Evidence for causality between GaN RF HEMT degradation and the EC-0.57 eV trap in GaN

The degradation of industry-supplied GaN high electron mobility transistors (HEMTs) subjected to accelerated life testing (ALT) is directly related to increases in concentrations of two defects with trap energies of E_C-0.57 and E_C-0.75 eV. Pulsed I-V measurements and constant drain current deep level transient spectroscopy were employed to evaluate the quantitative impact of each trap. The trap concentration increases were only observed in devices that showed a 1 dB drop in output power and not the result of the ALT itself indicating that these traps and primarily the E_C-0.57 eV trap are responsible for the output power degradation. Increases from the E_C-0.57 eV level were responsible for 80% of the increased knee walkout while the E_C-0.75 eV contributed only 20%. These traps are located in the drain access region, likely in the GaN buffer, and cause increased knee walkout after the application of drain voltage.     

Investigation of deep level defects in electron irradiated indium arsenide quantum dots embedded in a gallium arsenide matrix

Gallium arsenide diodes with and without indium arsenide quantum dots were electron irradiated to investigate radiation induced defects. Baseline and quantum dot gallium arsenide pn-junction diodes were characterized by capacitance–voltage measurements, and deep level transient spectroscopy. Carrier accumulation was observed in the gallium arsenide quantum dot sample at the designed depth for the quantum dots via capacitance–voltage measurements. Prior to irradiation, a defect 0.84 eV below the conduction band (E_C – 0.84 eV) was observed in the baseline sample which is consistent with the native EL2 defect seen in gallium arsenide. After 1 MeV electron irradiation three new defects were observed in the baseline sample, labeled as E3 (E_C – 0.25 eV), E4 (E_C – 0.55 eV), and E5 (E_C – 0.76 eV), consistent with literature reports of electron irradiated gallium arsenide. Prior to irradiation, the addition of quantum dots appeared to have introduced defect levels at E_C – 0.21, E_C – 0.38, and E_C – 0.75 eV denoted as QD–DX1, QD–DX2, and QD–EL2 respectively. In the quantum dot sample after 1 MeV electron irradiation, QD–E3 (E_C – 0.28 eV), QD–E4 (E_C – 0.49 eV), and QD–EL2 (E_C – 0.72 eV) defects, similar to the baseline sample, were observed, although the trap density was dissimilar to that of the baseline sample. The quantum dot sample showed a higher density of the QD–E4 defect and a lower density of QD–E3, while the QD–EL2 defect seemed to be unaffected by electron irradiation. These findings suggest that the quantum dot sample may be more radiation tolerant to the E3 defect as compared to the baseline sample.     

Effect of trap states on the electrical doping of organic semiconductors

We demonstrate that direct charge transfer (CT) from trap states of host molecules to the p-dopant molecules raises the doping effect of organic semiconductors (OS). Electrons of the trap states in 4,4′-N,N′-dicarbazolyl-biphenyl (CBP) (E_HOMO = 6.1 eV) are directly transferred to the p-dopant, 2,2′-(perfluoronaphthalene-2,6-diylidene) dimalononitrile (F6-TCNNQ) (ELUMO = 5.4 eV). This doping process enhances the conductivity of doped OS by different ways from the ordinary doping mechanism of generating free hole carriers and filling trap states of doped OS. Trap density and trap energy are analysed by impedance spectroscopy and it is shown that the direct charge transfer from deep trap states of host to dopants enhances the hole mobility of doped OS and the I–V characteristics of hole only devices.     

Similarities of lag phenomena and current collapse in field-plate AlGaN/GaN HEMTs with different types of buffer layers

We make a two-dimensional transient analysis of field-plate AlGaN/GaN high electron mobility transistors (HEMTs) with a Fe-doped semi-insulating buffer layer, which is modeled that as deep levels, only a deep acceptor located above the midgap is included (E_C − E_DA = 0.5 eV, E_C: energy level at the bottom of conduction band, E_DA: deep acceptor's energy level). And the results are compared with a case having an undoped semi-insulating buffer layer in which a deep donor above the midgap (E_C − E_DD = 0.5 eV. E_DD: the deep donor's energy level) is considered to compensate a deep acceptor below the midgap (E_DA − E_V = 0.6 eV, E_V: energy level at the top of valence band). It is shown that the drain-current responses when the drain voltage is lowered abruptly are reproduced quite similarly between the two cases with different types of buffer layers, although the time region where the slow current transients occur is a little different. The lags and current collapse are reduced by introducing a field plate. This reduction in lags and current collapse occurs because the deep acceptor's electron trapping is reduced under the gate region in the buffer layer. The dependence of drain lag, gate lag and current collapse on the field-plate length and the SiN layer thickness is also studied, indicating that the rates of drain lag, gate lag and current collapse are quantitatively quite similar between the two cases with different types of buffer layers when the deep-acceptor densities are the same.     

