Wu-Wei Tsai

Pentacene-Based Organic Thin Film Transistors for Ammonia Sensing

Non-invasive ammonia sensors are attractive alternatives for the diagnoses of a variety of chronic diseases such as liver cirrhosis and renal failure. A low cost pentacene-based organic thin film transistor (OTFT) fabricated by a novel and simple process was demonstrated to be highly sensitive and specific for ammonia gas. Various measurement parameters that reflected OTFT device characteristics for ammonia detection were investigated. Significant variations of the turn-on current, intrinsic mobility, and threshold voltage (Vth) were observed while subthreshold swing (S.S.) was almost unchanged to the alteration of ammonia concentration. The OTFT device detected 0.5 ~ 5 ppm concentration ammonia gas at room temperature, which is in the critical range that can distinguish between healthy person and paticents with liver cirrhosis and renal failure. The sensitivity of the device was further enhanced following a simple UV irradiation treatment to modify the functional groups on poly(methyl methacrylate) (PMMA) dielectric layer. Possible interference for ammonia detection such as humidity effect and selectivity among nitrogen, alcohol, carbon dioxide, acetone, methane and ammonia were also examined. We concluded that the proposed pentacene-based OTFT is a promising device for the future application in non-invasive medical diagnoses.

Effective Mobility Enhancement by Using Nanometer Dot Doping in Amorphous IGZO Thin‐Film Transistors

IO N With a high mobility ( > 10 cm 2 V − 1 s − 1 ) and a low threshold voltage ( < 5 V) in low-temperature processes, transparent oxide semiconductor thin-fi lm transistors (TOS TFTs) have drawn considerable attention due to their applications on fl exible displays, level shifters, drivers, and pixel-driving circuits for activematrix organic light-emitting-diode (AMOLED) displays. [ 1 − 3 ] In addition to display applications, amorphous indium gallium zinc oxide (a-IGZO) TFTs are also promising for the development of radio-frequency identifi cation (RFID) tags, smart cards, and other types of fl exible electronics. When TOS TFTs are developed for a low-power high-frequency circuit, high electron mobility and a low parasitic capacitance are required. Most TFTs fabricated with ZnO, SnO 2 , In 2 O 3 , IGZO, or other semiconducting oxide thin fi lms exhibit electron mobilities smaller than 35 cm 2 V − 1 s − 1 . [ 4–6 ] Recent reports on transparent oxide nanowire transistors (NWTs) have demonstrated high electron mobilities approximately 70 to 4000 cm 2 V − 1 s − 1 . [ 7–9 ] The quasi1D structure of NWTs may reduce low-angle carrier scattering to produce high electron mobility. [ 9 ] However, the fabrication process of NWTs has poor reproducibility and is still not practical for real-world applications. Because TOS transistors are transparent, developing TOS circuits on windows is appealing. Particularly, for modern buildings or trains with series of windows, TOS RFID circuits on windows can deliver various types of signals through a low-power transmission system. In this type of application, the dimension of the transparent transistor can be large because an integrated circuit on a small chip is not necessary. A low-cost production method for delivering a highperformance TOS transistor is a critical challenge. Here, a nanostructure to improve the effective mobility in a-IGZO TFTs is proposed. A large channel dimension of 1000 μ m, defi ned by a shadow mask, is utilized. The nanostructure is developed using a low-cost, lithography-free process to produce abundant nanometer-scale dot-like doping in a-IGZO channel. The new method, called nanodot doping (NDD) increases the effective electron mobility to a level 19 times higher than that of the control and the intrinsic electron mobility is also 10 times higher than that of the control. This study demo nstrates a process utilizing self-organized polystyrene spheres with a diameter of 200 nm to fabricate a porous gate structure. Ar plasma treatment through the porous gate performs dot-like doping on a-IGZO channel region. A top-gate

Polymer space-charge-limited transistor as a solid-state vacuum tube triode

We report the construction of a polymer space-charge-limited transistor (SCLT), a solid-state version of vacuum tube triode. The SCLT achieves a high on/off ratio of 3×105 at a low operation voltage of 1.5 V by using high quality insulators both above and below the grid base electrode. Applying a greater bias to the base increases the barrier potential, and turns off the channel current, without introducing a large parasitic leakage current. Simulation result verifies the influence of base bias on channel potential distribution. The output current density is 1.7 mA/cm2 with current gain greater than 1000.

