AlGaN/GaN Schottky barrier diodes on silicon substrates with various Fe doping concentrations in the buffer layers

This study demonstrated AlGaN/GaN Schottky barrier diodes (SBDs) for use in high-frequency, high-power, and high-temperature electronics applications. Four structures with various Fe doping concentrations in the buffer layers were investigated to suppress the leakage current and improve the breakdown voltage. The fabricated SBD with an Fe-doped AlGaN buffer layer of 8 × 10¹⁷ cm^− 3 realized the highest on-resistance (R_ON) and turn-on voltage (V_ON) because of the memory effect of Fe diffusion. The optimal device was the SBD with an Fe-doped buffer layer of 7 × 10¹⁷ cm^− 3, which exhibited a R_ON of 31.6 mΩ-cm², a V_ON of 1.2 V, a breakdown voltage of 803 V, and a buffer breakdown voltage of 758 V. Additionally, the low-frequency noise decreased when the Fe doping concentration in the buffer layer was increased. This was because the electron density in the channel exhibited the same trend as that of the Fe doping concentration in the buffer layer.     

Improved performance of polymer solar cells based on P3HT and ICBA using alcohol soluble titanium chelate as electron collection layer

We demonstrate efficient polymer solar cells (PSCs) based on poly(3-hexylthiophene) (P3HT) and fullerene derivatives ether Indene-C₆₀ Bisadduct (IC₆₀BA) or Indene-C₇₀ Bisadduct (IC₇₀BA)) by using solution-processed titanium(IV) oxide bis(2,4-pentanedionate) (TOPD) as electron collection layer (ECL) between the Al cathode and photoactive layer. The TOPD buffer layer was simply prepared by spin-coating isopropanol solution of TOPD on active layer and then baked at 80 °C for 15 min. The short-circuit current density (J_sc) and the open-circuit voltage (V_oc) of the devices can be simultaneously and significantly improved by optimizing the electron collection layer, the photoactive layer and the device fabrication conditions. The power conversion efficiency (PCE) of the P3HT:IC₆₀BA BHJ device with TOPD buffer layer reaches 5.0% under the illumination of AM1.5G, 100 mW/cm², which is increased by 27% in comparison with that (3.9%) of the device without TOPD buffer layer under the same experimental conditions. When IC₇₀BA was chosen instead of IC₆₀BA, the BHJ device could show better performance with PCE of 5.59%. The results indicate that TOPD is a promising electron collection layer for PSCs.     

New hole blocking material for green-emitting phosphorescent organic electroluminescent devices

The new amorphous molecular material, 2,5-bis(4-triphenylsilanyl-phenyl)-[1,3,4]oxadiazole, that functions as good hole blocker as well as electron transporting layer in the phosphorescent devices. The obtained material forms homogeneous and stable amorphous film. The new synthesized showed the reversible cathodic reduction for hole blocking material and the low reduction potential for electron transporting material in organic electroluminescent (EL) devices. The fabricated devices exhibited high performance with high current efficiency and power efficiency of 45 cd/A and 17.7 lm/W in 10 mA/cm², which is superior to the result of the device using BAlq (current efficiency: 31.5 cd/A and power efficiency: 13.5 lm/W in 10 mA/cm²) as well-known hole blocker. The ITO/DNTPD/α-NPD/6% Ir(ppy)₃ doped CBP/2,5-bis(4-triphenylsilanyl-phenyl)-[1,3,4]oxadiazole as both hole blocking and electron transporting layer/Al device showed efficiency of 45 cd/A and maximum brightness of 3000 cd/m² in 10 mA/cm².     

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.     

High electron mobility triazine for lower driving voltage and higher efficiency organic light emitting devices

The triazine compound 4,4′-bis-[2-(4,6-diphenyl-1,3,5-triazinyl)]-1,1′-biphenyl (BTB) was developed for use as an electron transport material in organic light emitting devices (OLEDs). The material demonstrates an electron mobility of ∼7.2 × 10⁻⁴ cm² V⁻¹ s⁻¹ at a field of 8.00 × 10⁵ V cm⁻¹, which is 10-fold greater than that of the widely used material tris(8-hydroxyquinoline) aluminum (AlQ₃). OLEDs with a BTB electron transport layer showed a ∼1.7–2.5 V lower driving voltage and a significantly increased efficiency, compared to those with AlQ₃. These results suggest that BTB has a strong potential for use as an OLED electron transport layer material.     