Reliability of submicron InGaAs/InP DHBT under thermal and electrical stresses

We report on the reliability of InGaAs/InP DHBT technology which has applications in very high-speed ICs (over 100 Gbits/s). This work presents the results of accelerated aging tests under thermal and electrical stresses performed on HBT up to 2000 h. Stress conditions consist in applying collector–emitter bias V_CE from 1.3 to 2.7 V and collector current densities J_C of 400 and 610 kA/cm². The corresponding junction temperatures T_J extends from 83 to 137 °C. The base current ideality factor η_B increase and the current gain β decrease have revealed a degradation of the base–emitter junction. The normalized current gain β_norm drop has occurred earlier for higher V_CE and/or higher T_J. A 20% decrease of β_norm chosen as the failure criterion leads to an activation energy of 1.1 eV.     

The influence of high energy electron irradiation on the Schottky barrier height and the Richardson constant of Ni/4H-SiC Schottky diodes

The influence of high energy electron (HEE) irradiation from a Sr-90 radio-nuclide on n-type Ni/4H–SiC samples of doping density 7.1×10¹⁵ cm⁻³ has been investigated over the temperature range 40–300 K. Current–voltage (I–V), capacitance–voltage (C–V) and deep level transient spectroscopy (DLTS) were used to characterize the devices before and after irradiation at a fluence of 6×10¹⁴ electrons-cm⁻². For both devices, the I–V characteristics were well described by thermionic emission (TE) in the temperature range 120–300 K, but deviated from TE theory at temperature below 120 K. The current flowing through the interface at a bias of 2.0 V from pure thermionic emission to thermionic field emission within the depletion region with the free carrier concentrations of the devices decreased from 7.8×10¹⁵ to 6.8×10¹⁵ cm⁻³ after HEE irradiation. The modified Richardson constants were determined from the Gaussian distribution of the barrier height across the contact and found to be 133 and 163 A cm⁻² K⁻² for as-deposited and irradiated diodes, respectively. Three new defects with energies 0.22, 0.40 and 0.71 eV appeared after HEE irradiation. Richardson constants were significantly less than the theoretical value which was ascribed to a small active device area.     

Deep trap-induced dynamic on-resistance degradation in GaN-on-Si power MISHEMTs

Identification and characterization of a single, deep trap causing large increases in the on-resistance of GaN-on-Si power metal-insulator-semiconductor-high electron mobility transistors (MISHEMTs) is reported. This is achieved by using HEMT-based deep level optical spectroscopy (DLOS) and related methods in conjunction with high voltage off-state V_DS switching up to 400 V. A trap with an activation energy of ~ E_C − 2 eV that is physically located in the drain-access region of the MISHEMT is shown to be the primary source of an increase of the dynamic on-resistance increase by as much as ~ 9 times at 400 V operation. Comparisons of trap signatures extracted from the MISHEMT with capacitance-based DLOS measurements of simple Schottky-diode test-structures showing the same, dominant trap signature suggests that the physical defect is located within the GaN buffer and is not a surface or insulator-related defect. A buffer trap based model is presented to explain the observed on-resistance degradation effects in the MISHEMTs during high voltage switching.     

A detailed comparative study on the main electrical parameters of Au/n-Si and Au/PVA:Zn/n-Si Schottky barrier diodes

