S. Steudel

Low-voltage gallium–indium–zinc–oxide thin film transistors based logic circuits on thin plastic foil: Building blocks for radio frequency identification application

In this work a technology to fabricate low-voltage amorphous gallium-indium-zinc oxide thin film transistors (TFTs) based integrated circuits on 25 µm foils is presented. High performance TFTs were fabricated at low processing temperatures (<150 °C) with field effect mobility around 17 cm2 /V s. The technology is demonstrated with circuit building blocks relevant for radio frequency identification applications such as high-frequency functional code generators and efficient rectifiers. The integration level is about 300 transistors.

A comprehensive model for bipolar electrical switching of CuTCNQ memories

The generally observed bipolar electrical switching of Cu\CuTCNQ\metal memories (metal=Al,Yb,Ti) between two stable resistance states is shown to occur at the CuTCNQ\metal interface and not in the bulk of CuTCNQ. The switching is explained by a model involving electrochemical formation and dissolution of Cu filaments at the interface. In this mechanism, CuTCNQ acts as solid ionic conductor and source for the Cu+ cations. The model also explains earlier reported findings of bipolar switching in CuTCNQ devices, including the apparently contradictory observation that neutral TCNQ appears in the low-resistance state.

An Inductively-Coupled 64b Organic RFID Tag Operating at 13.56MHz with a Data Rate of 787b/s

RFID systems operating at a base carrier frequency of 13.56 MHz can use low-cost inductive antennas on foil. In parallel to this coil, a capacitor on foil is used for matching the resonance frequency at 13.56 MHz. This LC-antenna detects the signal transmitted by the reader and energizes the organic rectifier with an AC-voltage at 13.56 MHz. From this voltage the rectifier generates the DC supply voltage for the 64 b organic transponder chip, which drives the modulation transistor between the on and off state with a 64b code sequence.

Bias stress in pentacene transistors measured by four probe transistor structures

This paper deals with operational lifetime measurements of organic field-effect transistors. The organic semiconductor under study is pentacene. We apply DC stress conditions on these structures, and monitor the output characteristics of the TFTs during stress as well as during recovery after stress. The transistor structures have been modified to incorporate two voltage-measurement probes in the channel in addition to the source and drain contacts. This results in a 4-probe configuration, that allows us to measure the voltage drop in the intrinsic transistor channel separately from the voltage drop over the source and the drain contact regions. This phenomenological study is a first step towards a comprehensive model for degradation of bias stress in organic field-effect transistors.

A 2V Organic Complementary Inverter

A complementary organic thin-film transistor technology uses pentacene and F16CuPc as the p-type and n-type materials, respectively. The semiconductors are patterned by vacuum deposition through an integrated shadow mask, while tilting the substrate. Organic complementary inverters are realized that display an almost ideal inverter curve at a supply voltage of 2V, showing a gain of 14 and a noise margin of 0.65V

Pulsed Excitation of OLEDs With a Remote Metallic Cathode

In this paper, we report on the behavior of organic LEDs (OLEDs) with a remote metallic cathode under pulsed excitation. Devices comprising poly(triarylamine) as the hole-transporting layer, Alq3:DCM2 as the light-emitting layer, and PTCDI-C13H27 as the electron-transporting layer are analyzed and their performance is compared to conventional OLED device structures comprising the same organic materials. We demonstrate that the dependence of the light intensity on the applied pulsewidth is mainly determined by charge injection into the active light-emitting layer and show that pulses down to 1 ¿s can be applied to the device without affecting the light intensity. This latter observation suggests that pulsed excitation may be able to reduce the accumulation of triplets in the device. In this way, triplet-state losses, which are major loss mechanisms with respect to the high photon density required for an electrically pumped organic laser, will be suppressed.

Off-current reduction in p-type SnO thin film transistors

SnO is one of the few candidates for p-type oxide thin film transistors (TFTs) because it retains a reasonable high hole mobility in a nanocrystalline film. However, the high off-current of SnO TFT limits its usefulness. In this work, SnO TFTs were fabricated using thermal evaporation under ultra-high vacuum. In order to decrease the off-current in p-type SnO thin film transistors (TFTs), we used yttrium to reduce n-type minority charges in the channel. The on/off ratio of the TFT increases from 102 to 5 × 104 and the mobility of the TFT in the saturated regime reduces from 1.6 to 1.4 cm2/V s when the SnO channel is doped with 1 wt. % of Y. Grazing incidence X-ray diffraction and X-ray photoelectron spectroscopy reveal that the reduction of SnO2 in the Y-doped SnO TFT channel is the main reason for the improvement in the TFT characteristics.SnO is one of the few candidates for p-type oxide thin film transistors (TFTs) because it retains a reasonable high hole mobility in a nanocrystalline film. However, the high off-current of SnO TFT limits its usefulness. In this work, SnO TFTs were fabricated using thermal evaporation under ultra-high vacuum. In order to decrease the off-current in p-type SnO thin film transistors (TFTs), we used yttrium to reduce n-type minority charges in the channel. The on/off ratio of the TFT increases from 102 to 5 × 104 and the mobility of the TFT in the saturated regime reduces from 1.6 to 1.4 cm2/V s when the SnO channel is doped with 1 wt. % of Y. Grazing incidence X-ray diffraction and X-ray photoelectron spectroscopy reveal that the reduction of SnO2 in the Y-doped SnO TFT channel is the main reason for the improvement in the TFT characteristics.

