Frederik Ante

Contact Resistance and Megahertz Operation of Aggressively Scaled Organic Transistors

Bottom-gate, top-contact organic thin-film transistors (TFTs) with excellent static characteristics (on/off ratio: 10(7) ; intrinsic mobility: 3 cm(2) (V s)(-1) ) and fast unipolar ring oscillators (signal delay as short as 230 ns per stage) are fabricated. The significant contribution of the transfer length to the relation between channel length, contact length, contact resistance, effective mobility, and cutoff frequency of the TFTs is theoretically and experimentally analyzed.

Mixed self-assembled monolayer gate dielectrics for continuous threshold voltage control in organic transistors and circuits.

Mixed alkyl/fluoroalkyl phosphonic acid self-assembled monolayers have been prepared as ultra-thin dielectrics in low-voltage organic thin-film transistors and complementary circuits. Mixed monolayers enable continuous threshold-voltage tuning simply by adjusting the molecular mixing ratio Continuous threshold-voltage control makes it possible to place the switching voltage of the circuits at precisely half the supply voltage, producing the maximum noise margin.

Contact Doping and Ultrathin Gate Dielectrics for Nanoscale Organic Thin-Film Transistors

Organic thin-fi lm transistors (TFTs) are of interest for electronic applications on fl exible plastic substrates, such as rollable or foldable active-matrix displays, [ 1 ] conformable sensor arrays, [ 2 ] and fl exible identifi cation tags. [ 3 ] Due to the relatively small intrinsic fi eld-effect mobility in most conjugated organic semiconductors ( 10 MHz) are highly desirable. Such high frequencies are indeed feasible, provided the lateral dimensions of the organic TFTs are suffi ciently small (about 100 nm). However, TFTs with such small lateral dimensions will suffer from a variety of detrimental short-channel effects, unless a number of important scaling requirements are observed in the design and fabrication of the transistors. Here we report on the successful fabrication and detailed analysis of organic TFTs with channel lengths and gate overlaps of about 100 nm in which the short-channel effects are greatly suppressed by area-selective contact doping (using a strong organic dopant) and by aggressive gate-dielectric scaling (using a 5.7 nm-thick, low-temperature-processed gate insulator based on a molecular self-assembled monolayer). As a result, these nanoscale organic TFTs have off-state drain currents below 1 pA, on/ off current ratios near 10 7 , as well as clean linear and saturation characteristics. The transconductance of these transistors reaches 0.4 S m − 1 , which is the largest transconductance reported for organic TFTs with patterned gate electrodes. The gate electrodes and source/drain contacts of organic TFTs are usually defi ned by photolithography, [ 1 , 3 , 4 ]

Top-Gate ZnO Nanowire Transistors and Integrated Circuits with Ultrathin Self-Assembled Monolayer Gate Dielectric

A novel approach for the fabrication of transistors and circuits based on individual single-crystalline ZnO nanowires synthesized by a low-temperature hydrothermal method is reported. The gate dielectric of these transistors is a self-assembled monolayer that has a thickness of 2 nm and efficiently isolates the ZnO nanowire from the top-gate electrodes. Inverters fabricated on a single ZnO nanowire operate with frequencies up to 1 MHz. Compared with metal-semiconductor field-effect transistors, in which the isolation of the gate electrode from the carrier channel relies solely on the depletion layer in the semiconductor, the self-assembled monolayer dielectric leads to a reduction of the gate current by more than 3 orders of magnitude.

A 3.3 V 6-Bit 100 kS/s Current-Steering Digital-to-Analog Converter Using Organic P-Type Thin-Film Transistors on Glass

A 3.3 V 6-bit binary-weighted current-steering digital-to-converter converter (DAC) using low-voltage organic p-type thin-film transistors (OTFTs) is presented. The converter marks records in speed and compactness owing to an OTFT fabrication process that is based on high-resolution silicon stencil masks. The chip has been fabricated on a glass substrate and consumes an area of 2.6× 4.6 mm2. The converter has a maximum update rate of 100 kS/s and a maximum output voltage swing of 2 V. The measured DNL and INL at an update rate of 1 kS/s are - 0.69 and 1.16 LSB, respectively. A spurious-free dynamic range (SFDR) of 32 dB has been measured for output sinusoids at 31 Hz (update rate of 1 kS/s) and 3.1 kHz (update rate of 100 kS/s).

