is a promising way in modern technology. In our research group, we explore the GAA structure of poly-Si TFTs and try to find the better performance. I have surveyed spe-cial structures of poly-Si TFTs in many professional journals. Very clear, the trend is that the number of such research activities increases constantly. The circuit simulation using mixed-mode methods is popular and interesting. It is necessary and accurate because the compact models of novel devices are lacked. The mixed mode methods contains devices and circuits simulation and we develop a device coupling circuit technique to explore the behavior. It is my motivation to study this topic in my master thesis.
1.3 Outline
The outline of the thesis is as follows. The device structures and physics of the poly-Si TFTs are shown in Chap. 2. In Chaps. 3 and 4, I discuss the relative models and the detail of numerical methods. The simulation of poly-Si TFTs with different structures from 5μm to 300 nm are listed in Chap. 5. Besides, I also consider the positions and the size of grain boundary. Moreover, the performance of 90 nm poly-Si TFTs using quantum mechanical models is also explored. I study the active-matrix driving circuit (2T1C) with 300 nm gate-all-around poly-Si TFTs and analyze the stability of circuit in Chap. 6. Our adaptive circuit simulation is mixed-mode method and compact model is needless. The chapter 7 includes a summary of this work and suggestion for future work. A source code for the numerical
10 Chapter 1 : Introduction
solution of the 2D Poisson equation is listed in Appendix A and the explored mixed mode code is listed in Appendix B.
Chapter 2
Device Structures and Physics
T
he chapter focuses on devices of poly-Si TFTs. For each structure, the fundamental advantages and disadvantages of device will be described. If we have better driving capability of poly-Si TFTs, the panel will become brightly colored. The off-state current of poly-Si TFTs, referred to as leakage current, is due to traps in the channel. To improve the performance of poly-Si TFTs, we explore and compare the electrical characteristics of the poly-Si TFTs with different gate structures. By evaluating the transfer and output characteristics, the gate-all-around poly-Si TFTs demonstrate better driving capability. It mainly results from that the gate-all-around poly-Si TFTs with a large coverage ratio, and can improve short channel effects. Before simulating, we must know the device structures and physics.11
12 Chapter 2 : Device Structures and Physics
2.1 Device Structures
There are some ways to fabricate different structures of poly-Si TFTs. The first production of active-matrix LCDs for pocket TV in 1984 [60]. It was used the top gate poly-Si TFTs with the high temperature process. Since 1980s, the low temperature poly-Si crystallization methods have been developed. Because of the simple structures, single-gate poly-Si TFTs is the popular devices in recent years, shown in Fig. 2.1(a). Based on the technology, large size, integrated driving LCDs has been in production using the top gate poly-Si TFTs [61].
The concept of the double-gate transistor dates back to 1984 [61]. The double-gate (DG) has two gates. One is above and the other is below the channel of poly-Si. If the second gate is connected to first gate, it results in excellent controlling the potential of channel. The structure is shown in Fig. 2.1(b). S. Zhang et al. develope the self-aligned double gate TFT technology. According to their proposed process for DG, they find out that the On-state current of DG is twice as big as SG. Although the high transconductance and suppression of kink effect have been observed in the DG poly-Si TFTs, a high leakage current is observed. This is improved by adding the LDD regions into the DG. Kumer et al. [10] explore the kink-free I-V characteristics, low leakage current, and higher On-state current compared to conventional poly-Si TFTs.
We know that the variation of the threshold voltage is serious problem when channel length narrows down to sub-micron scale. The threshold voltage will decide the On-state
2.1 : Device Structures 13
current and we must make it constant as well as possible. Here, we study the new structure of poly-Si TFTs in Fig. 2.2 and Fig. 2.3.
There are two major types in gate-all-around structure. One is circle-shape gate, and the other is square-shape gate. We study the square-shape gate in this thesis. However, one major difference is that the square-shape gate is full of electric field in the corner.
Its controlling ability is inferior to circle-shape gate, but the defects cannot obscure the virtues. It still provides the better driving force than the conventional poly-Si TFTs. We will discuss the details in chapter 5. The structures of gate-all-around (GAA) can suppress the variation induced by grain boundaries in the channel. In addition to improve the average performance, it is compared with the conventional single gate TFT (SG) [14] [17]. The results clearly confirm this effect in terms of the current(I)- voltage(V) characteristics. The high performance enables reduction of the size and provides a large On-state current. The fabrication of GAA is shown in Fig. 2.3(c). The GAA poly-Si TFTs show high channel conductance and features of shield structure peculiar to the double-gate poly-Si TFTs. It can also eliminate an anomalous leakage current which appears in the sub-micron regime.
Since the GAA poly-Si TFTs require only one additional mask layer if comparing with the conventional single-gate poly-Si TFTs.
14 Chapter 2 : Device Structures and Physics
(a)
(b)
Figure 2.1: (a) A n-type of single top gate structure and (b) a
double-gate structure. The source and drain of the device are phosphorous doping and the channel is boron doping.
2.1 : Device Structures 15
(a)
(b)
Figure 2.2: (a) The gate-all-around structure (b) and the 2D cross section along the red dash line.
16 Chapter 2 : Device Structures and Physics
(b)
(c) (a)
Figure 2.3: (a) Another gate-all-around structure, the square-shape-gate device, (b) the 2D cross section of square-shape gate along the red line, and (c) the SEM micrograph of GAA [17].