The new opportunities that the use of organic semiconductors offer to device fabrication in the field of large area, low-cost and flexible electronics, have recently increased the research efforts on these materials. Recent results have shown that devices based on some organic semiconductor can be easily compared with those based amorphous silicon. Indeed, the performance obtained with pentacene-based organic thin film transistors (OTFTs) make possible that OTFTs are used as drivers of organic light emitting diodes in high-resolution active matrix displays. Today pentacene TFTs can be fabricated using inorganic and organic materials as substrates. The applications in microelectronics require very reliable devices with relative good on current stability: infact today, one of the main challenges, related to the processing of organic semiconductor, is the possibility to find a protective coating enabling patterning of the active layer as well as preventing aging effects caused by air and humidity. The use of conventional optical lithography to pattern pentacene active layers has been already demonstrated, in combination with different passivation film, including PVA, parylene.

Nanowires (NWs) are going to become the building blocks of several kinds of future devices, and for this reason they are presently among the most pursued topics in solid-state physics. This newly started activity on the growth of silicon nanowires has the objective to develop growth protocols compatible with low-temperature device processes and able to produce NWs with excellent electronic properties. NWs are generally grown after the vapour–liquid–solid model, by using a metal nanoparticle (NP), the catalyst, that induces and dictates the growth. The NP is then found at the free end of the NW. The most commonly used metal is gold. However, the catalyst may diffuse into the wires inducing unwished changes of the NW electronic properties. In particular, Au should be avoided when growing Si NWs or integrating III-V NWs with Si substrates, being Au a very efficient carrier trap in Si. Growths at low temperatures with catalysts alternative to gold or without the use of any external catalyst are then the first practical goals to achieve.

Preliminary results are shown in the figure that shows a SEM image of Si nanowires grown without catalyst at 430 °C, a temperature considerably lower than those indicated in literature for catalyst-free Si NWs.

The physics underlying the devices behaviour is investigated by using the 2D and 3D numerical simulation program DESSIS. Transport in amorphous and poly-crystalline silicon can be simulated by using the effective medium approach, which introduce a density of acceptor and donor states, exponentially distributed respectively from conduction and valence band, throughout the material energy gap. Moreover, the effect of interface states between semiconductor/oxide and semiconductor/ semiconductor interfaces can be simulated. This approach has been used also for the simulations of organic based (pentacene) devices.

The simulation software allow the use of different models for the semiconductor equations: drift-diffusion, thermodynamic model (which accounts for the device temperature field), hydrodynamic model (which accounts for the energy exchange between carriers and lattice). Among the physical mechanisms that can be included in the simulations we note: impact ionization mechanisms (needed for output characteristics simulation), bulk and interface traps models, generation-recombination models.

Several topics have been investigated:
Transport and reliability of polycrystalline silicon TFTs with advanced drain architecture
Transport in presence of grain boundaries
Self heating effects
Floating Body Effects
Short Channel Effects
Negative Bias – Temperature Instabilities (NBTI)
Aging and Hysteretic effects in OTFT

Temperature distribution inside the polysilicon active layer of a TFT biased in the self-heating regime

 

Impact ionization rate at the drain junction of a polysilicon TFT for different values of the lateral penetration of the doping.