We have fabricated Au/n-Si and Au/PVA:Zn/n-Si Schottky barrier diodes (SBDs) to investigate the effect of organic interfacial layer on the main electrical characteristics. Zn doped poly(vinyl alcohol) (PVA:Zn) was successfully deposited on n-Si substrate by using the electrospinning system and surface morphology of PVA:Zn was presented by SEM images. The current–voltage (I–V) characteristics of these SBDs have been investigated at room temperature. The experimental results show that interfacial layer enhances the device performance in terms of ideality factor (n), zero-bias barrier height (Φ_B0), series resistance (R_s), and shunt resistance (R_sh) with values of 1.38, 0.75 eV, 97.64 Ω, and 203 MΩ whereas those of Au/n-Si SBD are found as 1.65, 0.62 eV, 164.15 Ω and 0.597 MΩ, respectively. Also, this interfacial layer at metal/semiconductor (M/S) interface leads to a decrease in the magnitude of leakage current and density of interface states (N_ss). The values of N_ss range from 1.36×10¹² at E_c—0.569 eV to 1.35×10¹³ eV^?1 cm^?2 at E_c—0.387 eV for Au/PVA:Zn/n-Si SBD and 3.34×10¹² at E_c—0.560 eV to 1.35×10¹³ eV^?1 cm^?2 at E_c—0.424 eV for Au/n-Si SBD. The analysis of experimental results reveals that the existence of PVA:Zn interfacial layer improves the performance of such devices.     

Region-dependent behavior of I–V characteristics in n-ZnO:Al/p-Si contacts

The discrepancy of rectifying characteristics in n-ZnO:Al/p-Si heterojunctions from diode to diode was demonstrated by region dependent dark I–V characteristics, where the junction is laterally cut to sequentially decrease the area. Further investigation shows that the junction (2.1×2.1 cm²) with the barrier height Φ=0.693 eV consists of one part (2.1×1.4 cm²) with Φ=0.695 eV and the other part (2.1×0.7 cm²) with Φ=0.686 eV. It is found that reverse currents saturate with different values of 3.6×10^?3, 2.5×10^?3 and 1.58×10^?3 A for the light I–V curves of the three junctions with the same areas. To explain this peculiarity, the probable reason is discussed in terms of carrier transportation through the spatially fluctuating barrier.     

Investigation of TiO2 on AlGaAs prepared by liquid phase deposition and its application

The study explored titanium dioxide (TiO₂) on aluminum gallium arsenide (AlGaAs) prepared by liquid phase deposition (LPD) at 40 °C. The leakage current density was about 8.4 × 10^?6 A/cm² at 1 MV/cm. The interface trap density (D_it) and the flat-band voltage shift (ΔV_FB) were 2.3 × 10¹² cm^?2 eV^?1 and 1.2 V, respectively. After rapid thermal annealing (RTA) in the ambient N₂ at 350 °C for 1 min, the leakage current density, D_it, and ΔV_FB were improved to 2.4 × 10^?6 A/cm² at 1 MV/cm, 7.3 × 10¹¹ cm^?2 eV^?1, and 1.0 V, respectively. Finally, the study demonstrates the application to the AlGaAs/InGaAs metal–oxide–semiconductor pseudomorphic high-electron-mobility transistor (MOS-PHEMT). The results indicate the potential of the proposed device with a LPD-TiO₂ gate oxide for power application.     

2 MeV ion irradiation effects on AlGaN/GaN HFET devices

AlGaN/GaN heterostructure field effect transistors (HFETs) were irradiated with 2 MeV protons, carbon, oxygen, iron and krypton ions with fluences ranging from 1 × 10⁹ cm^?2 to 1 × 10¹³ cm^?2. DC, pulsed I–V characteristics, loadpull and S-parameters of the AlGaN HFET devices were measured before and after irradiation. In parallel, a thick GaN reference layer was also irradiated with the same ions and was characterized by X-ray diffraction, photoluminescence, Hall measurements before and after irradiation. Small changes in the device performance were observed after irradiation with carbon and oxygen at a fluence of 5 × 10¹⁰ cm^?2. Remarkable changes in device characteristics were seen at a fluence of 1 × 10¹² cm^?2 for carbon, oxygen, iron and krypton irradiation. Similarly, remarkable changes were also observed in the GaN layer for irradiations with fluence of 1 × 10¹² cm^?2. The results found on devices and on the GaN layer were compared and correlated.     

Low power flexible organic thin film transistors with amorphous Ba0.7Sr0.3TiO3 gate dielectric grown by pulsed laser deposition at low temperature

We deposited amorphous Ba_0.7Sr_0.3TiO₃ (BST) on silicon and plastic substrate under 110 °C by pulsed laser deposition (PLD) and use it as the dielectric of the organic transistor. Depends on the thickness of BST layer, the highest mobility of the devices can achieve 1.24 cm² V^?1 s^?1 and 1.01 cm² V^?1 s^?1 on the silicon and polyethylene naphthalate (PEN) substrate, respectively. We also studied the upward and downward bending tests on the transistors and the dielectric thin films. We found that the BST dielectric pentacene transistor can maintain the mobility at 0.5 cm² V^?1 s^?1 or higher while the bending radius is around 3 mm in both upward and downward bending. Our finding demonstrates the potential application of PLD growth high-k dielectric in the large area organic electronics devices.     