D–A copolymer with high ambipolar mobilities based on dithienothiophene and diketopyrrolopyrrole for polymer solar cells and organic field-effect transistors

Donor–acceptor (D–A) type conjugated polymers have been developed to absorb longer wavelength light in polymer solar cells (PSCs) and to achieve a high charge carrier mobility in organic field-effect transistors (OFETs). PDTDP, containing dithienothiophene (DTT) as the electron donor and diketopyrrolopyrrole (DPP) as the electron acceptor, was synthesized by stille polycondensation in order to achieve the advantages of D–A type conjugated polymers. The polymer showed optical band gaps of 1.44 and 1.42 eV in solution and in film, respectively, and a HOMO level of 5.09 eV. PDTDP and PC₇₁BM blends with 1,8-diiodooctane (DIO) exhibited improved performance in PSCs with a power conversion efficiency (PCE) of 4.45% under AM 1.5G irradiation. By investigating transmission electron microscopy (TEM), atomic force microscopy (AFM), and the light intensity dependence of J_SC and V_OC, we conclude that DIO acts as a processing additive that helps to form a nanoscale phase separation between donor and acceptor, resulting in an enhancement of μ_h and μ_e, which affects the J_SC, EQE, and PCE of PSCs. The charge carrier mobilities of PDTDP in OFETs were also investigated at various annealing temperatures and the polymer exhibited the highest hole and electron mobilities of 2.53 cm² V⁻¹ s⁻¹ at 250 °C and 0.36 cm² V⁻¹ s⁻¹ at 310 °C, respectively. XRD and AFM results demonstrated that the thermal annealing temperature had a critical effect on the changes in the crystallinity and morphology of the polymer. The low-voltage device was fabricated using high-k dielectric, P(VDF-TrFE) and P(VDF-TrFE-CTFE), and the carrier mobility of PDTDP was reached 0.1 cm² V⁻¹ s⁻¹ at V_d = −5 V. PDTDP complementary inverters were fabricated, and the high ambipolar characteristics of the polymer resulted in an output voltage gain of more than 25.     

High current conduction with high mobility by non-radiative charge recombination interfaces in organic semiconductor devices

We report a unique non-radiative p-n-p junction structure to provide high current conduction with high mobility in organic semiconductor devices. The current conduction was improved by increasing p-n junctions made with intrinsic p-type hole transport layer and n-type electron transport layer. The excellent hole mobility of 5.3 × 10^?1 cm²/V s in this p-n-p device configuration is measured by the space charge limited current method with an electric field of 0.3 MV/cm. Enhanced current conduction of 248% at 4.0 V was observed in fluorescent blue organic light-emitting diodes with introduction of non-radiative p-n-p-n-p junction interfaces. Thereupon, the power efficiency at 1000 cd/m² was improved by 22% and the driving voltage also was reduced by 17%, compared to that of no interface device. Such high current conduction with high mobility is attributed to the carrier recombination at p-n-p interfaces through coulombic interaction. This non-radiative p-n-p junction structure suggested in this report can be very useful for many practical organic semiconductor device applications.     

Ambipolar transport in transparent and flexible all-organic heterojunction field effect transistors at ambient conditions

We report on the realization of fully flexible and transparent n-type and ambipolar all-organic OFETs. A double layer, pentacene-C60 heterojunction, was used as the semiconductor layer. The contacts were made with poly(ethylenedioxythiophene)/poly(styrenesulfonate) (PEDOT:PSS) and patterned by means of Soft Lithography MicroContact Printing (μCP). Interestingly, as demonstrated by atomic force microscopy and X-ray diffraction investigations, growing C60 on a pre-deposited pentacene buffer layer leads to a clear improvement in the morphology and crystallinity of the deposited film allowing to obtain n-type conduction despite the very high electron injection barrier at the interface between PEDOT:PSS and C60. As a result, it was possible to realize n-type and ambipolar all-organic OFETs by optimizing the thicknesses of the pentacene buffer layer. All devices, measured in air, worked in accumulation mode with mobilities up to 1 × 10⁻² cm²/V s and 3.5 × 10⁻⁴ cm²/V s for p-type and n-type regimes, respectively. This is particularly interesting because it demonstrates, also for n-type and ambipolar transistors, the possibility of avoiding problems normally associated to metal contacts: the lack of mechanical robustness, flexibility, and the unfeasibility of realizing contacts with low cost techniques like printing or soft lithography. These results confirm the importance of the substrate properties for the ordered growth of organic semiconductors, which determines the transport properties of organic materials.     

