What is flexible electronics
Nowadays, the “Internet of Things” (IoT) is the driving force behind the rapid development in the field of flexible electronics. Basic technologies such as RFID labels, in synergy with sensor networks, offer the potential for innovative applications; be it in the health sector as wearable electronics for monitoring vital parameters or in the food industry in the area of quality monitoring. Thin film transistors (TFTs) are a key element of this technology as they are responsible for modulating the current in the system. In order to arouse the market's interest in these portable gadgets, in addition to being cost-effective to manufacture, they also have to be characterized by low power consumption.
Figure 1 shows the schematic structure of a TFT. In this construction, the substrate does not form an electrically active element and is not decisive for the functionality of the TFT. For this reason, polymers such as PET or PP were discovered as possible substrates in order to be able to use processes suitable for mass production such as roll-to-roll processes. It should be noted, however, that the use of these substrates place various new demands on the integration process, since polymers generally have only limited thermal and chemical resistance.
The active elements of a TFT form the electrodes (source, drain, gate), the insulation layer (dielectric) and the semiconductor layer used. The semiconductor area between the source and drain electrodes is called the channel area of the TFT. In this channel, the size of the current flow can be influenced by the applied electric field in the gate area (gate electrode with associated insulation layer). Thin-film transistors work in accumulation and depending on the type of charge carrier that is responsible for the transport of current, a distinction can be made between n-channel (majority charge carriers: electrons) and p-channel (majority charge carriers: holes).
In general, low-resistance metals are used as electrode materials in order to minimize power losses that may occur. The choice of the source / drain material is particularly important because it has to be adapted to the requirements of the active semiconductor layer in order to minimize possible charge carrier injection barriers.
In order to enable the device to operate at low voltages, dielectrics with a high dielectric constant (high-k), because this is proportional to the field-induced charge carrier density in the channel area. In the literature, inorganic dielectrics such as Al2O3, HfO2 or TiO2 known, which can be produced using different process technologies (atomic layer deposition, electron beam evaporation, reactive sputtering, ...). However, it is primarily their mechanical properties such as flexibility and surface roughness that limit their scope of application. As a further starting point, organic-inorganic nanocomposites are used, which combine the flexibility of organic polymers with the high permittivity of the inorganic components and are therefore used in the sensor technology field.
Since 2005, the sensor technology department has been working intensively on the integration of thin-film transistors based on nanostructured material systems (ZnO, CuO) and organic semiconductor materials (pentacene, DNTT, DPh-BTBT, C8-BTBT) within the framework of industrial collaborations and funded research projects. The integration process developed by the sensor technology department enables various components to be manufactured independently of the substrate. This enabled the integration of both individual transistors and the first logic circuits (inverters) on silicon, glass and film substrates with only a small amount of parameter variation.
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