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We analysed the electrical characteristics and the stability of gate overlapped lightly doped drain (GOLDD) thin-film transistors (TFTs) with different channel length, n- region doping concentration and lateral doping profile at the junctions. A reduction of kink effect and an increase of device stability have been observed with the increase of the lateral doping profile. These results are explained by numerical simulation of electrical characteristics and hot carrier induced degradation. We found that different doping profiles produce, after bias stress, different interface state distributions across the channel/n- and n-/n+ junctions.

A new approach to control the lateral-growth mechanism of polycrystalline silicon films through appropriate spatial modulation of the absorbed laser energy and with a two-pass excimer laser annealing process is presented. In the first pass, spatial modulation of the light intensity has been achieved by irradiating the precursor amorphous silicon films through a patterned mask in contact with the sample, triggering the lateral growth of grain in excess of 1 µm. During the second pass of the process, the film is reirradiated without the mask and the laterally grown grains can be used as seeds and can be propagated to crystallize the film uniformly. With optimized mask patterns the sample area can be fully covered with laterally grown grains.

We have studied the light degradation and the thermal annealing of a-Si:H at different light intensities and temperatures monitoring both the defect density and the photoconductivity. This large set of data is used as a benchmark for the models of the metastability in a-Si:H. We show that an entire class of these models (which predict stretched exponential kinetics) is not in good agreement with our data. We discuss the mathematical and physical basis of these models pointing out some weak points that could explain these failures. The data are in better agreement with the “recombination induced bond breaking" models if an accurate description of annealing processes is included. The annealing rate must be computed considering a broad distribution of energy barriers. It must be also taken into account that the energy barrier for the annealing of a metastable defect can be reduced by a monomolecular trapping event at the defect itself. Our calculations show that this reduction amounts to an energy of about half the gap and lasts for a time of the order of 1 ps.

Top-gate a-Si:H thin fihn transistors (TFTs), with Pd as a gate electrode and silicon nitride as a gate insulator, have been fabricated and successfully operated as hydrogen sensors. The sensitivity mechanism of such devices resides in the catalytic behavior of Pd towards H2-molecules and in the formation of a H-dipole at the Pd/gate insulator interface, which influences the threshold voltage of the transistor. The measured H-response is similar to that relative to a c-Si Pd-gate MOSFET. The obtained results show possible promising applications of the use of a-Si:H TFTs in the field of chemical sensors.

Short-channel hydrogenated amorphous silicon thin-film transistors have been fabricated by using electron-beam lithography with the shortest channel length of L=0.2 µm. As source-drain contacts, Cr or Al have been used. In the case of Cr-contact field-effect mobility, µFE, degradation for decreasing L has been observed, while the devices with Al-contacts show µFE around 0.7 cm2/V ·s independently of the channel length. This result is related to low parasitic resistance shown by our devices and attributed to the Al-diffusion in the source-drain contact regions. The calculated maximum operating frequency for the shortest channel devices is about 250 MHz.

In this paper we show that the experimentally observed increase in the field-effect mobility in superlattice TFTs can be accounted for by a simple classical model based on Poisson's equation, including the presence of interface states at every interface. The field-effect mobility enhancement is a direct consequence of the different potential and electric field distibutions among the various layers, if compared with a single layer structure with the same total active layer thickness. The results of the calculations fit well experimental data already reported. Furthermore, the analysis of the single layer structure shows a similar field-effect mobility enhancement, suggesting the reduction of the active layer thickness as a key factor.