Operational frequency degradation induced trapping in scaled GaN HEMTs

Cut-off frequency increase from 12.1 GHz to 26.4 GHz, 52.1 GHz and 91.4 GHz is observed when the 1 μm gate length GaN HEMT is laterally scaled down to L_G = 0.5 μm, L_G = 0.25 μm and L_G = 0.125 μm, respectively. The study is based on accurately calibrated transfer characteristics (I_D-V_GS) of the 1 μm gate length device using Silvaco TCAD. If the scaling is also performed horizontally, proportionally to the lateral (full scaling), the maximum drain current is reduced by 38.2% when the gate-to-channel separation scales from 33 nm to 8.25 nm. Degradation of the RF performance of a GaN HEMT due to the electric field induced acceptor traps experienced under a high electrical stress is found to be about 8% for 1 μm gate length device. The degradation of scaled HEMTs reduces to 3.5% and 7.3% for the 0.25 μm and 0.125 gate length devices, respectively. The traps at energy level of E_T = E_V + 0.9 eV (carbon) with concentrations of N_IT = 5 × 10¹⁶cm^− 3, N_IT = 5 × 10¹⁷cm^− 3 and N_IT = 5 × 10¹⁸cm^− 3 are located in the drain access region where highest electrical field is expected. The effect of traps on the cut-off frequency is reduced for devices with shorter gate lengths down to 0.125 μm.     

Using metal/organic junction engineering to prepare an efficient organic base-modulation triode and its inverter

In this study, we investigated the influence of a buffer layer of molybdic oxide (MoO₃) at the metal/organic junction on the behavior of organic base-modulation triodes. The performance of devices featuring MoO₃/Al as the emitter electrode was enhanced relative to that of corresponding devices with Au and Ag, presumably because of the reduced in the contact barrier and the prevention of metal diffusion into the organic layer. The device exhibited an output current of ?16.1 μA at V_B = ?5 V and a current ON/OFF ratio of 10³. Using this architecture, we constructed resistance–load inverters that exhibited a calculated gain of 6.     

Solution processable poly(2,5-dialkyl-2,5-dihydro-3,6-di-2-thienyl-pyrrolo[3,4-c]pyrrole-1,4-dione) for ambipolar organic thin film transistors

Solution processable diketopyrrolopyrrole (DPP)-bithiophene polymers (PDBT) with long branched alkyl side chains on the DPP unit are synthesized. These polymers have favourable highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels for the injection and transport of both holes and electrons. Organic thin film transistors (OTFTs) using these polymers as semiconductors and gold as source/drain electrodes show typical ambipolar characteristics with very well balanced high hole and electron mobilities (μ_h = 0.024 cm² V^?1 s^?1 and μ_e = 0.056 cm² V^?1 s^?1). These simple and high-performing polymers are promising materials for ambipolar organic thin film transistors for low-cost CMOS-like logic circuits.     

Hole mobilities of thermally polymerized triaryldiamine derivatives and their application as hole-transport materials in organic light-emitting diodes (OLEDs)

This paper describes the synthesis of three triaryldiamine derivatives presenting two thermally polymerizable trifluorovinyl ether groups that can be polymerized through thermal curing to form perfluorocyclobutyl (PFCB) polymers. These PFCB polymers, studied using time-of-flight techniques for the first time, exhibited remarkable non-dispersive hole-transport properties, with values of μ_h of ca. 10^?4 cm² V^?1 s^?1. When we employed these thermally polymerized polymers as hole-transport layers (HTLs) in electroluminescence devices containing tris(8-hydroxyquinolate) aluminum (Alq₃) as the emission layer, we obtained high current densities (ca. 3400 mA cm^?2), impressive brightnesses (5 × 10⁴ cd m^?2), and high external quantum efficiencies (EQEs = 1.43%). These devices exhibited the same turn-on voltage, but higher EQEs, relative to those incorporating the vacuum-processed model compound N,N′-di(1-naphthyl)-N,N′-diphenylbenzidine (α-NPD) (EQE = 1.37%) as the HTL under the same device structure.     

