Yu-Syuan Lin

AlGaN/GaN HEMTs With Low Leakage Current and High On/Off Current Ratio

In this letter, we propose using an oxide-filled isolation structure followed by N₂/H₂ postgate annealing to reduce the leakage current in AlGaN/GaN HEMTs. An off-state drain leakage current that is smaller than 10^-9 A/mm (minimum 5.1 × 10^-10 A/mm) can be achieved, and a gate leakage current in the range of 7.8 ×10^-10 to 9.2 × 10^-11A/mm (<i>V</i> _GS from -10 to 0 V and <i>V</i> _DS = 10 V) is obtained. The substantially reduced leakage current results in an excellent on/off current ratio that is up to 1.5 × 10⁸. An improved flicker noise characteristic is also observed in the oxide-filled devices compared with that in the traditional mesa-isolated GaN HEMTs.

AlGaN/GaN Schottky Barrier Diodes on Silicon Substrates With Selective Si Diffusion for Low Onset Voltage and High Reverse Blocking

In this letter, a selective Si diffusion approach is proposed to improve both the forward and reverse characteristics of AlGaN/GaN Schottky barrier diodes on Si substrates. The Si diffusion layer forms a dual Schottky barrier anode structure, which results in a low Schottky barrier portion to reduce the onset voltage VON from 1.3 to 1.0 V (23%). In the same process step, the selectively diffused Si is adopted in the cathode to reduce the ohmic contact resistance RC and improve the breakdown voltage VBK. A low RC of 0.21 Ω·mm and enhanced VBK up to 20% (from 1250 to 1500 V) are demonstrated, which can be attributed to the alleviated electric-field peaks around the alloy spikes beneath the ohmic contact.

High current density InN∕AlN heterojunction field-effect transistor with a SiNx gate dielectric layer

InN∕AlN metal-insulator-semiconductor heterojunction field-effect transistors with a gate-modulated drain current and a clear pinch-off characteristic have been demonstrated. The devices were fabricated using high-quality InN (26nm)∕AlN (100nm) epifilms grown by plasma-assisted molecular-beam epitaxy on Si (111) substrates. The devices exhibited a current density higher than ∼530mA∕mm with a 5μm gate length. The pinch-off voltage was at ∼−7V with an associated drain leakage current less than 10μA∕mm. The observed high current density may be attributed to the high sheet carrier density due to the large spontaneous polarization difference between InN and AlN.

AlGaN/GaN HEMTs on Silicon With Hybrid Schottky–Ohmic Drain for High Breakdown Voltage and Low Leakage Current

In this letter, a hybrid Schottky-ohmic drain structure is proposed for AlGaN/GaN high-electron-mobility transistors on a Si substrate. Without additional photomasks and extra process steps, the hybrid drain design forms a Γ-shaped electrode to smooth the electric field distribution at the drain side, which improves the breakdown voltage and lowers the leakage current. In addition, the hybrid drain provides an auxiliary current path and decreases the on-resistance, in contrast to the devices with a pure Schottky drain. Compared with the conventional ohmic drain devices, the breakdown voltage could be improved up to 64.9%, and the leakage current is suppressed by one order of magnitude without degradation of the specific on-resistance.

Local environment surrounding S and Cd in CdS:O thin film photovoltaic materials probed by x-ray absorption fine structures

Local environments surrounding Cd and S in CdS:O thin films have been determined using extended x-ray absorption fine structure (EXAFS) and near-edge x-ray absorption fine structure (NEXAFS). As indicated by the Cd EXAFS, Cd atoms remain predominantly bonded with S. The S EXAFS and NEXAFS clearly demonstrate the presence of S–O bonds. The oxygen atoms actually combine with S to form SO3 and SO4 complexes. Combined with the transmission electron micrograph, these x-ray results suggest formation of oxygen-free CdS nanocrystals and provide an unambiguous explanation for the mystery of increased band gap that appears to violate the band anticrossing model.

Impacts of gate recess and passivation on AlGaN/GaN high electron mobility transistors

In this study, the impacts of gate recess and passivation on AlGaN/GaN high electron mobility transistors (HEMTs) were investigated. The trap-related characteristics were studied in detail by several different measurements including dc current– voltage, current collapse, gate lag, and flicker noise characterizations. With a Cl2/Ar-recessed gate, drain current collapse factors (� Imax )o f� 37:5 and � 6:9% were observed before and after SiN passivation. The gate lag measurements showed that the lagging phenomena almost disappear with SiN passivation for both Cl2- and Cl2/Ar-recessed devices. However, the flicker noise measurements revealed distinct noise levels of devices with different processes even after passivation. As the gate voltage (VG) changed from 2 to � 4 V, the devices recessed by Cl2 exhibited lower drain noise current densities (SID=ID 2 ranging from 2:8 � 10 � 14 to 1:7 � 10 � 12 Hz � 1 at 1 kHz) than those etched by Cl2/Ar mixture gas (SID=ID 2 ranging from 6:3 � 10 � 14 to 6:0 � 10 � 12 Hz � 1 at 1 kHz), whereas the devices without the recess process showed the lowest noise levels (SID=ID 2 ranging from 2:8 � 10 � 15 to 1:3 � 10 � 13 Hz � 1 at 1 kHz). It was found that SID=ID 2 increased monotonically when VG

