Tetra-phenyl porphyrin based thin film transistors

Long channel p-type field effect transistors were realized by using 5,10,15,20-tetra-phenyl porphyrin. The organic was deposited by spray coating from a chloroform solution onto thermally oxidized silicon substrates where chromium source and drain contacts were realized. Coplanar grid structures with a high geometric ratio were defined by fine line lithography in order to increase the collected current. Gate bias up to 100 V and drain to source bias up to 30 V were used. Leakage paths through the dielectric, also at low voltage, were evidenced by monitoring the gate current. Source to drain currents were evaluated considering the Kirchhoff law keeping into account the leakage currents. On these basis, a complete characterization of the transistors was carried out. Threshold voltage equal to 7.5 V was estimated on a 40 mm long device. Hole field effect mobility equal to 0.007 cm 2 /Vs was evaluated from zero-bias channel conductance. Besides, field dependent mobility as high as 0.012 cm 2 /Vs and transconductance equal to 6.5 mA/V were estimated from the analysis of the achieved results. The operation mechanism of the transistor and current modulation at low and high field is presented and discussed.