Monolayer Gate Research Articles

Solution Processed Self-Assembled Monolayer Gate Dielectrics for Low-Voltage Organic Transistors

AbstractLow-voltage organic transistors are sought for implementation in high volume low-power portable electronics of the future. Here we assess the suitability of three phosphonic acid based self-assembling molecules for use as ultra-thin gate dielectrics in low-voltage solution processable organic field-effect transistors. In particular, monolayers of phosphonohexadecanoic acid in metal-monolayer-metal type sandwich devices are shown to exhibit low leakage currents and high geometrical capacitance comparable to previously demonstrated self-assembled monolayer (SAM) type dielectrics but with a higher surface energy. The improved surface energy characteristics enable processing of a wider range of organic semiconductors from solution. Transistors based on a number of solution-processed organic semiconductors with operating voltages below 2 V are also demonstrated.

Ultralow-power organic complementary circuits

The prospect of using low-temperature processable organic semiconductors to implement transistors, circuits, displays and sensors on arbitrary substrates, such as glass or plastics, offers enormous potential for a wide range of electronic products. Of particular interest are portable devices that can be powered by small batteries or by near-field radio-frequency coupling. The main problem with existing approaches is the large power consumption of conventional organic circuits, which makes battery-powered applications problematic, if not impossible. Here we demonstrate an organic circuit with very low power consumption that uses a self-assembled monolayer gate dielectric and two different air-stable molecular semiconductors (pentacene and hexadecafluorocopperphthalocyanine, F16CuPc). The monolayer dielectric is grown on patterned metal gates at room temperature and is optimized to provide a large gate capacitance and low gate leakage currents. By combining low-voltage p-channel and n-channel organic thin-film transistors in a complementary circuit design, the static currents are reduced to below 100 pA per logic gate. We have fabricated complementary inverters, NAND gates, and ring oscillators that operate with supply voltages between 1.5 and 3 V and have a static power consumption of less than 1 nW per logic gate. These organic circuits are thus well suited for battery-powered systems such as portable display devices and large-surface sensor networks as well as for radio-frequency identification tags with extended operating range.

High-Performance Carbon Nanotube Field Effect Transistors with a Thin Gate Dielectric Based on a Self-Assembled Monolayer

Individual single-walled carbon nanotube (SWCNT) field effect transistors (FETs) with a 2 nm thick silane-based organic self-assembled monolayer (SAM) gate dielectric have been manufactured. The FETs exhibit a unique combination of excellent device performance parameters. In particular, they operate with a gate-source voltage of only -1 V and exhibit good saturation, large transconductance, and small hysteresis (<or=100 mV), as well as a very low subthreshold swing (60 mV/dec) under ambient conditions. The SAM-based gate dielectric opens the possibility of fabricating transistors operating at low voltages and constitutes a major step toward nanotube-based flexible electronics.

Diamond power devices. Concepts and limits

The prospects and limits of diamond power devices incorporating boron as dopant are discussed using the results of a theoretical/empirical analysis coupled to 2D-numerical simulation. To obtain full activation of the boron acceptor at room temperature, the concept of delta-doping is introduced and used as basic element. Two device concepts were chosen for this analysis: (i) RF power FET incorporating a monolayer boron delta-doped channel and gate recess and gate filed plate structure; and (ii) various types of vertical power diodes including a novel stacked delta-doped structure, which is proposed here for the first time. The analysis was performed considering the limiting material parameters known for single crystal diamond including recent data on the carrier mobility, the phenomena related to thermal activation of boron acceptor and carrier scattering along and across delta-doped layers, thermal and electrical breakdown limits. The results of this study reveal that the optimised delta-FET with the field plate of 1 μm may be capable to produce approximately 34 W/mm continues-wave RF power at 20 °C operation temperature. The projected RF power of a similar FET structure with the field plate of 2 μm is about 75 W/mm, which may be beyond the thermal limit. The proposed stacked delta-doped diode could have the best on-resistance to blocking voltage ratio than any other type of diamond and n-type SiC-based diodes up to 20 kV blocking voltage. Below 3 kV the projected characteristics of the stacked delta diode on diamond are also favourable in comparison to those of an ideal n-type GaN diode. These examples demonstrate that boron delta-doped layer is a very promising configuration for various diamond-based power electronic devices.

