Cheng P. Wen

High-Performance Normally-Off

This letter reports a normally-OFF Al₂O₃/GaN gate-recessed MOSFET using a low-damage digital recess technique featuring multiple cycles of plasma oxidation and wet oxide removal process. The wet etching process eliminates the damage induced by plasma bombardment induced in conventional inductively coupled plasma dry etching process so that good surface morphology and high interface quality could be achieved. The fully recessed Al₂O₃/GaN MOSFET delivers true enhancement-mode operation with a threshold voltage of +1.7 V. The maximum output current density is 528 mA/mm at a positive gate bias of 8 V. A peak field-effect mobility of 251 cm²/V·s is obtained, indicating high-quality Al₂O₃/GaN interface.

{\rm Al}_{2}{\rm O}_{3}/{\rm GaN}

A mechanism of the formation of the bulges on the surface of Ti/Al/Ni/Au Ohmic contact in AlGaN/GaN high electron mobility transistors is proposed. According to the analysis of TEM images and corresponding electron dispersive x-ray spectra, the bulges were found to consist of Ni–Al alloy in the body and Au–Al alloy surrounding. We deduce that the bulges were formed due to Ni–Al alloy aggregation in some local areas during the rapid thermal annealing process, which accounts for the rough surface morphology.

MOSFET Using a Wet Etching-Based Gate Recess Technique

In this letter, we investigated the behaviors of surface- and buffer-induced current collapse in AlGaN/GaN high-electron mobility transistors (HEMTs) using a soft-switched pulsed I-V measurement with different quiescent bias points. It is found that the surface- and buffer-related current collapse have different relationship with the gate and drain biases (VGS0,VDS0) during quiescent bias stress. The surface-induced current collapse in devices without passivation monotonically increases with the negative VGS0, suggesting that an electron injection to the surface from gate leakage is the dominant mechanism and the Si3N4 passivation could effectively eliminate such current collapse. The buffer-induced current collapse in devices with intentionally carbon-doped buffer layer exhibits a different relationship with VGS0 after surface passivation. The buffer-related current collapse shows a bell-shaped behavior with VGS0, suggesting that a hot electron trapping in the buffer is the dominant mechanism. The soft-switched pulsed I-V measurement provides an effective method to distinguish between the surface- and buffer-related current collapse in group III-nitride HEMTs.

Analysis of surface roughness in Ti/Al/Ni/Au Ohmic contact to AlGaN/GaN high electron mobility transistors

In this letter, a plasma-free etch stop structure is developed for GaN HEMT toward enhancement-mode operation. The self-terminated precision gate recess is realized by inserting a thin AlN/GaN bilayer in the AlGaN barrier layer. The gate recess is stopped automatically at the GaN insertion layer after high-temperature oxidation and wet etch, leaving a thin AlGaN barrier to maintain a quantum well channel that is normally pinched off. With addition of an Al2O3 gate dielectric, quasi normally OFF GaN MOSHEMTs have been fabricated with high threshold uniformity and low ON-resistance comparable with the normally ON devices on the same wafer. A high channel mobility of 1400 cm2/V·s was obtained due to the preservation of the high electron mobility in the quantum-well channel under the gate.

Investigation of Surface- and Buffer-Induced Current Collapse in GaN High-Electron Mobility Transistors Using a Soft Switched Pulsed \(I-V\) Measurement

A test structure, which is called the ridge-furrow structure, is used to evaluate the leakage current of the Schottky contact on the mesa edge of an AlGaN/GaN heterostructure. The mesa edge leakage currents were measured at different temperatures from 300 to 500 K and analyzed. The conduction-band-edge energy distribution at the mesa edge is simulated by the Integrated Systems Engineering Technology Computer-Aided Design. Based on the simulation results, the electric field strength can be obtained as a function of reverse bias voltage. The mesa edge leakage current is found to agree with the predicted characteristics, which is based on the Frenkel-Poole emission model. Therefore, we believe that the Frenkel-Poole emission dominates the mesa edge leakage at temperatures between 300 and 500 K.