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.     

Fabrication of single crystal field-effect transistors with phenacene-type molecules and their excellent transistor characteristics

Single crystal field-effect transistors (FETs) using [6]phenacene and [7]phenacene show p-channel FET characteristics. Field-effect mobilities, μs, as high as 5.6 × 10^?1 cm² V^?1 s^?1 in a [6]phenacene single crystal FET with an SiO₂ gate dielectric and 2.3 cm² V^?1 s^?1 in a [7]phenacene single crystal FET were recorded. In these FETs, 7,7,8,8-tetracyanoquinodimethane (TCNQ) was inserted between the Au source/drain electrodes and the single crystal to reduce hole-injection barrier heights. The μ reached 3.2 cm² V^?1 s^?1 in the [7]phenacene single crystal FET with a Ta₂O₅ gate dielectric, and a low absolute threshold voltage |V_TH| (6.3 V) was observed. Insertion of 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F₄TCNQ) in the interface produced very a high μ value (4.7–6.7 cm² V^?1 s^?1) in the [7]phenacene single crystal FET, indicating that F₄TCNQ was better for interface modification than TCNQ. A single crystal electric double-layer FET provided μ as high as 3.8 × 10^?1 cm² V^?1 s^?1 and |V_TH| as low as 2.3 V. These results indicate that [6]phenacene and [7]phenacene are promising materials for future practical FET devices, and in addition we suggest that such devices might also provide a research tool to investigate a material’s potential as a superconductor and a possible new way to produce the superconducting state.     

Dependence of annealing on stability of transparent amorphous InGaZnO thin film transistor

Bottom-gate transparent IGZO–TFT had been successfully fabricated at relatively low temperature (200 °C). The devices annealing for 4 h at 200 °C exhibit good electrical properties with saturation mobility of 8.2 cm²V^?1s^?1, subthreshold swing of 1.0 V/dec and on/off current ratio of 5×10⁶. The results revealed that the stability of TFT devices can be improved remarkably by post-annealing treatment. After applying positive gate bias stress of 20 V for 5000 s, the device annealing for 1 h shows a larger positive V_th shift of 4.7 V. However, the device annealing for 4 h exhibits a much smaller V_th shift of 0.04 V and more stable.     

Source/drain metallization effects on the specific contact resistance of indium tin zinc oxide thin film transistors

We report on the specific contact resistance of interfaces between thin amorphous semiconductor Indium Tin Zinc Oxide (ITZO) channel layers and different source/drain (S/D) electrodes (Al, ITO, and Ni) in amorphous oxide thin film transistors (TFTs) at different channel lengths using a transmission line model. All the contacts showed linear current–voltage characteristics. The effects of different channel lengths (200–800 μm, step 200 μm) and the contact resistance on the performance of TFT devices are discussed in this work. The Al/ITZO TFT samples with the channel length of 200 μm showed metallic behavior with a linear drain current-gate voltage (I_D–V_G) curve due to the formation of a conducting channel layer. The specific contact resistance (ρ_C) at the source or drain contact decreases as the gate voltage is increased from 0 to 10 V. The devices fabricated with Ni S/D electrodes show the best TFT characteristics such as highest field effect mobility (16.09 cm²/V·s), ON/OFF current ratio (3.27×10⁶), lowest sub-threshold slope (0.10 V/dec) and specific contact resistance (8.62 Ω·cm² at V_G=0 V). This is found that the interfacial reaction between Al and a-ITZO semiconducting layer lead to the negative shift of threshold voltage. There is a trend that the specific contact resistance decreases with increasing the work function of S/D electrode. This result can be partially ascribed to better band alignment in the Ni/ITZO interface due to the work function of Ni (5.04–5.35 eV) and ITZO (5.00–6.10 eV) being somewhat similar.     

