced Dislocation Mobility by Electron-Beam Irradiation in GaP

Mobility of α-dislocations under stress in Te-doped n-GaP has been measured by cathodoluminescence microscopy at high temperatures. The mobility is remarkably enhanced by electron-beam irradiation. The activation energy of the dislocation velocity without irradiation is 1.48 eV, while that under the electron-beam irradiation is only 0.37 eV.

RF Performance of Diamond Metel–Semiconductor Field-Effect Transistor at Elevated Temperatures and Analysis of its Equivalent Circuit

Temperature dependent DC and RF characteristics of p-type diamond metal?semiconductor field-effect transistors (MESFETs) on hydrogen-terminated surfaces are investigated. The device is thermally stable up to 100 ?C, because it does not deteriorate at all at higher temperatures. Temperature coefficients of transconductance (gm), drain conductance (gds), gate?source capacitance (Cgs), gate?drain capacitance (Cgd), cut-off frequency ( fT), and maximum drain current (Ids) were obtained from small-signal equivalent circuit analysis. The cut-off frequency ( fT) is almost totally independent of temperature. Intrinsic gm, gds, and Cgs decrease with increasing temperature. Cgd is almost totally independent of temperature. The threshold voltage shifts to the negative side with increasing temperature. We propose a band model of an Al-gate contact/H-terminated diamond to explain the temperature dependence of these components.

DC and RF Characteristics in Al2O3/Si3N4 Insulated-Gate AlGaN/GaN Heterostructure Field-Effect Transistors

Al2O3/Si3N4 insulated-gate AlGaN/GaN heterostructure field-effect transistors (HFETs) have been fabricated, where excellent RF characteristics have been obtained in addition to the low gate leakage current as the result of employing the metal–insulator–semiconductor (MIS) structure. In an HFET with a gate length (Lg) of 0.1 µm, the cutoff frequency ( fT) and maximum oscillation frequency ( fmax) were estimated to be 70 and 90 GHz, respectively. The drain current density (Id) and transconductance (gm) were 1.30 A/mm and 293 mS/mm, respectively. The gate leakage current (Ig) was as low as 4×10-5 A/mm even at a forward bias voltage of +3 V.