Ya-Hsi Hwang

Effect of low dose γ-irradiation on DC performance of circular AlGaN/GaN high electron mobility transistors

The changes in direct current performance of circular-shaped AlGaN/GaN high electron mobility transistors (HEMTs) after 60Co γ-irradiation doses of 50, 300, 450, or 700 Gy were measured. The main effects on the HEMTs after irradiation were increases of both drain current and electron mobility. Compton electrons induced from the absorption of the γ-rays appear to generate donor type defects. Drain current dispersions of ∼5% were observed during gate lag measurements due to the formation of a virtual gate between the gate and drain resulting from the defects generated during γ-irradiation.

High breakdown voltage in AlN/GaN metal–insulator–semiconductor high-electron-mobility transistors

The breakdown characteristics of AlGaN/GaN based metal–insulator–semiconductor high-electron-mobility transistors (MISHEMTs) using a 10 nm thick AlN gate insulator and passivation layer deposited plasma enhanced atomic layer deposition. The AlN was effective in significantly reducing gate leakage current relative to Schottky gate devices and showed only small decreases in drain current during gate lag measurements. The devices exhibited a strong dependence of gate breakdown voltage on source–drain distance, reaching a value of 2000 V for a source–drain distance of 40 μm limited by the measurement instrument. The specific on-state resistance was 1.3 and 10.9 mΩ cm2 for the devices with the gate–drain distance of 7.5 and 37.5 μm, respectively. The saturation drain current was inversely dependent on source–drain distance and the on–off ratios were in excess of 108 due to the low gate leakage current in the MISHEMTs.

Novel approach to improve heat dissipation of AlGaN/GaN high electron mobility transistors with a Cu filled via under device active area

A through Si-substrate via-hole under the active area of GaN-based HEMTs grown on Si substrates is proposed to reduce the maximum junction temperature. Due to the large lattice mismatch between Si and GaN, an AlN nucleation layer and an AlGaN transition layer are required to grow GaN layers on Si substrates. This AlN nucleation layer is very defective and thermally resistive. The proposed through Si-substrate via-hole offers access to this AlN nucleation layer from the back side of the wafer. By removing this highly thermally resistive layer and plating the via hole with copper, the maximum junction temperature can be reduced from 146 to 120 °C at a power density of 5 W/mm. Besides reducing the maximum junction temperature of the HEMT, this through Si-substrate via-hole can be electrically connected to the source contact and act as a backside source field plate to reduce the maximum electric field around the gate edges and thereby increase the drain breakdown voltage. If this through Si-substrate via-hole...

GaN metal–insulator–semiconductor high-electron-mobility transistor with plasma enhanced atomic layer deposited AlN as gate dielectric and passivation

AlGaN/GaN based metal–insulator–semiconductor high-electron-mobility transistors (HEMTs) using a plasma enhanced atomic layer deposited 10 nm AlN as the gate insulator and passivation layer were demonstrated. A refractive index of 1.92 for the deposited AlN was measured using an ellipsometer, which was slightly lower than that of bulk AlN. The deviation of the refractive index from the ideal value was caused by AlN surface oxidation, and this was confirmed by X-ray photoelectron spectroscopy and Auger depth profiling analyses. The HEMT drain current was modulated with gate voltages ranging from −3 to +4 V. The HEMT exhibited an on-off ratio of 3.3 × 108 due to the low gate leakage current and a maximum saturation drain current of 600 mA/mm. Beside reducing the gate leakage current, the effectiveness of the HEMT passivation was confirmed by gate pulse measurements, which showed only a 7% decrease of the drain current.

Improvement of drain breakdown voltage with a back-side gate on AlGaN/GaN high electron mobility transistors

The effect of a back gate on the dc performance of AlGaN/GaN high electron mobility transistor was investigated. The back gate was fabricated directly under the device active area by etching off the Si substrate, AlN nucleation layer, and graded AlGaN transition layer and depositing Ni/Au-based gate metal on the exposed GaN buffer layer. The reverse bias gate leakage current decreased from 3.9 × 10−5 to 1.2 × 10−5 mA/mm by applying −10 V at the back gate. Because of the suppression of gate leakage current by the back gate, the drain on/off ratio improved from 1.8 × 105 to 1.2 × 106 and the subthreshold swing from 204 to 137 mV/dec. Moreover, the drain breakdown voltage could be improved by 40% when the back gate was biased at −25 V.