A GaN HEMT Structure Allowing Self-Terminated, Plasma-Free Etching for High-Uniformity, High-Mobility Enhancement-Mode Devices

The temperature dependence of current collapse (CC) in AlGaN/GaN high-electron mobility transistors on silicon substrate is studied in this paper. Devices without and with Si3N4 passivation are used to investigate the behavior of surface- and buffer-induced CC, respectively. It is found that the degree of surface-induced CC in unpassivated devices has a weak temperature dependence, which is induced by the cancelling out between enhanced carrier injection based on surface hopping and enhanced emission when the temperature is increased. On the other hand, the degree of buffer-induced CC in the Si3N4 passivated devices is reduced at higher temperature since the energy of hot electrons is reduced due to the phonon scattering and the trapping of hot electrons in the buffer is mitigated. Temperature-dependent transient measurement is also carried out to investigate the recovery process for these two type of CC. Two types of trap levels are identified in the unpassivated and Si3N4 passivated devices, respectively. The trap level E1 with an activation energy of 0.08 eV is supposed to be related to the surface trapping, while E2 with an activation energy of 0.22 eV is located in the buffer layer.

The Leakage Current of the Schottky Contact on the Mesa Edge of AlGaN/GaN Heterostructure

A novel stacked Ti/Al based Ti/Al/Ti/Al/Ti/Al/Ti/Al/Ni/Au Ohmic contact structure is optimized. Compared with the conventional alloyed Ti/Al/Ni/Au Ohmic contact structure, the novel Ohmic contact structure can obtain much lower contact resistance and specific contact resistivity. Through analysis of x-ray diffraction spectra, cross-section transmission electron microscopy images and corresponding electron dispersive x-ray spectroscopy spectra in the novel stacked Ti/Al based Ohmic structure, the reactions between metals and the AlGaN layer were proven to be stable, uniform and continuous, which produced smooth contact interface. In addition, the top Au layer was prevented from diffusing downwards to the metal/AlGaN interface, which degraded the Ohmic performance.

Temperature Dependence of the Surface- and Buffer-Induced Current Collapse in GaN High-Electron Mobility Transistors on Si Substrate

High-field carrier transport properties probed by pulsed current-voltage characteristics in n-type GaN epilayers have been investigated at room temperature. The threshold electric field of the Gunn effect is experimentally determined to be about 400 kV/cm with an electron peak velocity of about 1.9×107 cm/s. The current-controlled negative differential resistance effect induced by the Gunn-type instability in n-type GaN is observed, which takes the early electric breakdown of the GaN epilayers in charge.

Analysis on the new mechanisms of low resistance stacked Ti/Al Ohmic contact structure on AlGaN/GaN HEMTs

We report on an AlGaN/GaN metal-oxide-semiconductor high-electron-mobility transistor (MOS-HEMT) using thermal oxidation of electron-beam deposited aluminum as the gate dielectric. This novel dielectric deposition process is simple, and less expensive than electron cyclotron resonance (ECR) plasma oxidation of Al or atomic layer deposited (ALD) Al2O3. The X-ray Photoelectron Spectroscopy (XPS) Ols spectrum showed that the Al2O3 with a bandgap of 7.8 eV was obtained in this specimen. The resulted MOS-HEMT exhibits several orders of magnitude lower gate leakage, larger drain saturation current and larger gate voltage swing compared to a conventional AlGaN/GaN high-electron-mobility transistor (HEMT) of similar design. The MOS-HEMT is therefore a viable alternative to regular HEMTs for high-power, high-frequency and high-temperature applications.

Current-controlled negative differential resistance effect induced by Gunn-type instability in n-type GaN epilayers

Current collapse (drain current dispersion, gradual power saturation, or memory effect) encountered during microwave GaN HEMT power amplifier operation remains to be a major reliability and stability issue for the highly promising, emerging III-nitride, polar semiconductor based technology. Current collapse leads to bias condition induced memory effect, which is particularly detrimental to broad band RF amplifiers operating with large peak-to-average signal level. Loss of polarization induced, surface mobile holes, accompanied by the reduction of transistor channel electron density have recently been identified as the root cause of these undesirable, transient effects exhibited by the polar semiconductor based transistors, in which charge carriers are primarily originated from the built-in electric dipoles of the nitride material. A novel ohmic metal contact scheme, presenting low energy barrier for both the transistor channel electrons, and the surface mobile holes, is found to be effective in eliminating the undesirable, time and signal strength dependence behavior of the transistor performance characteristics. A GaN MISHEMT with such compound ohmic contacts is found to be capable of operating at higher saturation current density, compared to that of a GaN HEMT with traditional ohmic contacts. The new finding represents a major breakthrough in polar semiconductor device technology for microwave power amplification.