Ultrathin CdSe nanowire field-effect transistors

We report the fabrication, and electrical and optical characterization, of solution-liquid-solid (SLS) grown CdSe nanowire field-effect transistors. Ultrathin nanowires (7–12 nm diameters) with lengths between 1 μm and 10 μm were grown by the SLS technique. Al-CdSe-Al junctions are then defined over oxidized Si substrate using photolithography. The nanowires, which were very resistive in the dark, showed pronounced photoconductivity even with a visible light source with resistance decreasing by a factor of 2–100 for different devices. Field-effect devices fabricated by a global backgating technique showed threshold voltages between −7.5 V and −2.5 V and on-to-off channel current ratios between 10³ and 10⁶ at room temperature. Channel current modulation with gate voltage is observed with the current turning off for negative gate bias, suggesting unintentional n-type doping. Further, optical illumination resulted in the loss of gate control over the channel current of the field-effect transistor.     

The color change in polychromatic organic light-emitting field-effect transistors: Optical filtering versus reemission

In this contribution the color conversion process of a polychromatic organic light-emitting field-effect transistor (OLET) is revisited on the basis of an analytic device model. The device of interest consists of a color conversion layer out of rubrene on top of a monochromatic light-emitting transistor based on poly(9,9-di-n-octyl-fluorene-alt-benzothiadiazole) (F8BT). The model describes the relation of color coordinate and emission intensity – set by the applied drain and gate biases – linking the optoelectronic response of the employed monochromatic OLET to the optical processes occurring in the color conversion layer. The model shows that the color shift is rather due to partial absorption of the F8BT emission by rubrene than, as was claimed earlier, due to a color conversion process by absorption and reemission in the conversion layer. In addition to the earlier publication, it will be demonstrated that such a device allows for an independent electrical tunability of emission intensity and color coordinate within the color span of the F8BT and the rubrene spectrum being a unique feature of such a polychromatic light-emitting field-effect transistor.     

关于短沟道双栅无结型晶体管的仿真研究

We study the characteristics of short channel double-gate(DG) junctionless(JL) FETs by device simulation. OutputⅠ-Ⅴcharacteristic degradations such as an extremely reduced channel length induced subthreshold slope increase and the threshold voltage shift due to variations of body doping and channel length have been systematically analyzed.Distributions of electron concentration,electric field and potential in the body channel region are also analyzed.Comparisons with conventional inversion-mode(IM) FETs,which can demonstrate the advantages of JL FETs,have also been performed.     

Device performance and density of trap states of organic and inorganic field-effect transistors

Trap states, present in any semiconductor, have a large influence on charge transport as well as various other physical processes relevant for device performance. Therefore, quantifying the density of trap states (trap DOS) in a semiconductor is a crucial step towards understanding and improving field-effect transistors (FET), organic photovoltaic cells (OPV) and organic light-emitting diodes (OLED). We present a method to determine the free vs. total charge carrier density in FETs, and therefore the trap DOS, through full normalization of the transfer curve. We apply this method to many different materials, prepared under a wide range of processing conditions, e.g. organic single crystals, thermally evaporated and inkjet printed thin-films, leading to various degrees of order/disorder. They are compared to inorganic thin-films. A quantitative analysis of the spectral density of the trap DOS reveals the trap DOS in p- and n-type semiconductors to be very similar provided they have a similar morphology. The variation by 3 orders of magnitude is dominated by the degree of crystalline order. Further, the trap DOS in organic materials is essentially the same as in inorganic materials, again, provided they have a similar morphology. Surprisingly, ink-jet printed organic polymers have a relatively low trap DOS, which is in between the one of organic single crystals and thin-films.     

ZnO纳米线场效应管的制备及光电特性

本文采用水热法生长的ZnO纳米线制得悬浮ZnO纳米线场效应晶体管,该场效应管的跨导为0.396μS,迁移率为50.17cm^2/Vs,VGS = 0 V时电阻率为0.96×102Ωcm,开关态电流比（Ion/Ioff）为10^5。该效应管在紫外下曝光（2.5μw/cm2）后显示了穿通现象和开启电压漂移（从-0.6V 到 +0.7V）,且由于漏极感应势垒的降低,使得源漏电流减少一半（从560nA到320nA）。悬浮ZnO纳米线场效应管的这些性质揭示了它的一些内在属性和器件方面的应用。     

Fabrication and photoelectrical characteristics of ZnO nanowire field-effect transistors

rease by almost half of the source-drain current (Ids, from 560 nA to 320 nA) due to drain-induced barrier lowering.Continued work is underway to reveal the intrinsic properties of suspended ZnO nanowires and to explore their device applications.     

