Unified analytic model for current-voltage behavior in amorphous oxide semiconductor TFTs

We present a simple and semi-physical analytical description of the current-voltage characteristics of amorphous oxide semiconductor thin-film transistors in the above-threshold and sub-threshold regions. Both regions are described by single unified expression that employs the same set of model parameter values directly extracted from measured terminal characteristics. The model accurately reproduces measured characteristics of amorphous semiconductor thin film transistors in general, yielding a scatter of < 4%. © 1980-2012 IEEE.

Amorphous oxide semiconductor TFTs for displays and imaging

© 2013 IEEE. This paper reviews the mechanisms underlying visible light detection based on phototransistors fabricated using amorphous oxide semiconductor technology. Although this family of materials is perceived to be optically transparent, the presence of oxygen deficiency defects, such as vacancies, located at subgap states, and their ionization under illumination, gives rise to absorption of blue and green photons. At higher energies, we have the usual band-to-band absorption. In particular, the oxygen defects remain ionized even after illumination ceases, leading to persistent photoconductivity, which can limit the frame-rate of active matrix imaging arrays. However, the persistence in photoconductivity can be overcome through deployment of a gate pulsing scheme enabling realistic frame rates for advanced applications such as sensor-embedded display for touch-free interaction.

Modeling current-voltage behaviour in oxide TFTs combining trap-limited conduction with percolation

Current-voltage behaviour of oxide TFTs is modeled based on trap-limited conduction and percolation theories. The mobility has a power-law dependence, in which percolation controls the exponent while trap states determine constant term in the power law. The proposed model, which is fully physically-based, provides a good agreement with measured transistor characteristics as well as transient operations of fabricated pixel test circuits for oxide-based OLED displays. © 2013 Society for Information Display.

Optical and Carrier Transport Properties of Cosputtered Zn–In–Sn–O Films and Their Applications to TFTs

The optical and carrier transport properties of amorphous transparent zinc indium tin oxide (ZITO)(a-ZITO) thin films and the characteristics of the thin-film transistors TFTs were examined as a function of chemical composition. The as-deposited films were very conductive and showed clear free carrier absorption FCA . The analysis of the FCA gave the effective mass value of 0.53 me and a momentum relaxation time of 3.9 fs for an a-ZITO film with Zn:In:Sn = 0.35:0.35:0.3. TFTs with the as-deposited channels did not show current modulation due to the high carrier density in the channels. Thermal annealing at 300°C decreased the carrier density and TFTs fabricated with the annealed channels operated with positive threshold voltages VT when Zn contents were 25 atom % or larger. VT shifted to larger negative values, and subthreshold voltage swing increased with decreasing the Zn content, while large on–off current ratios 107–108 were kept for all the Zn contents. The field effect mobilities ranged from 12.4 to 3.4 cm2 V−1 s−1 for the TFTs with Zn contents varying from 5 to 48 atom %. The role of Zn content is also discussed in relation to the carrier transport properties and amorphous structures.

Novel transparent and flexible transistors for radiation harsh environment

La scoperta dei semiconduttori amorfi ha segnato l’era della microelettronica su larga scala rendendo possibile il loro impiego nelle celle solari o nei display a matrice attiva. Infatti, mentre i semiconduttori a cristalli singoli non sono consoni a questo tipo di applicazioni e i s. policristallini presentano il problema dei bordi di grano, i film amorfi possono essere creati su larga scala (>1 m^2) a basse temperature (ad es. <400 °C) ottenendo performance soddisfacenti sia su substrati rigidi che flessibili. Di recente la ricerca sta compiendo un grande sforzo per estendere l’utilizzo di questa nuova elettronica flessibile e su larga scala ad ambienti soggetti a radiazioni ionizzanti, come lo sono i detector di radiazioni o l’elettronica usata in applicazioni spaziali (satelliti). A questa ricerca volge anche la mia tesi, che si confronta con la fabbricazione e la caratterizzazione di transistor a film sottili basati su ossidi semiconduttori ad alta mobilità e lo studio della loro resistenza ai raggi X. La micro-fabbricazione, ottimizzazione e caratterizzazione dei dispositivi è stata realizzata nei laboratori CENIMAT e CEMOP dell’Università Nova di Lisbona durante quattro mesi di permanenza. Tutti i dispositivi sono stati creati con un canale n di ossido di Indio-Gallio-Zinco (IGZO). Durante questo periodo è stato realizzato un dispositivo dalle ottime performance e con interessanti caratteristiche, una delle quali è la non variazione del comportamento capacitivo in funzione della frequenza e la formidabile resistenza alle radiazioni. Questo dispositivo presenta 114 nm di dielettrico, realizzato con sette strati alternati di SiO2/ Ta2O5. L’attività di ricerca svolta al Dipartimento di Fisica e Astronomia di Bologna riguarda prevalentemente lo studio degli effetti delle radiazioni ionizzanti su TFTs. Gli esperimenti hanno rivelato che i dispositivi godono di una buona stabilità anche se soggetti alle radiazioni. Infatti hanno mostrato performance pressoché inalterate anche dopo un’esposizione a 1 kGy di dose cumulativa di raggi X mantenendo circa costanti parametri fondamentali come la mobilità, il threshold voltage e la sub-threshold slope. Inoltre gli effetti dei raggi X sui dispositivi, così come parametri fondamentali quali la mobilità, si sono rivelati essere notevolmente influenzati dallo spessore del dielettrico.

High mobility top-gate and dual-gate polymer thin-film transistors based on diketopyrrolopyrrole-naphthalene copolymer

In this letter, the performance characteristics of top-gate and dual-gate thin-film transistors (TFTs) with active semiconductor layers consisting of diketopyrrolopyrrole-naphthalene copolymer are described. Optimized top-gate TFTs possess mobilities of up to 1 cm 2 /V s with low contact resistance and reduced hysteresis in air. Dual-gate devices possess higher drive currents as well as improved subthreshold and above threshold characteristics compared to single-gate devices. We also describe the reasons that dual-gate devices result in improved performance. The good stability of this polymer combined with their promising electrical properties make this material a very promising semiconductor for printable electronics.

Charge carrier velocity distributions in high mobility polymer field-effect transistors

In this letter, the velocity distributions of charge carriers in high-mobility polymer thin-film transistors (TFTs) with a diketopyrrolopyrrole- naphthalene copolymer (PDPP-TNT) semiconductor active layer are reported. The velocity distributions are found to be strongly dependent on measurement temperatures as well as annealing conditions. Considerable inhomogeneity is evident at low measurement temperatures and for low annealing temperatures. Such transient transport measurements can provide additional information about charge carrier transport in TFTs which are unavailable using steady-state transport measurements.