Determination of the laterally homogeneous barrier height of palladium Schottky barrier diodes on n-Ge (1 1 1)

We have studied the experimental linear relationship between barrier heights and ideality factors for palladium (Pd) on bulk-grown (1 1 1) Sb-doped n-type germanium (Ge) metal-semiconductor structures with a doping density of about 2.5×10¹⁵ cm^?3. The Pd Schottky contacts were fabricated by vacuum resistive evaporation. The electrical analysis of the contacts was investigated by means of current–voltage (I–V) and capacitance–voltage (C–V) measurements at a temperature of 296 K. The effective barrier heights from I–V characteristics varied from 0.492 to 0.550 eV, the ideality factor n varied from 1.140 to 1.950, and from reverse bias capacitance–voltage (C^?2–V) characteristics the barrier height varied from 0.427 to 0.509 eV. The lateral homogenous barrier height value of 0.558 eV for the contacts was obtained from the linear relationship between experimental barrier heights and ideality factors. Furthermore the experimental barrier height distribution obtained from I–V and (C^?2?V) characteristics were fitted by Gaussian distribution function, and their mean values were found to be 0.529 and 0.463 eV, respectively.     

A study on phase transformation of SnOx thin films prepared by reactive magnetron sputtering

In the paper, SnO_x thin films were deposited by reactive magnetron sputtering from a tin target in O₂ containing working gas. The evolution from Sn-containing SnO to tetravalent SnO₂ films was investigated. The films could be classified into three groups according to their optical band gaps, which are E_g<2.5 eV, E_g=3.0–3.3 eV and E_g>3.7 eV. The electric measurements show that high conductivity can be obtained much easier in SnO₂ than in SnO films. A high electron mobility of 15.7 cm² V⁻¹ s⁻¹, a carrier concentration of 1.43×10²⁰ cm⁻³ and a resistivity of 2.8×10⁻³ Ω cm have been achieved in amorphous SnO₂ films. Films with the optical band gap of 3.0–3.3 eV remain amorphous though the substrate temperature is as high as 300 °C, which implies that °btaining high mobility in p-type SnO is more challenging in contrast to n-type SnO₂ films.     

Selective activation of failure mechanisms in packaged double-heterostructure light emitting diodes using controlled neutron energy irradiation

This paper demonstrates the feasibility of creating specific defects in double-heterostructure AlGaAsGaAs commercial light emitting diode by neutron irradiation. Using controlled neutron energy, only one failure mechanism can be activated. Defects are located in the side of the chip and increase the leakage current driven by the well-known Pool–Frenkel effect with E_c ? E_T = 130 meV electron trap energy level. The maximal amplitude of optical spectrum also reveals a drop about 20% associated to the rise of leakage current.     

Strain sensitivity of gate leakage in strained-SOI nMOSFETs: A benefit for the performance trade-off and a novel way to extract the strain-induced band offset

The impact of biaxial stress on gate leakage is investigated on fully-depleted silicon-on-insulator (FD-SOI) nMOS transistors, integrating either a standard gate stack or an advanced high-κ/metal gate stack. It is demonstrated that strained devices exhibit significantly reduced leakage currents (up to ?90% at E_ox = 11 MV/cm for σ_tensile = 2.5 GPa). This specific effect is used to extract the conduction band offset ΔEc induced by strain and is shown to be accurate enough to monitor stress in MOSFETs. This new technique is much less sensitive to gate oxide defects than the method based on the threshold voltage shift ΔV_T. This accurate experimental extraction allowed us to pick out realistic values for the deformation potentials in silicon (Ξ_u = 8.5 eV and Ξ_d = ?5.2 eV), among the published values.     