Interface effect in pentacene field-effect transistors from high energy proton beam irradiation

We report the effect of irradiation using 10 MeV high energy proton beams on pentacene organic field-effect transistors (OFETs). The electrical characteristics of the pentacene OFETs were measured before and after proton beam irradiation with fluence (dose) conditions of 10¹², 10¹³, and 10¹⁴ cm⁻². After proton beam irradiation with fluences of 10¹² or 10¹³ cm⁻², the threshold voltage of the OFET devices shifted to the positive gate voltage direction with an increase in the current level and mobility. In contrast, for a high proton beam fluence condition of 10¹⁴ cm⁻², the threshold voltage shifted to the negative gate voltage direction with a decrease in the current level and mobility. It is evident from the electrical characteristics of the pentacene OFETs treated with a self-assembled monolayer that these experimental observations can be attributed to the trapped charges in the dielectric layer and pentacene/SiO₂ interface. Our study will enhance the understanding of the influence of high energy particles on organic field-effect transistors.     

Alternating dithienobenzoxadiazole-based conjugated polymers for field-effect transistors and polymer solar cells

Three new alternating copolymers derived from dithienobenzoxadizole (DTfBO) and different thiophene-based π-spacers, including terthiophene, quarterthiophene, and dithienyl flanked thienothiophene, were successfully synthesized. The DTfBO-based polymers possess optical band-gaps in the range of 1.84–1.89 eV and exhibit relatively deep HOMO levels between −5.36 eV and −5.50 eV. Due to strong interchain aggregation, DTfBO-based polymers could not be well dissolved in chlorobenzene at room temperature, but they could be processed with hot chlorobenzene solutions of ∼100 °C. Evolutions of UV absorption spectra of polymer solutions during heating process could differentiate their different aggregation ability, among which a repeating unit based on a DTfBO and a terthiophene could supply the strongest inter-chain interaction. Notably, the three DTfBO-based polymers displayed high field-effect hole mobilities between 0.21 and 0.54 cm²/(V s). In polymer solar cells (PSCs) with the three polymers as the donors, high open-circuit voltages between 0.87 and 0.93 V could be realized. For active layer thickness of 80 nm, the PSCs displayed power conversion efficiency (PCE) between 2.85% and 5.07%. A very high fill factor of 75.4% was achieved for the polymer comprising dithienyl flanked thienothiophene. With thicker ative layers of 250 nm, the three DTfBO-based polymers exhibited comparable PCEs of ∼5.61% due to obviously increased short-circuit currents. Our results suggest that DTfBO, a big coplanar heterocycle, is a promising building block to construct high mobility conjugated polymers for efficient thick-film PSCs.     

Submicron-meter tunneling field-effect poly-Si thin-film transistors with a thinned channel layer

Submicron-meter poly-Si tunneling-effect thin-film transistor (TFT) devices with a thinned channel layer have been investigated. With reducing the gate length to be shorter than 1 μm, the poly-Si TFT device with conventional MOSFET structure is considerably degraded. The tunneling field-effect transistor (TFET) structure can be employed to alleviate the short channel effect, thus largely suppressing the off-state leakage. However, for a poly-Si channel layer of 100 nm thickness, the TFET structure causes a small on-state current, which may not provide well sufficient driving current. By reducing the channel layer thickness to be 20 nm, the on-state current for the TFET structure can be largely increased, due to the enhanced bending of energy band for a thinned channel layer. As a result, for the TFET poly-Si TFTs at a gate bias of 5 V and a drain bias of 3 V, a 20-nm channel layer leads to an on-state current of about 1 order larger than that by a 100-nm channel layer, while still keeping an off-state leakage smaller than 0.1 pA/μm. Accordingly, the submicron-meter TFET poly-Si TFT devices with a thinned channel layer would show good feasibility for implementing high packing density of poly-Si TFT devices.     