Solving the technology barriers in flexible AMOLED displays

In this paper, we present some of the technology challenges and process temperature trade-offs when realizing AM OLED displays on thin flexible plastic films that can be mechanically bent to a roll radius of ~1 cm. We furthermore present complementary approaches to realize low-power, high resolution OLED displays using self-aligned IGZO TFT architecture; a novel driving method using a compact 2T-1C pixel engine.

Organic transistor technology options for device performance versus technology options for increased circuit reliability and yield on foil

Last year, advances in organic device technology (such as device scaling, high K-dielectrics [1], … ) enabled a substantial progress in performance. This resulted in an increase in the data rate of plastic transponder circuits from about 2 kbit/s [2] to EPC-compatible speeds (50 kbit/s). The organic semiconductor pentacene deposited from solution was replaced by a better-performing vapor-phase deposited pentacene, with a mobility being a factor of 3 higher. The isolation of pentacene transistors in this new process is achieved by an integrated shadow mask, shown in Fig. 1, that results in a reliable isolation of the semiconductor area testified by off-currents below 10pA. We replaced the organic polymer (low-k) dielectric with a 100-nm thick high-k dielectric, sputtered Al2O3, resulting in an 8-fold higher specific accumulation capacitance. That, in turn, allowed for downscaling the transistor channel length from 5µm to 2µm, while maintaining a high output resistance in saturation - and therefore also high inverter gain and noise margins. A cross-section of this process flow is depicted in Fig. 1. Fig. 2 shows a micrograph image of a 5-stage ring oscillator realized in this technology on foil optimized for speed. This organic thin-film circuit technology allows to design with lower overlap capacitance and to downscale the transistor channel length, within the boundaries achievable by existing high-throughput manufacturing tools (e.g. steppers used in backplane manufacturing). In this work, we varied the channel lengths (L) of the TFTs in the circuits between 20µm and 2µm and limited the gate-source and gate-drain overlap capacitances by decreasing the width of the finger-shaped source and drain contacts, that fully overlap the gate, from 5µm to 2µm. Fig. 2 shows a micrograph image of one transistor. Typical transfer curves of transistors fabricated in this technology, having L = 5µm and 2µm, are depicted in Fig. 3 and Fig. 4 respectively. The transistors are normally-on and their charge carrier (hole) mobility exceeds 0.5cm2/Vs. Fig. 5 shows typical transfer curves of a zerovgs-load inverters with channel lengths of 5µm and 2µm. The ratio between drive and load transistor is 10∶1. The inverters have high gains and noise margins. The stage delay (τD) of inverters in this technology is plotted as a function of the supply voltage in Fig. 6. The stage delay is determined from 19-stage ring oscillators. The figure shows τD for inverters with channel lengths from 20µm to 2µm and gate-overlap of the transistor-fingers ranging from 5µm to 2µm. Stage delays below 1µs, and as low as 400ns, are shown at VDD=10V. To our knowledge, no plastic technology was shown before with such speeds at these low power voltages. The effect of decreasing the overlap capacitance is also shown for the two smaller channel lengths: shrinking the overlap from 5µm to 2µm improves the stage delay by a factor of 1.5 to 2. We proceeded with the design and realization of 8bit organic RFID transponder chips, having a channel length of 2µm and either 5µm or 2µm finger widths. Figs. 7 and 8 show the photographs of the 6″ wafer and a zoom of the die, Fig. 9 shows the corresponding schematic. Fig. 11 depicts the output signal of both types of transponders. In agreement with the two-fold faster inverter stage delay for the 2µm fingers, the data rate of this transponder is also twice as high as that of the design with 5µm fingers. The obtained data rate of the 8bit transponder with channel length and fingers of 2µm is a record 50kb/s.

Organic circuit components for pentacene RF-ID tags

Summary form only given. RF-ID tags with organic semiconductors require the development of several types of devices and components. In particular, apart from logic gates, a tag also requires memory and power management. In a passive tag, that draws energy from the RF field of the reader coil, the power management includes a rectifier to generate the DC supply voltage for the circuit. In this presentation, we discuss our progress in logic, memory and rectifiers using organic semiconductors. For logic and rectifying diodes, we employ evaporated pentacene. Memory devices are based the cross-point concept, with impedance switching of the charge-transfer complex material CuTCNQ.