Critical Evaluation of Organic Thin-Film Transistor Models

Thin-film transistors (TFTs) represent a wide-spread tool to determine the charge-carrier mobility of materials. Mobilities and further transistor parameters like contact resistances are commonly extracted from the electrical characteristics. However, the trust in such extracted parameters is limited, because their values depend on the extraction technique and on the underlying transistor model. We propose a technique to establish whether a chosen model is adequate to represent the transistor operation. This two-step technique analyzes the electrical measurements of a series of TFTs with different channel lengths. The first step extracts the parameters for each individual transistor by fitting the full output and transfer characteristics to the transistor model. The second step checks whether the channel-length dependence of the extracted parameters is consistent with the model. We demonstrate the merit of the technique for distinct sets of organic TFTs that differ in the semiconductor, the contacts, and the geometry. Independent of the transistor set, our technique consistently reveals that state-of-the-art transistor models fail to reproduce the correct channel-length dependence. Our technique suggests that contemporary transistor models require improvements in terms of charge-carrier-density dependence of the mobility and/or the consideration of uncompensated charges in the transistor channel.Comment: 20 pages, 10 figure

Ultraflexible Organic Active Matrix Sensor Sheet for Tactile and Biosignal Monitoring

Abstract Flexible sensors are currently the subject of intensive research, as they allow cost‐effective and environmentally friendly production of large‐area, flexible, and when fabricated on ultrathin substrates, highly conformable devices. Among many intriguing applications, tactile and biosignal monitoring, where lightweight sensors with high wearing comfort are particularly interesting, is focused on here. The required spatiotemporal resolution of the signals is achieved by integrating the sensors in an active matrix configuration. Organic ferroelectric transducers of high uniformity, characterized, for example, by a sensitivity spread of only 1.5%, are combined with similarly uniform ultralow noise level organic thin film transistors operating below 5 V, showing, for example, a threshold voltage variation of just 0.13 V, in a 12 × 12 sensor array. The transistors transition frequency of up to 160 kHz (saturation range) and 17 kHz (linear range) allows for a high spatiotemporal resolution of ≈3 mm at a frame rate of 1400 fps. The thickness of only 2.8 µm renders the organic active matrix sensor sheet ultraflexible and therefore virtually imperceptible on the human skin. Real‐time monitoring of tactile modes in a subset of 8 × 3 pixels and of the pulse wave including heart rate and blood pressure using four sensors of the matrix is demonstrated

Critical Evaluation of Organic Thin-Film Transistor Models

The thin-film transistor (TFT) is a popular tool for determining the charge-carrier mobility in semiconductors, as the mobility (and other transistor parameters, such as the contact resistances) can be conveniently extracted from its measured current-voltage characteristics. However, the accuracy of the extracted parameters is quite limited, because their values depend on the extraction technique and on the validity of the underlying transistor model. We propose here a new approach for validating to what extent a chosen transistor model is able to predict correctly the transistor operation. In the two-step fitting approach we have developed, we analyze the measured current-voltage characteristics of a series of TFTs with different channel lengths. In the first step, the transistor parameters are extracted from each individual transistor by fitting the output and transfer characteristics to the transistor model. In the second step, we check whether the channel-length dependence of the extracted parameters is consistent with the underlying model. We present results obtained from organic TFTs fabricated in two different laboratories using two different device architectures, three different organic semiconductors and five different materials combinations for the source and drain contacts. For each set of TFTs, our approach reveals that the state-of-the-art transistor models fail to reproduce correctly the channel-length-dependence of the transistor parameters. Our approach suggests that conventional transistor models require improvements in terms of the charge-carrier-density dependence of the mobility and/or in terms of the consideration of uncompensated charges in the carrier-accumulation channel