IEEE JOURNAL OF SOLID-STATE CIRCUITS, VOL. zyxwvutsrqponmlk 25, NO. 2, APRIL 1990 531 2-MHz Clocked LCD Drivers on Glass IGOR DE RYCKE, ANDRE VAN CALSTER, JAN VANFLETEREN, JOHAN DE BAETS, JAN DOUTRELOIGNE, HERBERT DE SMET, AND PETER VETTER zyxw Absstruct -In this paper a poly-CdSe thin-film transistor zyxwvutsrq (TIW) LCD driver circuit is described, which is integrated on the display itself, reduc- ing the interconnections to the display. The driver can zyxwvutsr be used to drive the rows as well as the columns of active and passive matrix LCD’s. Drivers made in a 25-pm technology operate at frequencies up to 2 MHz. Simulations indicate a maximum operating frequency of 8 MH2 for a zyxwvuts 12.5- km technology. By using a time-multiplexing addressing scheme gray scales can be obtained. In order to address high-resolution displays with gray scales, parallel operating drivers zyxwvutsrqpo can be used, requiring only one extra connection per parallel driver. I. INTRODUCTION grated drivers using a complementary poly-Si technology with shift registers operating at up to 2 MHz [9]. In this paper the performance of complete poly-CdSe drivers, consisting of shift registers, latches, and buffers, will be discussed. 11. DESIGN OF DRIVER CIRCUITS Poly-CdSe TFT’s are n-channel field-effect transistors; thus, TFT driver circuits can be derived from NMOS LSI circuits. The block diagram of the TFT driver is shown in Fig. 1. The driver consists of three parts: the shift register, the latch, and the buffer. ATRIX addressing schemes have become very pop- M ular in recently developed flat-panel displays. How- A. Inverter Des,gn ever, matrix addressing involves a large number of inter- connections between the display and the external driving circuits. Therefore it would be most attractive if the driver circuits could be placed directly on the display, in order to minimize the interconnection problem. An attractive way of doing so is integrating the driver circuits by thin-film transistor (TFT) processing on the display. As these driver circuits have to be able to handle video, only TFT pro- cesses based on polycrystalline semiconductors can be used. Commonly used polycrystalline TFT processes are the poly-Si technology and the poly-CdSe technology. Al- though the poly-Si process can rely on known IC pro- cesses, poly-CdSe is the only semiconductor that yields films with large carrier mobilities (50-160 cm2/V.s) on low-temperature glass, without any laser annealing steps. In the past poly-CdSe was claimed to be unstable and to have no standard processing. These problems have now been solved satisfactorily and a number of photolitho- graphic processes have been developed [1]-[4]. Integrated drivers using poly-Si and poly-CdSe have been published. Most of the designs are based on scanner circuits [5] (poly-CdSe), [6] (poly-Si), with operating frequencies of zyxwvu 500 kHz [6]. Other designs use a decoder-kbuffer for addressing the display columns and frequencies up to 1.25 MHz are reported [7], [8]. Still others reported on inte- Manuscript received January 17, 1989; revised November 10, 1989. I. De Rycke and H. De Smet are supported by the IWONL. J. Doutreloigne is supported by the NFWO. The authors are with the Laboratory of Electronics, Ghent State University, B-9000 Ghent, Belgium. IEEE Log Number 8933722. From Fig. 1 it is seen that the complete driver circuit is based on inverters, and will work well if the basic inverter is designed properly. To design the inverter we followed the guidelines outlined by Elmasry [lo]. The characteristics of our n-channel TFT’s are as fol- lows. The dc output characteristics ZDs(VG,, VD,) in the “linear” region of the TFT are described by [ll], [12] W and where N, is the number of grain-boundary traps, Nb is the background doping of the CdSe film (in ~ m - ~ ) , Nd is the dose of evaporated donors (in cm-*), t, is the semicon- ductor thickness, Cu, is the oxide capacitance per unit area, pb is the electron mobility due to scattering at the grain boundaries, E, is the permittivity of the semiconduc- tor, and 4 is the elementary charge. From (1) it follows that the “linear” IDS( VG,) character- istics (VD, small) are strongly nonlinearly dependent on zyx V,,, especially at the onset of conduction (V,, ). This is due to the exponential influence of the grain barrier volt- age Vb. Only when V,, >> 0 does V, become neglectably 00lS-9200/90/0400-0531$01.00 01990 IEEE