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Transient drain current of polymer transistors

 

We have measured transient drain current in millisecond range for some poly(thiophene) transistors with different channel length. Under fixed gate bias and with source grounded, a voltage pulse is applied periodically to the drain and the current waveform is recorded. The drain current of a 50  µm channel device is almost proportional to the drain bias at 0.2 ms after the drain bias application. As the time progresses, the transient output curve starts to show a sign of saturation at a large drain bias. For a shorter-channel device, this initial resistorlike characteristic is less apparent. Two mechanisms are considered to account for this behavior. First, we analyze contribution from the channel charges that exist in excess of the steady-state distribution. Although the linear dependency on the drain bias is correctly predicted, its time dependence underestimates the observed drain current. This suggests that the number of mobile carriers in the channel is more than those balancing the gate potential during the early stage. Second, we consider relaxation of this excess current. Its decay is well fit by a stretched exponential function. Using an empirical expression for the data at a certain low drain bias, transient drain current at other drain biases is calculated. The resultant output curves more or less reproduce the measurement.

 
citation

Fujieda, I.; Street, R. A. Transient drain current of polymer transistors. Journal of Applied Physics. 2009 March 11; 105 (5): 054503.