Papers by Author: Toshio Hirao

Abstract: P-channel pentacene field effect transistorswith a Si/SiO₂/pentacene/Au structure were fabricated, and were gamma-ray irradiated with a Co⁶⁰ source. The changes of the drain current I_D vs. source/drain voltage V_SD (I_D - V_SD) characteristics were measured after every 200 Gy in silicon (Gy_Si) irradiations up to the total dose of 1200 Gy_Si. The drain current I_D continuously decreased to less than 10 % of that before irradiation after 1200 Gy_Si irradiation. The threshold voltage V_th continuously decreased up to 800 Gy_Si, started to saturate above 800 Gy_Si, and recovered above 1000 Gy_Si. The mobility m continued to decrease up to 1200 Gy_Si. Those behaviors were explained by accumulation of positive trapped charge within the gate insulator SiO₂ near the interface, continuous increase of interface traps near the interface between the SiO₂ and pentacene, and build up of electrons in the channel regions. These behaviors were discussed in comparisons with previously reported results on ultraviolet (UV) light irradiation experiments on similarly structured pentacene-based transistors.

Abstract: Electrical characterization of ⁶⁰Co γ-ray radiation effects on pentacene-based organic thin-film-transistors having two kinds of gate insulator have been carried out. For transistors with SiO₂ gate insulator, the threshold voltage shifts are consistent with positive charge trapping in the oxide and a “rebound” effect is observed. This “rebound” effect is attributed to the negatively charged interface traps generated during irradiation. For polyimide gate insulator, the threshold voltage continually decreases with an increasing total-dose. At total-dose of 1200 Gy (Si), for the SiO₂ gate insulator, the field-effect mobility decreased by almost 80%, and for polyimide gate insulator, it decreased by 40%.

Abstract: This paper investigates the transient induced currents by energetic carbon ions in 6H-SiC MOSFETs and the carrier dynamic response due to such a heavy ion collision is simulated by Technology Computer Aided Design (TCAD). It was found that a heavy ion strike induces a bipolar effect on the transistor, whereby the current transients can vary in both polarities. And this has been attributed to the inherent in the MOSFET is a parasitic bipolar junction transistor.

Abstract: We investigated the energy spectra in p+n 6H-SiC diodes by a wide variety of charged particles with energies up to several hundred MeV. Though Pulse Height Defect (PHD) was detected when the samples were irradiated with high energy heavy ions (322MeV-Kr and 454MeV-Xe), linearity between pulse height (peak channel) and ion energy up to 150MeV was observed.

Abstract: Transient currents in 6H-SiC n+p diodes and those in Si PIN diodes are compared, and the carrier dynamic response to a heavy ion collision is analyzed using Technology Computer Aided Design (TCAD). In case of 6H-SiC n+p diodes, it is found that the contribution of the ambipolar-diffusion current to total transient current is extremely weak compared to that in Si PIN diodes, since plasma disruption is accelerated by the Auger recombination process in the former.

Abstract: The charge generated in 6H-SiC n+p diodes by gold (Au) ion irradiation at an energy of 12 MeV was evaluated using the Transient Ion Beam Induced Current (TIBIC). The signal peak of the transient current increases, and the fall-time decreases with increasing applied reverse bias. The value of collected charge experimentally obtained is smaller than the ideal value. The Charge Collection Efficiency (CCE) of 6H-SiC n+p diodes irradiated with Au ions is approximately 50 % in spite that the CCE of 100 % is obtained in the case of oxygen (O) ion irradiation.

Abstract: The charge generated in 6H-SiC n+p diodes by oxygen (O) ion irradiation at energies between 6 and 15 MeV was evaluated using the Transient Ion Beam Induced Current (TIBIC). The signal peak of the transient current increases, and the fall-time decreases with increasing applied reverse bias. The value of collected charge increases with increasing applied reverse bias, and the saturation of the collected charge was observed in high reverse bias regions (e.g. above 70 V in the case of 12MeV O-irradiation). The charge generated in the deeper region than the depletion layer is collected due to the "funneling effect". Almost all charge generated in n+p SiC diodes by O-irradiation between 6 and 15 MeV is collected when the length of the depletion layer becomes longer than the projection range of ions.