The generalized Einstein relation (GER) can unify various theoretical models and predict charge transport in OSCs with various crystallinities, by altering the variance of the density of states and the delocalization degree in a Gaussian-distributed density of states.
Many advanced materials have been developed for organic field-effect transistors (OFETs) or thin-film transistors (TFTs) based on organic and organic hybrid materials. However, although many new OFETs exhibit superior characteristic parameters (such as high mobility), most of them show nonideal performances that have strongly limited progress in the design of molecules, the understanding of transport mechanisms, and the circuit applications of OFETs. In this review, the device physics of ideal and nonideal OFETs is discussed first to understand the factors that limit effective mobility in semiconducting channels, distort the potential distribution, or reduce the drift electric field. Then, recent advances in optimizing the material combinations, device structures, and fabrications of OFETs toward ideal transistors are discussed. Based on the good control of materials and interfaces, some new and novel concepts to utilize the nonideal properties of OFETs to build low-power circuits and integrated sensors are also discussed.
Revealing the intrinsic electrical properties is the basis of understanding new functional materials and developing their applications. However, in nonideal field-effect transistors (FETs), conventional current-voltage characterizations do not accurately probe charge transport, particularly for newly developed semiconductors. Here, a generalized gated four-probe (G-GFP) technique is developed, which detects dynamic changes in carrier accumulation and transport. The technique is suitable for exploring the intrinsic properties of semiconductors in FETs with arbitrary contacts and in any operational regimes above the threshold. Application to simulated transistors confirms its accuracy in probing the evolution of channel potential, drift field, and gate-dependent carrier mobility for devices with a contact-limited operation and disordered semiconductors. Comparative experiments are performed based on FETs with various materials, device structures, and operational temperatures. The G-GFP technique proves to exclude the various injection properties, to detect in situ how carriers are accumulated, and to clarify carrier mobility of the semiconductors. In particular, the well-known "double-slope" features in the current-voltage relations are controllably generated and their origins are identified. The approach could be used to explore electronic properties of newly developed materials such as organic, oxide, or 2D semiconductors.
Thin-film transistors (TFTs) and field-effect transistors (FETs) are basic units to build functional electronic circuits and investigate transport physics. In conventional TFTs or FETs, performance in terms of current level, on−off ratio, and the sensitivity of detection is limited by homogeneous semiconducting layers. In this paper, we develop TFTs with submicron heterostructures by using a strategy based on near-field photolithography. We use an array of totalreflective polydimethylsiloxane pyramids or trenches as a soft photomask in photolithography to induce multiple reflections and diffractions to focus the light. The textured feature enables the generation of gaps, dots, and grids at the nanoscale, with dimensions as small as sub-100 nm on substrates at the centimeter scale. We demonstrated the very high performance oxide TFTs on the nanoscale and periodic degenerately doped heterojunctions, and they yielded a nearly 20fold increase in transconductance and apparent device mobility. The on−off ratio was higher than 10 9 , with notably enhanced output current and clear scaling effect with channel length. We also built nanostructured wide-gap/narrow-gap heterojunctions to balance the high on−off ratio and sensitive photoresponse in a unidirectional phototransistor. This study shows the viability of programming a variety of nanoscale submicron patterns or interfaces in TFTs and FETs to significantly enlarge the scope of research on multifunctional TFTs and FETs.
Indium tin oxide (ITO) is generally used as an electrode material but has recently been demonstrated to be a competitive candidate for use in semiconductor layers in high-performance thin-film transistors (TFTs), due to its high mobility and strong resistance to wet-etching. Here, we demonstrate TFTs using solution-processed, ultra-thin ITO films with outstanding switching performance. These devices exhibit a mobility of up to 15 cm2 V−1 s−1 and a high on-off ratio of 108. Because the device exhibits significant instability under stress tests, moderate doping with Ga as a dopant is introduced to form Ga-doped ITO TFTs. The resulting device has much enhanced stability, near-zero turn-on voltage, and a high on-off current ratio of 108. Through further involvement of an AlOx dielectric layer, the Ga-doped ITO TFTs exhibit a high apparent mobility of more than 40 cm2 V−1 s−1 and operate at small gate voltages (3 V). Remarkably, the device maintains an on-off ratio of over 104 at drain voltages as small as 1 mV.
In organic thin-film transistors (OTFTs), using high-k dielectrics could increase the accumulated carrier numbers at low operation voltage (i.e. the bulk effect), but may induce extra dipolar disorders to affect the electronic states in organic semiconductors (i.e. the interfacial effect). The two effects are considered together to model charge transport in OTFTs and to give quantitative calculations of the impacts from using high-k dielectrics or hybrid dielectrics, especially the relations between mobility and Fermi-energy, carrier concentrations, or gate-voltage. In experimental studies, zirconium oxide (ZrO 2 ) and yttrium oxide (Y 2 O 3 ) are fabricated by solution process for high-k dielectrics. The oxide insulating films are modified by low-k polymer layers and the resulting OTFTs show low operation voltage and high performance in ambient test. The highest mobility reaches 2.38 cm 2 V −1 s −1 for ZrO 2 based OTFT and 3.56 cm 2 V −1 s −1 for Y 2 O 3 based OTFT, and the on/off ratio is 1.4×10 4 for ZrO 2 devices and 5.4×10 4 for Y 2 O 3 devices. In general, the trends of experimentally measured mobility are consistent with the theoretical calculations, showing the high-k/low-k hybrid dielectrics mainly improve the carrier mobility in the regime of low carrier density.
In article number 1903889, Chuan Liu and co‐workers review the origins and critical factors that lead to organic field‐effect transistors (OFETs) with deviations from the ideal device models in terms of device physics. The recent progress in the optimization strategies and new perspectives for utilizing nonideal OFETs are also presented.
BackgroundThis research focused on evaluating the correlation between platelet count and sepsis prognosis, and even the dose-response relationship, in a cohort of American adults.MethodPlatelet counts were recorded retrospectively after hospitalization for patients admitted to Beth Israel Deaconess Medical Center’s intensive care unit between 2008 and 2019. On admission to the intensive care unit, sepsis patients were divided into four categories based on platelet counts (very low < 50 × 109/L, intermediate-low 50 × 109–100 × 109/L, low 100 × 109–150 × 109/L, and normal ≥ 150 × 109/L). A multivariate Cox proportional risk model was used to calculate the 28-day risk of mortality in sepsis based on baseline platelet counts, and a two-piece linear regression model was used to calculate the threshold effect.ResultsThe risk of 28-day septic mortality was nearly 2-fold higher in the platelet very low group when compared to the low group (hazard ratios [HRs], 2.24; 95% confidence interval [CI], 1.92–2.6). Further analysis revealed a curvilinear association between platelets and the sepsis risk of death, with a saturation effect predicted at 100 × 109/L. When platelet counts were below 100 × 109/L, the risk of sepsis 28-day death decreased significantly with increasing platelet count levels (HR, 0.875; 95% CI, 0.84–0.90).ConclusionWhen platelet count was less than 100 × 109/L, it was a strong predictor of the potential risk of sepsis death, which is declined by 13% for every 10 × 109/L growth in platelets. When platelet counts reach up to 100 × 109/L, the probability of dying to sepsis within 28 days climbs by 1% for every 10 × 109/L increase in platelet count.
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