Short channel effects in polysilicon thin film transistors G. Fortunato T , A. Valletta, P. Gaucci, L. Mariucci, S.D. Brotherton IFN-CNR, Via Cineto Romano 42, 00156-Roma, Italy Available online 2 March 2005 Abstract Short channel effects have been investigated in n-channel polysilicon thin film transistors. Transconductance degradation has been observed when reducing the channel length L and this effect has been explained with the presence of a parasitic resistance related to residual implant damage. Threshold voltage slighlty decreases for channel length around L=1 Am and simulations confirm that the roll-off of the threshold voltage is expected for Lb1 Am. In the output characteristics a stronger kink effect has been observed at short L which, from a comprehensive analysis of the current components, can be attributed to an enhanced parasitic bipolar transistor action. D 2005 Elsevier B.V. All rights reserved. Keywords: Polysilicon TFTs; Short channel effects; Kink effect; Parasitic bipolar transistor 1. Introduction Polycrystalline silicon (polysilicon) thin film transistor (TFT) technology is emerging as a key technology for both active matrix liquid crystal displays (AMLCDs) [1] and active matrix organic light emitting displays (AMOLEDs) [2], allowing the integration of both active matrix and driving circuitry on the same substrate. Integration of other functions should lead to system-on-panel (SOP) technol- ogy, requiring, however, a significant improvement in the performance of current polysilicon TFTs. The biggest leverage in circuit performance can be obtained by the reducing channel length, L, from the typical, current values of L=3–5 Am to L=1 Am, or less. Therefore, short channel effects in scaled down polysilicon TFTs will have to be controlled in order to allow proper operation of the devices. In this work we have studied the electrical characteristics of devices with channel lengths down to 1 Am, by combining experimental data with two-dimensional numerical simulations. 2. Experimental results The n-channel TFTs used in this work were fabricated at Philips Res. Lab [3] on about 40 nm thick films of excimer laser crystallised polysilicon, using a self-aligned (SA) architecture, in which the source and drain dopant (P) was implanted through the gate oxide, and was activated by a second pass through the excimer laser. The gate oxide was deposited in a PECVD system using SiH 4 and N 2 O, and ranged in thickness t ox from t ox =45 nm to t ox =150 nm. In Fig. 1a,b typical I d –V g characteristics (Fig. 1a) and normalised transconductance (Fig. 1b), g m L, are shown. It appears evident that for decreasing channel length L the device transconductance is seriously degraded. In order to investigate the short channel effects in such devices we analysed the threshold voltage, V t , dependence upon L. The threshold voltage V t has been determined as the gate voltage giving a drain current of I d =W/L 10 7 A, a current level sufficiently low to reduce the effects of parasitic resistance. In Fig. 2 the plot of V t vs. L is shown for three different gate oxide thicknesses and, as can be seen, only a minor V t decrease with decreasing L is observed in the devices with thicker gate oxide. Finally, output characteristics have been measured for different L and compared at relatively low V g (around threshold 0040-6090/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2005.01.069 T Corresponding author. E-mail address: fortunato@ifn.cnr.it (G. Fortunato). Thin Solid Films 487 (2005) 221 – 226 www.elsevier.com/locate/tsf