Potential of low-voltage organic transistors with high on-state drain current for temperature sensor development

Organic transistors with high on-state drain current at gate and drain voltages of −2 V fabricated on polyethylene naphthalate foils were investigated for sensor development. Two aspects were studied: (a) the ability of such transistors to raise the sensitivity of a temperature sensor and (b) the bias stress stability of the transistors subjected to square voltage pulses that turned them on and off repeatedly. To demonstrate the first aspect, the voltage-amplifying ability of the organic transistor was used to increase the response to the temperature, ordinarily achieved with a thermistor. To achieve voltage amplification, the transistor must have on-state drain current of at least 20 μA at gate and drain voltages of −2 V. Two transistors with on-state drain current of ~60 and ~120 μA were tested, leading to voltage gain of −2.8 and −4.9 V/V, respectively, thus increasing the sensitivity of the temperature sensor by a factor of up to 5. To study the second aspect, the same square voltage pulses were concurrently applied to the gate and drain electrodes, causing the transistor to turn on and off repeatedly. The turn-on and turn-off voltages were −2 and 0 V respectively and four different pulse periods were used: T of 5, 20, 40 and 60 s. For each T, 1000 pulses with turn-on time of 1 s and varying turn-off times were applied to the transistors, leading to the aggregate net stress time of 1000 s in all cases. The changes in the on-state drain current, threshold voltage, and field-effect mobility depended on T, in spite of the net stress time being the same. The reduction in the on-state drain current did not exceed 17%, stabilization was also observed after about 500 cycles in some cases, and the maximum drop occurred for medium T, thus making T = 60 s a favorable condition for sensor operation.