Submicron low-voltage organic transistors and circuits enabled by high-resolution silicon stencil masks

Using high-resolution silicon stencil masks and employing a resist-free, solvent-free, low-temperature (90 °C) fabrication process we have fabricated low-voltage top-contact organic thin-film transistors (TFTs) with a channel length down to 0.8 µm. To observe the scaling requirements and to alleviate short-channel effects, a thin gate dielectric with a thickness of 5.3 nm is employed. The p-channel TFTs have a record static performance, with a transconductance of 1 S/m, an on/off current ratio of 108, and a subthreshold swing of 100 mV/dec. Unipolar inverters based on p-channel TFTs with a channel length of 1 µm and gate-to-source and gate-to-drain overlaps of 2 µm respond to input signals with frequencies up to 1 MHz. Combining air-stable p-channel and n-channel TFTs we have also realized organic complementary ring oscillators with record low-voltage dynamic performance (signal delay of 30 µs per stage at a supply voltage of 3 V).

Logic circuits based on individual semiconducting and metallic carbon-nanotube devices

Nanoscale transistors employing an individual semiconducting carbon nanotube as the channel hold great potential for logic circuits with large integration densities that can be manufactured on glass or plastic substrates. Carbon nanotubes are usually produced as a mixture of semiconducting and metallic nanotubes. Since only semiconducting nanotubes yield transistors, the metallic nanotubes are typically not utilized. However, integrated circuits often require not only transistors, but also resistive load devices. Here we show that many of the metallic carbon nanotubes that are deposited on the substrate along with the semiconducting nanotubes can be conveniently utilized as load resistors with favorable characteristics for the design of integrated circuits. We also demonstrate the fabrication of arrays of transistors and resistors, each based on an individual semiconducting or metallic carbon nanotube, and their integration on glass substrates into logic circuits with switching frequencies of up to 500 kHz using a custom-designed metal interconnect layer.

A 3.3V 6b 100kS/s current-steering D/A converter using organic thin-film transistors on glass

Organic thin-film transistors (OTFT) processed at low-temperatures offer prospects for a vast number of integrated circuit applications in mechanically flexible, inexpensive, large-area and biomedical electronics [1]. In addition, the low-voltage operation capability of recent OTFTs makes them well-suited for battery-powered or radio frequency-coupled portable devices [2]. In such applications, data conversion to interface the digital processors with the analog world is an essential necessity. Here, we demonstrate a compact 6b current-steering D/A converter (DAC) circuit, built in OTFT technology, which is 1000× faster and 30× smaller than the previously published data for a 6b DAC [3]. These considerable improvements result from an OTFT fabrication process based on silicon stencil masks that provide submicron channel length capability and excellent transistor matching [4], [5].

Top-contact organic transistors and complementary circuits fabricated using high-resolution silicon stencil masks

The maximum operating frequency of a field-effect transistor is inversely proportional to its lateral dimensions. Organic thin-film transistors (TFTs) with dimensions of ∼1 µm or less have been fabricated by photolithography [1], electron-beam lithography (EBL) [2], nano-imprint lithography (NIL) [3], sub-femtoliter inkjet printing (SIJ) [4] and self-aligned inkjet printing (SAP) [5]. Some of these methods (EBL, SIJ, SAP) have small throughput, others (EBL, NIL, photolithography) involve solvents or high process temperatures. Since high-mobility small-molecule organic semiconductors often undergo phase transitions when exposed to solvents or heat [6,7], these methods are in general not suitable to pattern source and drain contacts on top of such semiconductors. As an alternative, high-resolution stencil masks offer the possibility to pattern top contacts with high throughput and without the need for solvents or elevated temperatures. For example, Jin et al. reported top-contact pentacene TFTs with a channel length of 1.8 µm fabricated by using a global silicon back gate and a high-resolution silicon nitride stencil mask [8]. For devices with short channel lengths, top-contact organic TFTs usually provide better performance than bottom-contact TFTs [9].

Deterministic and continuous control of the threshold voltage and noise margin of organic thin-film transistors and organic complementary circuits using mixed phosphonic acid self-assembled monolayer gate dielectrics

For many applications of organic thin-film transistors (TFTs), high-capacitance gate dielectrics that can be processed at low temperature are of interest. Several approaches exist, including vapor-deposited metal oxides, ultra-thin polymers, self-assembled nanodielectrics, and thin hybrid dielectrics based on alkyl phosphonic acid self-assembled monolayers (SAMs) on plasma-oxidized aluminum gates. None of these approaches, however, provides deterministic and continuous control of the threshold voltage. A few reports of low-voltage (¿5 V) organic complementary circuits with symmetric switching threshold and large noise margins exist, but the TFTs were not air-stable and had to be operated in an inert gas, which is not practical for real applications. Here we show how the threshold voltage of air-stable, low-voltage organic TFTs and the switching threshold and noise margin of air-stable, low-voltage, low-power organic complementary circuits can be reproducibly tuned over a wide range by using a mixed alkyl/fluoroalkyl phosphonic acid self-assembled monolayer (SAM) as a high-capacitance gate dielectric.