Degradation mechanisms of Ti/Al/Ni/Au-based Ohmic contacts on AlGaN/GaN HEMTs

The degradation mechanism of Ti/Al/Ni/Au-based Ohmic metallization on AlGaN/GaN high electron mobility transistors upon exposure to buffer oxide etchant (BOE) was investigated. The major effect of BOE on the Ohmic metal was an increase of sheet resistance from 2.89 to 3.69 Ω/◻ after 3 min BOE treatment. The alloyed Ohmic metallization consisted 3–5 μm Ni-Al alloy islands surrounded by Au-Al alloy-rings. The morphology of both the islands and ring areas became flatter after BOE etching. Energy dispersive x-ray analysis and Auger electron microscopy were used to analyze the compositions and metal distributions in the metal alloys prior to and after BOE exposure.

Effect of proton irradiation on thermal resistance and breakdown voltage of InAlN/GaN high electron mobility transistors

InAlN/GaN high electron mobility transistors were irradiated from the front side with 340 keV protons to a dose of 5 × 1013 cm−2. Raman thermography showed that the irradiated devices had higher channel temperatures than unirradiated control devices, but only by ∼10% under typical biasing conditions. Accordingly, the irradiated devices have higher thermal resistance (400 °C/W) compared to reference devices (350 °C/W), based on the slope of the power versus channel temperature line. However, increases of 42% in off-state drain breakdown voltage (VBR) and of >92% in critical voltage (Vcri) were observed for the proton irradiated HEMT. This is ascribed to the reduction of the peak electric field at the gate edges by ∼50% through the introduction of negative trap charges created from vacancies generated by the proton irradiation.

Study of the effects of GaN buffer layer quality on the dc characteristics of AlGaN/GaN high electron mobility transistors

The effect of buffer layer quality on dc characteristics of AlGaN/GaN high electron mobility (HEMTs) was studied. AlGaN/GaN HEMT structures with 2 and 5 μm GaN buffer layers on sapphire substrates from two different vendors with the same Al concentration of AlGaN were used. The defect densities of HEMT structures with 2 and 5 μm GaN buffer layer were 7 × 109 and 5 × 108 cm−2, respectively, as measured by transmission electron microscopy. There was little difference in drain saturation current or in transfer characteristics in HEMTs on these two types of buffer. However, there was no dispersion observed on the nonpassivated HEMTs with 5 μm GaN buffer layer for gate-lag pulsed measurement at 100 kHz, which was in sharp contrast to the 71% drain current reduction for the HEMT with 2 μm GaN buffer layer.

Enhancement of AlGaN/GaN high-electron mobility transistor off-state drain breakdown voltage via backside proton irradiation

Proton irradiation from the backside of the samples were employed to enhance off-state drain breakdown voltage of AlGaN/GaN high electron mobility transistors (HEMTs) grown on Si substrates. Via holes were fabricated directly under the active area of the HEMTs by etching through the Si substrate for subsequent backside proton irradiation. By taking the advantage of the steep drop at the end of proton energy loss profile, the defects created by the proton irradiation from the backside of the sample could be precisely placed at specific locations inside the AlGaN/GaN HEMT structure. There were no degradation of drain current nor enhancement of off-state drain voltage breakdown voltage observed for the irradiated AlGaN/GaN HEMTs with the proton energy of 225 or 275 keV, for which the defects created by the proton irradiations were intentionally placed in the GaN buffer. HEMTs with defects placed in the 2 dimensional electron gas (2DEG) channel region and AlGaN barrier using 330 or 340 keV protons not only showed degradation of drain current, but also exhibited improvement of the off-state drain breakdown voltage. FLOODS TCAD finite-element simulations were performed to confirm the hypothesis of a virtual gate formed around the 2DEG region to reduce the peak electric field around the gate edges and increase the off-state drain breakdown voltage.

Role of Electric Field, Defects and Radiation Damage in Determining Reliability in AlGaN/GaN High Electron Mobility Transistors

AlGaN/GaN high electron mobility transistors are gaining commercial acceptance for use in high power and high frequency applications, but the degradation mechanisms that drive failure in the devices are not completely understood. Since some of these mechanisms are current or field driven, reliability studies must go beyond the typical Arrhenius accelerated life tests. In this chapter we summarize recent work on electric field or current driven degradation in devices with different gate metallization, device dimensions, electric field mitigation techniques (such as source field plates), and the effect of defects either already present or introduced by exposure to ionizing radiation. Some of the mechanisms now established include gate or Ohmic metal reaction, hot carrier trap generation, stress-induced crack generation and oxidation reactions involving the AlGaN layer.