3314 IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. 55, NO. 11, NOVEMBER 2008 UV Illumination Technique for Leakage Current Reduction in a-Si:H Thin-Film Transistors Yiming Li, Member, IEEE, Chih-Hong Hwang, Chung-Le Chen, Shuoting Yan, and Jen-Chung Lou Abstract—The high photoconductivity of hydrogenated amor- phous silicon thin-film transistors (a-Si:H TFTs) is responsible for the leakage current under illumination—particularly in pro- jectors and displays with high-intensity backlight illumination. This work investigates a leakage current reduction approach, in which the inverted staggered a-Si:H TFTs are exposed to the ultraviolet (UV) laser. An 85% reduction in the leakage current in a-Si:H TFTs is experimentally observed. The general SPICE model (such as the RPI model) lacks the proper term to capture the photo-induced phenomena; therefore, the physical mechanisms that are associated with the illumination of a-Si:H TFTs under UV, including the energy state and the density of traps, are ana- lyzed using device simulation. The I –V characteristics of the inverted staggered a-Si:H TFTs under different magnitudes of UV exposure are calibrated with experimentally measured data. The preliminary results show the change of trap states in amorphous silicon film and a shift of the Fermi level with UV illumination. UV illumination may induce traps in the active layer of the device and thereby reduce the OFF-state leakage current. Index Terms—Amorphous silicon thin-film transistors (a-Si:H TFTs), band-to-band tunneling, device simulation and character- ization, leakage current, trap-assisted tunneling, ultraviolet (UV) illumination. I. INTRODUCTION H YDROGENATED amorphous silicon thin-film transis- tors (a-Si:H TFTs) have recently been used widely as switching devices in large-area electronics such as active matrix liquid crystal displays (LCDs) [1] and memory devices [2]. When the TFT turns on, both the liquid crystal capacitance and the associated capacitance are charged; they have to main- tain sufficient voltage for the rotation of the liquid crystal. Unfortunately, an a-Si:H TFT has high photoconductivity [3], which may result in a high leakage current under visible light illumination, particularly those projectors and displays with Manuscript received May 28, 2008. Current version published October 30, 2008. This work was supported in part by the National Science Council under Contracts NSC-96-2221-E-009-210 and NSC-96-2752-E-009-003-PAE and in part by the InnoLux Display Corporation, Science-Based Industrial Park, Chu-Nan 350, Miao-Li County, Taiwan, R.O.C., under a 2006–2008 Grant. The review of this brief was arranged by Editor H. S. Tae. Y. Li is with the Department of Communication Engineering, the Mod- eling and Simulation Center, and the Parallel and Scientific Computing Laboratory, National Chiao Tung University, Hsinchu 300, Taiwan, R.O.C. (e-mail: ymli@faculty.nctu.edu.tw). C.-H. Hwang is with the Department of Communication Engineering, National Chiao Tung University, Hsinchu 300, Taiwan, R.O.C. C.-L. Chen and J.-C. Lou are with the Institute of Electronics, National Chiao Tung University, Hsinchu 300, Taiwan, R.O.C. S. Yan is with the Technology Development Division, InnoLux Display Corporation, Chu-Nan 350, Taiwan, R.O.C. Color versions of one or more of the figures in this brief are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TED.2008.2005133 high-intensity backlight illumination. The leakage current thus causes a voltage drop, which may induce an insufficient rotation angle of the liquid crystal. Various leakage current mechanisms in poly-Si TFTs have been reported [4]–[11]. Recently, the incorporation of fluorine and chlorine into a-Si:H has been pro- posed to suppress the OFF-state leakage current by increasing the acceptorlike density of states in a-Si:H (fluorine) material to shift the Fermi level toward the valence band edge [9]–[11]. Observations of the increase in acceptorlike states motivated this exploration of the mechanism by which the OFF-state leakage current of a-Si:H TFTs is reduced. The influence of prolonged illumination with intense light (600–900 nm) on the metastable changes in a-Si:H film has been reported elsewhere [12], [13]. However, little attention has been paid to study the ultraviolet (UV) illumination-induced metastable increase in hydrogenated materials. The effect of UV exposure on the passivation quality of SiN x /a-Si:H stacked layers in solar cell fabrication has been examined [14]. The increase of trap density reduces the carrier lifetime caused by UV illumination, which may reduce the OFF-state leakage current in a-Si:H TFTs. This brief presents a leakage current reduction approach, in which inverted staggered a-Si:H TFTs are exposed to a UV laser. The UV illumination may produce traps in the active layer of the device and thus reduce the OFF-state leakage current. A general SPICE model, such as the well-known RPI model [15], [16], for simulating the a-Si:H TFT circuit lacks a term for photo-induced phenomena. Therefore, the physical mechanism that governs the characteristics of the device should be studied qualitatively and quantitatively. To further study the metastable changes in a-Si:H film, caused by UV illumination, a set of ther- modynamic transport equations coupled with trap state models is simultaneously solved using our own simulation platform [17]–[19]. The calculated current–voltage (I –V ) characteristics of the inverted staggered a-Si:H TFTs illuminated with different intensities of UV are calibrated with experimentally measured results. The shift in the threshold voltage, the reduction of the leakage current, and the shift in the Fermi level are observed and discussed. This brief is organized as follows. Section II introduces the experimental measurement and the physical models used for numerical simulation. In Section III, the measurement and simulation results are calibrated for the best accuracy of the analysis. The observed results and the associated phenomena are then studied. Finally, conclusions are drawn. II. EXPERIMENT AND SIMULATION Fig. 1(a) shows the inverted staggered a-Si:H TFTs exposed to a UV laser (355 nm) from the topside of the a-Si:H layer. 0018-9383/$25.00 © 2008 IEEE