Effects of combined NO and forming gas annealing on interfacial properties and oxide reliability of 4H-SiC MOS structures

The effects of NO and forming gas post oxidation annealing treatments on the interfacial properties and reliability of thermal oxides grown on n-type 4H-SiC (0001) Si face have been investigated in this study. The results show that forming gas annealing (FGA) treatment has limited effect on interface trap density (D_it) while it results in an improvement of the insulating properties of thermal oxide with uniform high FN barrier height (2.56 eV), high field-to-breakdown (10.71 MV/cm) and charge-to-breakdown (0.078 C/cm²). On the other hand, NO annealing causes a drastic reduction in D_it in the entire energy level, but in the case of reliability, it is not so effective as FGA, with lower barrier height (2.52 eV), field-to-breakdown (10.08 MV/cm), charge-to-breakdown (0.025 C/cm²) and worse uniformity of oxide. The combined NO&FGA treatment was also studied. It leads to a significant reduction in interface trap density further, especially in deep energy level (E_C-E_T ≥ 0.4 eV). As for reliability, it brings about uniform barrier height (2.69 eV), field-to-breakdown (10.15 MV/cm) and charge-to-breakdown (0.024 C/cm²). Taking interfacial properties and reliability into account, combined NO&FGA treatment is a promising POA technique for fabrication of high-quality SiC MOS devices.     

Determination of contact parameters of Ni/n-GaP Schottky contacts

The electrical analysis of Ni/n-GaP structure has been investigated by means of current–voltage (I–V), capacitance–voltage (C–V) and capacitance–frequency (C–f) measurements in the temperature range of 120–320 K in dark conditions. The forward bias I–V characteristics have been analyzed on the basis of standard thermionic emission (TE) theory and the characteristic parameters of the Schottky contacts (SCs) such as Schottky barrier height (SBH), ideality factor (n) and series resistance (R_s) have been determined from the I–V measurements. The experimental values of SBH and n for the device ranged from 1.01 eV and 1.27 (at 320 K) to 0.38 eV and 5.93 (at 120 K) for Ni/n-GaP diode, respectively. The interface states in the semiconductor bandgap and their relaxation time have been determined from the C–f characteristics. The interface state density N_ss has ranged from 2.08 × 10¹⁵ (eV^?1 m^?2) at 120 K to 2.7 × 10¹⁵ (eV^?1 m^?2) at 320 K. C_ss has increased with increasing temperature. The relaxation time has ranged from 4.7 × 10^?7 s at 120 K to 5.15 × 10^?7 s at 320 K.     

Effective mobility in amorphous organic transistors: Influence of the width of the density of states

The temperature dependence of poly(3-hexylthiophene-2,5-diyl) (P3HT)/polystyrene (PS) blend organic transistor current/voltage (I–V) characteristics has been experimentally and theoretically studied. The planar transistors, realized by drop casting the P3HT/PS ink, exhibit high mobilities (over 5 × 10⁻³ cm² V⁻¹ s⁻¹) and good overall characteristics. A transistor model accounting for transport mechanisms in disordered organic materials was used to fit the measured characteristics. Using a single set of parameters, the measured effective mobility versus gate bias, either increasing or decreasing with the gate bias depending on temperature, is well reproduced over a wide temperature range (130–343 K). A Gaussian density of states width of 0.045 eV was determined for this P3HT/PS blend. The transistor I–V characteristics are very well described considering disordered material properties within a self-consistent transistor model.     

Photo-electrochemical characterization of porous material Fe-FSM-16. Application for hydrogen production

We describe in the present work the photo-electrochemical characterization of iron/folded-sheets mesoporous materials (Fe-FSM-16, Si/Fe=60) synthesized by microwave-assisted hydrothermal (M-H) method and its application for the hydrogen evolution upon visible light. The mesoporous catalyst consists of small Fe₂O₃ particles (～2 nm) spread on SiO₂ with specific surface area of ～800 m² g^?1. The capacitance measurements reveal an iron deficiency and the oxide exhibit p type conductivity with activation energy of 0.07 eV. The optical gap of the hematite (α-Fe₂O₃) is evaluated at 3.24 eV from the diffuse reflectance spectrum. The flat band potential V_fb (?0.54 V_SCE) and the holes density N_D (9.56×10¹⁴ cm^?3) of the hematite are obtained respectively by extrapolating the linear part to C^?2=0 and the slope of the Mott Schottky plot. At pH=7, the conduction band (?0.47 V_SCE) is suitably positioned with respect to the H₂O/H₂ level (?0.59 V_SCE) leading to a spontaneous water reduction. The oxide is stabilized by hole consumption involving SO₃^2? and S₂O₃^2? species and spectacular improvement of the hydrogen evolution is reported with evolution rates of ～461 and 163 μ mol respectively.