Reinhold Rödel

Contact properties of high-mobility, air-stable, low-voltage organic n-channel thin-film transistors based on a naphthalene tetracarboxylic diimide

Air-stable bottom-gate, top-contact n-channel organic transistors based on a naphthalene diimide exhibiting electron mobilities up to 0.8 cm2/Vs at low voltages were fabricated. Transistors with channel lengths of 1 μm show a transconductance of 60 mS/m, but are significantly limited by the contact resistance. Transmission line measurements in combination with contact resistance models were applied to investigate this influence. Both contact resistance and contact resistivity are proportional to the inverse gate overdrive voltage. Organic complementary ring oscillators were fabricated on a flexible plastic substrate showing record signal delays down to 17 μs at a supply voltage of 2.6 V.

S-Parameter Characterization of Submicrometer Low-Voltage Organic Thin-Film Transistors

This letter presents the first comprehensive experimental studies on the frequency response of staggered low-voltage organic thin-film transistors (OTFTs) using S-parameter measurements. The transistors utilize air-stable dinaphtho-thieno-thiophene as the organic semiconductor with various channel lengths and gate overlaps. A peak cutoff frequency of 3.7 MHz for a channel length of 0.6 μm, gate overlap of 5 μm, and a supply voltage of 3 V is achieved. In view of the low supply voltage and air-stability, this result marks a record achievement in OTFT technology. The channel length dependence of the cutoff frequency is described in a compact model and a close correspondence to the measured data of OTFTs with variable device dimensions is shown. Moreover, the cutoff frequencies at different gate biases are found to be proportional to the dc transconductance.

Accurate Capacitance Modeling and Characterization of Organic Thin-Film Transistors

This paper presents analysis of the charge storage behavior in organic thin-film transistors (OTFTs) by means of admittance characterization, compact modeling, and 2-D device simulation. The measurements are performed for frequencies ranging from 100 Hz to 1 MHz and bias potentials from zero to -3 V on top-contact OTFTs that employ air-stable and high-mobility dinaphtho-thieno-thiophene as the organic semiconductor. It is demonstrated that the dependence of the intrinsic OTFT gate-source and gate-drain capacitances on the applied voltages agrees very well with Meyers capacitance model. Furthermore, the impact of parasitic elements, including fringe current and contact impedance, is investigated. The parameters used for the simulation and modeling of all the dynamic characteristics correspond closely to those extracted from static measurements. Finally, the implications of the admittance measurements are also discussed relating to the OTFTs dynamic performance, particularly the cutoff frequency and the charge response time.

A comprehensive study of charge trapping in organic field-effect devices with promising semiconductors and different contact metals by displacement current measurements

A systematic and comprehensive study on the charge-carrier injection and trapping behavior was performed using displacement current measurements in long-channel capacitors based on four promising small-molecule organic semiconductors (pentacene, DNTT, C10-DNTT and DPh-DNTT). In thin-film transistors, these semiconductors showed charge-carrier mobilities ranging from 1.0 to 7.8 cm2 V−1 s−1. The number of charges injected into and extracted from the semiconductor and the density of charges trapped in the device during each measurement were calculated from the displacement current characteristics and it was found that the density of trapped charges is very similar in all devices and of the order 1012 cm−2, despite the fact that the four semiconductors show significantly different charge-carrier mobilities. The choice of the contact metal (Au, Ag, Cu, Pd) was also found to have no significant effect on the trapping behavior.

High voltage surface potential measurements in ambient conditions: Application to organic thin-film transistor injection and transport characterization

A scanning surface potential measurement technique suited for thin-film devices operating under high voltages is reported. A commercial atomic force microscope has been customized to enable a feedback-controlled and secure surface potential measurement based on phase-shift detection under ambient conditions. Measurements of the local potential profile along the channel of bottom-gate organic thin-film transistors (TFTs) are shown to be useful to disentangle the contributions from the channel and contacts to the device performance. Intrinsic contact current-voltage characteristics have been measured on bottom-gate, top-contact (staggered) TFTs based on the small-molecule semiconductor dinaphtho[2,3-b:2′,3-f]thieno[3,2-b]thiophene (DNTT) and on bottom-gate, bottom-contact (coplanar) TFTs based on the semiconducting polymer polytriarylamine (PTAA). Injection has been found to be linear in the staggered DNTT TFTs and nonlinear in the coplanar PTAA TFTs. In both types of TFT, the injection efficiency has been ...

Submicron-channel-length organic thin-film transistors on flexible substrates

A process for the fabrication of organic thin-film transistors (TFTs) with channel lengths as short as 0.5 μm on flexible plastic substrates has been developed. The TFTs are fabricated in the bottom-gate, top-contact (inverted staggered) architecture and employ vacuum-deposited small-molecule organic semiconductors and a low-temperature-processed gate dielectric that is sufficiently thin to allow the TFTs to operate with voltages of about 2 to 3 V. For a channel length of 0.5 μm, the TFTs have effective field-effect mobilities of 0.08 cm2/Vs (p-channel TFTs) and 0.03 cm2/Vs (n-channel TFTs), on/off current ratios of at least 106, and subthreshold swings of about 140 mV/decade. For 11-stage complementary and unipolar ring oscillators based on TFTs with a channel length of 1 μm, signal propagation delays per stage as short as 6.6 μs and 420 ns have been measured at a supply voltage of 3 V.

Low-voltage organic transistors for flexible electronics

A process for the fabrication of bottom-gate, top-contact (inverted staggered) organic thin-film transistors (TFTs) with channel lengths as short as 1 μm on flexible plastic substrates has been developed. The TFTs employ vacuum-deposited small-molecule semiconductors and a low-temperature-processed gate dielectric that is sufficiently thin to allow the TFTs to operate with voltages of about 3 V. The p-channel TFTs have an effective field-effect mobility of about 1 cm2/Vs, an on/off ratio of 107, and a signal propagation delay (measured in 11-stage ring oscillators) of 300 ns per stage. For the n-channel TFTs, an effective field-effect mobility of about 0.06 cm2/Vs, an on/off ratio of 106, and a signal propagation delay of 17 μs per stage have been obtained.

Low-voltage organic field-effect transistors for flexible electronics

A process for the fabrication of bottom-gate, top-contact (inverted staggered) organic thin-film transistors (TFTs) with channel lengths as short as 1 μm on flexible plastic substrates has been developed. The TFTs employ vacuum-deposited small-molecule semiconductors and a low-temperature-processed gate dielectric that is sufficiently thin to allow the TFTs to operate with voltages of about 3 V. The p-channel TFTs have an effective field-effect mobility of about 1 cm2/Vs, an on/off ratio of 107, and a signal propagation delay (measured in 11-stage ring oscillators) of 300 ns per stage. For the n-channel TFTs, an effective field-effect mobility of about 0.06 cm2/Vs, an on/off ratio of 106, and a signal propagation delay of 17 μs per stage have been obtained.