Test System for Thin Film Transistor Parameter Extraction in Active Matrix Backplanes

Thin film transistor (TFT) active matrix backplanes are used in large area electronic systems, such as displays and image sensors. With backplanes being fabricated on wearable and flexible substrates, the possibilities of operational faults in backplanes have increased. These faults could either be hard faults, such as line opens or shorts or could be softer faults, such as time dependent variations in the TFT transfer characteristics. Real time diagnosis of these faults require built-in-self-test systems. While many such systems have been demonstrated to diagnose hard faults, an easily realizable system to identify soft faults, such as variations in transistor transconductance remain an open challenge. In this paper, we discuss a system that extracts the transconductance by charging and then discharging the pixel capacitor at various gate voltages for an active matrix liquid crystal display backplane. This permits a plot of the time averaged current versus the gate voltage from which the spatial variation of transconductance can be extracted. The details of the design are discussed and a proof of concept with a 3×4 amorphous silicon backplane is demonstrated.