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
