The team, from the National Institute of Standards and Technology (NIST), Wake Forest University and Penn State University, see errors up to 10x.
The problem is the use of silicon transistor models.
Typically, said NIST, measurements are analysed as though they were of silicon field-effect transistors – where drain current response to gate voltage change provides a good measure of how fast charge carriers move through the channel.
“Organic semiconductors are more prone to non-ideal behaviour, because the relatively weak inter-molecular interactions that make them attractive for low-temperature processing also limit the ability to engineer efficient contacts as one would for state-of-the-art silicon devices,” sais NIST thin-film electronics engineer David Gundlach. “Since there are so many different organic materials under investigation for electronics applications, we decided to step back and do a measurement check on the conventional wisdom.”
Using industry’s workhorse techniques, according to Gundlach, the team measured single-crystal rubrene FETs.
They saw that, as well as gate voltage, resistance at the source electrode significantly influenced subsequent electron flow in the channel, and hence mobility.
“In effect, contact resistance at the source electrode creates the equivalent of a second valve that controls the entry of current into the transistor channel,” said NIST. “Unaccounted for in the standard theory, this valve can overwhelm the gate and become the dominant influence on transistor behaviour.”
At low gate voltages, source contact resistance can dominate device operation, leading to model-based estimates of charge-carrier mobility that may be more than 10 times higher than the actual value.
Nature Communications has more, in ‘The effect of gated contacts on organic field-effect transistor operation and parameterization‘.