Benzopyrazine-fused tetracene derivatives: Thin-film formation at the crystalline mesophase for solution-processed hole transporting devices

Benzopyrazine-fused tetracene (TBPy) and its disulfide (TBPyS) bearing alkoxy groups (OCH₃, OC₈H₁₇) were designed and synthesized to obtain π-expanded tetracene derivatives. These derivatives are featured with long-wavelength light absorption property (λ_onset: up to 820 nm), photooxidative stability (half-lives (τ_1/2): 11 times longer than tetracene), and solubility for solution process. The methoxy compounds (TBPy-C1 and TBPyS-C1) were used for single-crystal X-ray crystallographic analysis and single-crystal organic field-effect transistor (OFET) devices showing relationship between packing structures and hole mobilities. The octyloxy compounds (TBPy-C8 and TBPyS-C8) were investigated on solution-processed thin-film formation and hole transport property in thin-film OFET devices. Crystalline mesophase of TBPy-C8 and TBPyS-C8 was characterized by differential scanning calorimetry analysis showing endothermic peaks at 98 and 198 °C on its second heating process and exothermic peaks at 177 and 76 °C on its second cooling process for TBPyS-C8, and played crucial roles in thin-films formation. Hole mobility of 1.7 × 10^?2 cm²/V s (with V_th = ?30 V and I_ON/I_OFF = 10⁴) was obtained for the thin-film OFET device of TBPyS-C8.     

Channel width effect for organic thin film transistors using TIPS-pentacene employed as a dopant of poly-triarylamine

The effects of the physical channel width on the characteristics of organic thin film transistors (OTFTs), made with 6,13-bis(triisopropyl-silylethynyl)-pentacene (TIPS-pentacene) embedded into poly-triarylamine (PTAA, hole conductor within an active channel), have been examined in this paper. The devices are estimated by measuring the drain-source current (I_DS) for different contact metals such as Au and Ag, at fixed gate and drain voltages. The results show that the threshold voltage (V_T) and I_DS increase with increasing channel width. Furthermore, it has been observed that the field effect mobility is dependent on V_T, which is influenced by the channel width. The OTFTs, produced using Au and Ag contacts, exhibited the highest values of mobility in the saturation regime, namely 5.44 × 10^?2 and 1.33 × 10^?2 cm²/Vs, respectively.     

Non-functionalized soluble diketopyrrolopyrrole: Simplest p-channel core for organic field-effect transistors

We report the synthesis, characterization and behavior in field-effect transistors of non-functionalized soluble diketopyrrolopyrrole (DPP) core with only a solubilizing alkyl chain (i.e. –C₁₆H₃₃ or –C₁₈H₃₇) as the simplest p-channel semiconductor. The characteristics were evaluated by UV–vis and fluorescence spectroscopy, X-ray diffraction, cyclic voltammetry (CV), thermal analysis, atomic force microscopy (AFM) and density functional theory (DFT) calculation. For top-contact field-effect transistors, two types of active layers were prepared either by a solution process (as a 1D-microwire) or thermal vacuum deposition (as a thin-film) on a cross-linked poly(4-vinylphenol) gate dielectric. All the devices showed typical p-channel behavior with dominant hole transports. The device made with 1D-microwiress of DPP-R18 showed field-effect mobility in the saturation region of 1.42 × 10^?2 cm²/V s with I_ON/I_OFF of 1.82 × 10³. These findings suggest that the non-functionalized soluble DPP core itself without any further functionalization could also be used as a p-channel semiconductor for low-cost organic electronic devices.     

Role of Schottky barrier height at source/drain contact for electrical improvement in high carrier concentration amorphous InGaZnO thin film transistors

We report the fabrication of bottom-gate thin film transistors (TFTs) at various carrier concentrations of an amorphous InGaZnO (a-IGZO) active layer from ~10¹⁶ to ~10¹⁹ cm⁻³, which exceeds the limit of the concentration range for a conventional active layer in a TFT. Using the Schottky TFTs configuration yielded high TFT performance with saturation mobility (μ_sat), threshold voltage (V_TH), and on off current ratio (I_ON/I_OFF) of 16.1 cm²/V s, −1.22 V, and 1.3×10⁸, respectively, at the highest carrier concentration active layer of 10¹⁹ cm⁻³. Other carrier concentrations (<10¹⁹ cm⁻³) of IGZO resulted in a decrease of its work function and increase in activation energy, which changes the source/drain (S/D) contact with the active layer behavior from Schottky to quasi Ohmic, resulting in achieving conventional TFT. Hence, we successfully manipulate the barrier height between the active layer and the S/D contact by changing the carrier concentration of the active layer. Since the performance of this Schottky type TFT yielded favorable results, it is feasible to explore other high carrier concentration ternary and quaternary materials as active layers.