High operational stability of solution-processed organic field-effect transistors with top-gate configuration

Electrical characteristics of top-gate field-effect transistors based on a wide range of solution-processed organic semiconductors are systematically investigated. The top-gate field-effect transistors based on different organic semiconductors—from an amorphous polymer semiconductor to a polycrystalline molecular semiconductor—exhibit higher operational stability compared with bottom-gate organic field-effect transistors reported in literature, in spite of significant difference in field-effect mobility. The correlation between charge transport and operational stability is discussed to gain insight into high operational stability of top-gate organic field-effect transistors.     

Contactless charge carrier mobility measurement in organic field-effect transistors

With the increasing performance of organic semiconductors, contact resistances become an almost fundamental problem, obstructing the accurate measurement of charge carrier mobilities. Here, a generally applicable method is presented to determine the true charge carrier mobility in an organic field-effect transistor (OFET). The method uses two additional finger-shaped gates that capacitively generate and probe an alternating current in the OFET channel. The time lag between drive and probe can directly be related to the mobility, as is shown experimentally and numerically. As the scheme does not require the injection or uptake of charges it is fundamentally insensitive to contact resistances. Particularly for ambipolar materials the true mobilities are found to be substantially larger than determined by conventional (direct current) schemes.     

Analytical solution of fundamental surface potential equations for symmetric double-gate metal-oxide-semiconductor field-effect transistors

Both the fundamental surface potential equations for undoped and doped symmetric double-gate MOSFETs are transcendental equations with exponentials, to which a general analytical solution is introduced. Given the lowest potential in the channel film, this solution can calculate the surface potentials of both undoped and doped symmetric double-gate MOSFETs accurately. This analytical approach could also be applied to solve other similar device equations with exponential transcendent structures.     

Downscaling of n-channel organic field-effect transistors with inkjet-printed electrodes

In this contribution we demonstrate for the first time a downscaled n-channel organic field-effect transistors based on N,N′-dialkylsubstituted-(1,7&1,6)-dicyanoperylene-3,4:9,10-bis(dicarboximide) with inkjet printed electrodes. First we demonstrate that the use of a high boiling point solvent is critical to achieve extended crystalline domains in spin-coated thin films and thus high electron mobility >0.1 cm² V⁻¹ s⁻¹ in top-gate devices. Then inkjet-printing is employed to realize sub-micrometer scale channels by dewetting of silver nanoparticles off a first patterned gold contact. By employing a 50 nm crosslinked fluoropolymer gate dielectric, ∼200 nm long channel transistors can achieve good current saturation when operated <5 V with good bias stress stability.     

Enhanced performance of room temperature ammonia sensors using morphology-controlled organic field-effect transistors

Developing electronic sensors for ammonia (NH₃) is very useful for environmental monitoring and diagnostic purposes. In this work, a highly sensitive, organic field-effect transistor (OFET) based, room temperature sensor for NH₃ has been fabricated using dinaphtho [2,3-b:2′,3′-f]thieno [3,2-b]thiophene (DNTT), which showed a fast response to low concentration of the analyte down to 100 ppb. A thin film of solution-processed polymethyl methacrylate (PMMA) has been used as the gate dielectric material and its hydrophobic surface promoted structured growth of organic semiconductor, DNTT, by inducing mass transfer. By controlling the thickness and thereby exploiting the growth dynamics of the semiconductor film, the sensor performance was improved. The sensitivity of the device towards 1 ppm of NH₃ was almost doubled with a thinner and porous film of DNTT as compared to that with a thick film. Morphological studies of the sensing layers, using atomic force microscopy (AFM), have established this structure-property relation. The variations in different transistor parameters have been studied with respect to different analyte concentrations. The p-channel devices in the enhancement mode showed depletion upon exposure to NH₃. The devices exhibited a fast response and good recovery to the initial state within 2 min.     