Increasing organic vertical carrier mobility for the application of high speed bilayered organic photodetector

The direct influence of the vertical carrier mobility on the frequency response of bilayered organic photodiodes (PDs) is investigated for the first time. With fullerene as the acceptor material, changing vertical hole mobility from 2.3×10−5 to 2.8×10−4 cm2/V s increases PD bandwidth from 10 to 80 MHz under a 4 V operation. The influence of deposition rate on vertical hole mobility of pentacene film is also discussed. Our results facilitate the application of bilayered organic PDs on the detection of very-high-frequency optical signals.

High output current in vertical polymer space-charge-limited transistor induced by self-assembled monolayer

We present a promising solution-processed vertical transistor which exhibits high output current, high on/off current ratio, and low operation voltage. Numerous poly(3-hexylthiophene) vertical channels are embedded in vertical nanometer pores. Treating the sidewalls of pores by self-assemble monolayer with long alkyl chains enhances the pore-filling and inter-chain order of poly(3-hexylthiophene). The channel current is therefore greatly increased. A grid metal inside the porous template controls the channel potential profile to turn on and turn off the vertical transistor. Finally, the transistor delivers an output current density as 50–110 mA/cm2 at 2 V with an on/off current ratio larger than 10 000.

Impacts of ammonia background flows on structural and photoluminescence properties of InN dots grown on GaN by flow-rate modulation epitaxy

Structural and photoluminescence (PL) properties of InN dots grown on GaN by metal organic vapor phase epitaxy using the flow-rate modulation technique, and their dependence on growth conditions, were investigated. An ammonia (NH3) background flow was intentionally supplied during indium deposition periods to control the kinetics of adatoms and hence the morphology of InN dots. Samples prepared under lower NH3 background flows generally exhibit narrower and more intense PL signals peaked at lower emission energies. The authors point out that the NH3 background flow is an important parameter that controls not only the nucleation process but also the emission property of InN dots.

Boron incorporation in Si1−xGex films grown by ultrahigh vacuum chemical vapor deposition using Si2H6 and GeH4

0.1% B2H6 diluted in hydrogen is used as the p‐type dopant gas in Si1−xGex grown by ultrahigh vacuum chemical vapor deposition (UHVCVD) using Si2H6 and GeH4. The boron concentration is evaluated by secondary ion mass spectrometry (SIMS). The boron concentration of Si1−xGex increases with the increase of the GeH4 flow rate, that is, Ge fraction, by keeping Si2H6 and B2H6 flow rates constant. The result may be due to the increase of the vacant surface sites which is caused by the increase of the hydrogen desorption rate when a higher Ge fraction epilayer is grown.

Vertical polymer phototransistor featuring photomultiplication due to base-field shielding

We introduce a vertical polymer phototransistor with low operational voltage (−1.5 V). A blended polymer layer with both acceptor and donor materials was used as a channel material in the vertical space-charge-limited transistor. Under illumination, we obtained external quantum efficiency (EQE) as high as 360% at 620 nm. We propose the effects of base-field shielding as a means to explain high EQE. This proposition has been supported by two-dimensional simulation of the device.

Solution-Processed Vertical Organic Transistors Fabricated by Nanoimprint Lithography

In this letter, we demonstrate the first vertical-channel organic transistor using nanoimprint technology to produce a base electrode with high-density nanometer pores to well control the channel current vertically flowing through the pores. The aspect ratios of nanopores, which determine the switching performance of the vertical transistor, are greatly enhanced by transferring the nanostructure to the underlying layers. Without pore accumulation, a low leakage current can be achieved. The vertical transistor delivers an on current of 0.35 mA/cm2 and an on/off current ratio of around 3000 at 1.8 V. The results prove the feasibility to produce low-voltage organic transistors over a large area with potentially low production cost.

P‐25: New Polymer‐Capped a‐IGZO TFT with High Sensitivity to Visible Light for the Development of Integrated Touch Sensor Array

We deposit a polymer layer (P3HT) on top of a-IGZO thin-film transistor (TFT) to absorbed visible light. Under white light, photoresponsivity of P3HT-capped device is about 30 times of that of standard device. These results are important for the development ofin-cell integrated touch sensor in a -IGZO TFT array.