Effects of gate electron concentration on the performance of pentacene organic thin-film transistors

Bottom-gated pentacene organic thin-film transistors (OTFTs) with NdTaON as high-k gate dielectric have been fabricated on substrates with different resistivities: 0.005 Ω ∼ cm, 0.3 ∼ 0.9 Ω cm, and 1 ∼ 5 Ω cm for n-Si wafers, and 35 Ω for ITO-coated glass. On the three n-Si substrates, the dielectric surface roughness and pentacene grain size are nearly the same, but the carrier mobility of the OTFTs show an obvious increase with decreasing resistivity, indicating that the gate electron concentration can affect the device performance. Despite the much larger dielectric surface roughness and smaller pentacene grain size, the OTFT on the ITO-coated glass shows the highest carrier mobility. These effects are attributed to remote phonon scattering on the channel carriers, which has been strongly screened by the electrons in the gate electrode. According to the measurement on the mobility degradation at high temperature, the remote phonon scattering is determined to be the dominant factor affecting the carrier mobility. As a result, the OTFT on ITO glass can achieve a high carrier mobility of 2.43 cm²/V s and a small threshold voltage of -0.10 V.