Low-voltage organic transistors with an amorphous molecular gate dielectric

Organic thin film transistors (TFTs) are of interest for a variety of large-area electronic applications, such as displays, sensors and electronic barcodes. One of the key problems with existing organic TFTs is their large operating voltage, which often exceeds 20 V. This is due to poor capacitive coupling through relatively thick gate dielectric layers: these dielectrics are usually either inorganic oxides or nitrides, or insulating polymers, and are often thicker than 100 nm to minimize gate leakage currents. Here we demonstrate a manufacturing process for TFTs with a 2.5-nm-thick molecular self-assembled monolayer (SAM) gate dielectric and a high-mobility organic semiconductor (pentacene). These TFTs operate with supply voltages of less than 2 V, yet have gate currents that are lower than those of advanced silicon field-effect transistors with SiO2 dielectrics. These results should therefore increase the prospects of using organic TFTs in low-power applications (such as portable devices). Moreover, molecular SAMs may even be of interest for advanced silicon transistors where the continued reduction in dielectric thickness leads to ever greater gate leakage and power dissipation.

Oligothiophene Organic Thin Film Transistors and Circuits

AbstractWe have investigated a series of α,α'-oligothiophenes with chromophore length ranging from four to six units, and with side chains comprised of ten, six, two, and zero alkyl units. We have fabricated top-contact and bottom-contact organic TFTs with thermally evaporated active layers and found that the TFT performance depends critically on the length of the side chains and on the contact configuration (with mobilities ranging from 0.07 to 1.1 cm2 / V-s), but is relatively insensitive to the chromophore length. We have also fabricated ring oscillators with didecylsexithiophene and measured a signal propagation delay of 30 μsec per stage. In addition, we have fabricated oligothiophene-TFTs with ultra-thin self-assembled monolayer (SAM) gate dielectrics, with the intent to evaluate if long alkyl side chains contribute usefully to the effective thickness of a SAM gate dielectric. We have measured carrier mobilities as large as 0.05 cm2 / V-s and subthreshold swing as low as 200 mV / decade.

PH-Sensitive phosphate lipid self-assembled monolayers on gold

Self-assembled monolayers (SAMs) of a phosphate lipid, bis(11-mercaptoundecyl)phosphoric acid, were formed on gold electrodes via chemisorption from ethanol solutions by altering the immersion time. The electrochemistry of the lipid monolayers on gold electrodes was examined by using methylviologen and dipropanesulphonate-4,4′-bipyridinium as a cationic and a zwitterionic redox-active marker, respectively. SAMs of n-dodecylmercaptan, 11-mercaptoundecane-1-ol and 11-mercaptoundecane-1-oic acid on gold electrodes which were fabricated by 24 h adsorption from ethanolic solutions of each thiol compound, could not block the redox reaction of methylviologen. However, SAMs of the phosphoric acid lipid on gold which were prepared by adsorption over 6 h exhibited pH-responsive ion-gating phenomena; at lower pHs (<pH 4), penetration of both methylviologen and its reduced form was almost inhibited (gate closed) and at higher pH, incorporation of the redox compound into the SAMs was possible (gate open). By using SAMs of the lipid fabricated by 40 min adsorption from ethanolic solution, similar gating behavior was observed for the oxidation reaction of the reduced form of 1,1′-dipropanesulphonate-4,4′-bipyridinium. In this case, the monolayer gate was closed at higher pH and opened at lower pH. The electrochemical response of SAMs of the lipid using methylviologen as a marker toward Ca 2+ ion is also described.