Temperature Dependence of High-Mobility Poly-Si Tft with Single Grain Boundary
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TEMPERATURE DEPENDENCE OF HIGH-MOBILITY POLY-SI TFT WITH SINGLE GRAIN BOUNDARY In-Hyuk Song, Cheon-Hong Kim and Min-Koo Han School of Electrical Engineering, Seoul National University, Seoul 151-742, Korea Phone: +82-2-880-7992, Fax: +82-2-883-0827, E-mail: [email protected]
ABSTRACT We have fabricated high-mobility TFTs with large lateral grains and investigated the variation of drain current with increasing temperature. The activation energy of drain current in large grain TFTs is found to be higher under the off-state and lower under the on-state than that in small grain TFTs. The field-effect mobility during the on-state is reduced with increasing temperature due to a lattice scattering inside grain. Because the proposed device has large lateral grain in the channel region, the lattice scattering inside the grain would be dominant, which is similar to single crystal Si MOSFETs.
INTRODUCTION XeCl excimer laser crystallization (ELC) of amorphous Si is widely used for poly-Si TFT. Recently, various efforts have been reported in order to control the SLG (super lateral growth) phenomenon and to increase the grain quality such as grain size and defect [1]. We have already reported a ELC method to enlarge the grain size by employing a selectively floating amorphous silicon (a-Si) thin film with a thin air-gap [2]. Large lateral grains exceeding 4 microns have been grown due to the lateral thermal gradient caused by the low thermal conductivity of the air. We have fabricated n-type self-aligned TFTs by the proposed laser crystallization method and a high mobility of 331cm2/Vsec was obtained because the device has only one grain boundary perpendicular to the current path in the channel. Other groups also have studied high mobility poly-Si TFT with single grain boundary in the channel [3]. In this paper, we report temperature dependence of drain current in high-mobility TFTs with large lateral grains. It is well known that the driving current of poly-Si TFTs is increased with increasing temperature due to the activated mobility property resulted from potential barrier height. However, drain current of the proposed TFT is decreased as temperature increases, which results in a negative activation energy of –0.025eV at VDS=0.1V and VGS=20V. The mobility of our device is also decreased with increasing temperature. In the proposed device with large lateral grains in the channel, the lattice scattering inside the grain would be dominant, which is similar to single crystal Si MOSFETs.
EXPERIMENTS AND RESULTS The proposed ELA process is illustrated in Fig. 1(a). It is well known that a lateral grain growth mechanism is responsible for the large grain and the lateral temperature gradient in a melted Si film
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would induce the lateral grain growth. In order to reduce a solidificat
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