Dielectric surface-polarity tuning and enhanced operation stability of solution-processed organic field-effect transistors

The electrical performance of triethylsilylethynyl anthradithiophene (TES-ADT) organic field-effect transistors (OFETs) was significantly affected by dielectric surface polarity controlled by grafting hexamethyldisilazane and dimethyl chlorosilane-terminated polystyrene (PS-Si(CH₃)₂Cl) to 300-nm-thick SiO₂ dielectrics. On the untreated and treated SiO₂ dielectrics, solvent–vapor annealed TES-ADT films contained millimeter-sized crystals with low grain boundaries (GBs). The operation and bias stability of OFETs containing similar crystalline structures of TES-ADT could be significantly increased with a decrease in dielectric surface polarity. Among dielectrics with similar capacitances (10.5–11 nF cm⁻²) and surface roughnesses (0.40–0.44 nm), the TES-ADT/PS-grafted dielectric interface contained the fewest trap sites and therefore the OFET produced using it had low-voltage operation and a charge-carrier mobility ∼1.32 cm² V⁻¹ s⁻¹, on–off current ratio >10⁶, threshold voltage ∼0 V, and long-term operation stability under negative bias stress.     

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.     

Direct investigations of the interface impedance of organic field-effect transistors with self-assembled-monolayer-modified electrodes

It has been demonstrated that the modification of electrodes with self-assembled monolayers (SAMs) reduces the contact resistance and improves the device performances of organic field-effect transistors (OFETs). However, it has been difficult to judge if the contact resistance was reduced by the change in the electronic properties or by the change in the morphology of the metal–organic interface caused by the SAM modification because they have been difficult to be separately assessed. We have directly investigated the local impedance and the potential difference at the electrode–channel interfaces of the OFETs with and without modification of the electrodes by a pentafluorobenzenethiol SAM using frequency-modulation scanning impedance microscopy (FM-SIM). The potential profile measurement and the FM-SIM measurement at the interface showed that the improvement of the field-effect mobility in the SAM-modified OFET was caused by the reduction of the energy level mismatch, namely, the hole injection barrier at the source–channel interface, presumably with the reduction of the hole trap sites at the source–channel interface.     

Electrical characterization of mica as an insulator for organic field-effect transistors

Electrical properties of conjugated polymer films, including poly(3-hexylthiophene)-2,5-diyl (P3HT), poly(3,3-didodecylquarterthiophene) (PQT-12), and poly(triarylamine) (PTAA), on mica substrates have been studied. The test structure was similar to a standard organic field-effect transistor but with a 150-μm-thick commercially available mica gate insulator/substrate, which allowed to obtain a field-effect mobility of P3HT as high as 0.08 cm²/Vs in the linear regime in ambient air. The influence of interface treatment, thermal annealing, and measurement conditions on the electrical properties of the P3HT films has been characterized and analyzed. We also studied the time dependence of the carrier concentration and mobility before and after a thermal annealing process. The results indicate that mica is a promising insulator for organic field-effect transistors, apart from already being one of the common thin-film materials widely used in electric capacitors.     

Electrical responses of short-channel organic transistor prepared by solution-processed organic crystal wire mask

We report a formation of a solution-grown single crystal wire mask for the fabrication of short-channel organic field-effect transistor with enhanced dynamic response time. The various channel length, ranging from submicrometer to a few micrometers, were obtained by controlling the concentration of solution and processing conditions. We fabricated p- and n-channel bottom-contact organic field-effect transistors using pentacene and PTCDI-C₁₃, respectively, and static and dynamic electrical characteristics of the devices were investigated. The highest and average field-effect hole mobility values were found to be 0.892 cm²/V s and 0.192 cm²/V s, respectively. The load type inverter based on the short-channel transistor connected with a 2 MΩ resistor showed a clear switching response when square wave input signals up to 1 kHz were applied at V_DD = −60 V.     

Three-terminal capacitance–voltage measurements of pentacene field-effect transistors during operation

We propose a modified measurement technique of capacitance for three-terminal devices and apply this method for the evaluation of the channel formation in pentacene field-effect transistors. An additional structure in the capacitance–voltage curves clearly shows the channel formation from the saturation to the linear region in an operating transistor which has not been directly observed in conventional methods. Based on the amount of accumulated charge in the channel region, the validity of the gradual channel approximation model and the usability of a buffer layer are discussed. This method enables the detailed investigation of carrier behaviors in organic transistors through appropriate evaluation of the channel formation process during operation.