Square-Gate AlGaN/GaN HEMTs With Improved Trap-Related Characteristics

In this brief, the trap-related characteristics of high-breakdown AlGaN/GaN high-electron-mobility transistors (HEMTs) were investigated. Compared with a conventional multifinger layout, the square-gate design presented reduced the current collapse from 19% to 6% and almost eliminated the gate lag. The flicker noise density and the gate leakage decreased from 1.16 times10^-10 to 1.17 times10^-11 1/Hz (<i>f</i> = 100 Hz) and from 7.36 times10^-5 to 1.80 times10^-6 A/mm ( <i>V</i> _GS = -4 V and <i>V</i> _DS = 100 V), respectively. The breakdown voltage was also improved from 350 to 650 V. With the channel area away from the defects generated by the mesa etching process, the square-gate AlGaN/GaN HEMTs demonstrated excellent performance with much less trapping effects.

Impact of STI Effect on Flicker Noise in 0.13-

This paper reports on the impact of shallow-trench isolation (STI) on flicker noise characteristics in 0.13-mum RF nMOSFETs. The drain noise current spectral density was measured in both triode and saturation regions for a more complete study. The devices with a relatively small finger width and a large finger number (W=1 mum/Nfinger=40 and W=5 mum/Nfinger=8) presented more pronounced generation-recombination (G-R) noise characteristics compared to those with W=10 mum/Nfinger=4. In addition, a wide noise level variation of more than one order of magnitude was associated with the more obvious G-R noise components. The observed trends can be explained by the nonuniform stress effect of STI and also the associated traps at the edge of the gate finger between STI and the active region. To further study the noise mechanism, the single-linger devices with different STI-to-gate distances [SA(SB)=0.6,1.2, and 10 mum] were investigated. The measured results provided a direct evidence of STI effect on flicker noise characteristics. The activation energy of the traps was extracted at various temperatures in a range from EC-0.397 to EC-0.54 eV. Moreover, the calculated standard deviation sigmadB showed a strong dependence of noise variation on device geometry (sigmadB=2.95 dB for W=1 mum/Nfinger=40 and sigmadB=1.54 dB for W=10 mum/Nfinger=4). The analysis suggests that the carrier number fluctuation model with the correlated mobility scattering is more suitable for the noise characteristics in these devices.

\mu \hbox{m}

Single-crystal atomic-layer-deposited (ALD) Y2O3 films 2 nm thick were epitaxially grown on molecular beam epitaxy (MBE) GaAs(001)-4 × 6 and GaAs(111)A-2 × 2 reconstructed surfaces. The in-plane epitaxy between the ALD-oxide films and GaAs was observed using in-situ reflection high-energy electron diffraction in our uniquely designed MBE/ALD multi-chamber system. More detailed studies on the crystallography of the hetero-structures were carried out using high-resolution synchrotron radiation X-ray diffraction. When deposited on GaAs(001), the Y2O3 films are of a cubic phase and have (110) as the film normal, with the orientation relationship being determined: Y2O3(110)[001][1¯10]//GaAs(001)[110][11¯0]. On GaAs(111)A, the Y2O3 films are also of a cubic phase with (111) as the film normal, having the orientation relationship of Y2O3(111)[21¯1¯][011¯]//GaAs(111)[2¯11][01¯1]. The relevant orientation for the present/future integrated circuit platform is (001). The ALD-Y2O3/GaAs(001)-4 × 6 has shown excellent electrical properties. These include small frequency dispersion in the capacitance-voltage (CV) curves at accumulation of ~7% and ~14% for the respective p- and n-type samples with the measured frequencies of 1 MHz to 100 Hz. The interfacial trap density (Dit) is low of ~1012 cm−2eV−1 as extracted from measured quasi-static CVs. The frequency dispersion at accumulation and the Dit are the lowest ever achieved among all the ALD-oxides on GaAs(001).

RF nMOSFETs

Two contact engineering approaches are proposed and investigated to enhance the breakdown voltage VBK in GaN-on-Si power devices, including the hybrid Schottky–ohmic-drain structure for AlGaN/GaN HEMTs and the selective Si-diffusion structure for AlGaN/GaN SBDs. With the hybrid Schottky–ohmic drain, the devices showed a zero onset voltage and reduced off-state leakage current by one order of magnitude, compared with that of the traditional ohmic-drain devices. The breakdown voltage was also enhanced with comparable on-resistance. For the SBDs with the selective silicon-diffusion layer underneath ohmic metal at the cathode, a low-contact resistance Rc of 0.2 Ω mm and a smooth ohmic metal morphology were obtained. The SBDs with the additional silicon diffusion layer showed enhanced VBK, compared with that of the conventional SBDs. The results demonstrate that the proposed contact engineering approaches are useful for the breakdown voltage enhancement of GaN-on-Si power devices.