A Comprehensive Study on Schottky Barrier Nanowire Transistors (SB-NWTs): Principle, Physical Limits and Parameter Fluctuations Liangliang Zhang, Zhaoyi Kang, Runsheng Wang and Ru Huang * Key Laboratory of Microelectronic Devices and Circuits，Institute of Microelectronics, Peking University, 100871 * Email: ruhuang@ pku.edu.cn Abstract P-type Schottky barrier nanowire transistors (p-SB-NWTs) are computational studied in this paper. We analyzed the working principle and physical limits on their performance in details. The impact of Schottky contact of SB-NWTs on the current drivability, gate control and RF performance are studied comparing with conventional silicon nanowire transistors (SNWTs). It is pointed out that the inferior performance of SB-NWTs can not be solved by changing the S/D or channel materials. On the other hand, small V t , F t and on-off ratio fluctuation caused by process variation on channel diameter are observed, which is an advantage of SB-NWTs. 1. Introduction With MOSFETs scaling down to nano-scale, gate-all-around silicon nanowire transistors (SNWTs) have been considered as one of the most promising transistor structures for future CMOS technology [1]. But one of the key problems of SNWTs is that they suffered a lot from the large parasitic resistance of their ultra-narrow S/D extension. As a result, Schottky Barrier Nanowires (SB-NWTs), which replace the doped S/D and extension to silicides/metal, was proposed with the original purpose to reduce the S/D series resistance [2]. However, so far, both experiments and simulation show weak performance of SB-NWTs compared with silicon nanowires [2]. We investigated the 10nm p-type SB-NWTs by extensive 3-D device simulation. With the following analysis in the paper, it is figured out that even though the silicides have relatively lower resistance than doped silicon, the Schottky contact formed between the silicides and channel silicon still exhibits large resistance and reduced the on-state current. The different current conduction mechanism compared with SNWTs results in smaller on-state current, larger SS, larger DIBL, lower F t and longer intrinsic delay for p-SB-NWTs. These weaknesses can not be solved by changing S/D or channel materials. Moreover, we observed the great immunity of the impact of channel diameter fluctuation of SB-NWTs. 2. Device structure and simulation The device studied in this paper is schematically shown in Fig.1, with 10nm gate length and 1nm thick SiO 2 as the gate oxide. The simulation is based on Synopsys TCAD Sentaurus Device simulation tools [3]. The 3D drift-diffusion model was used with density gradient model for quantum correction. Tunneling current through the Schottky barrier is considered by nonlocal tunneling model with a nonlocal distance of 5nm. The tunneling mass for holes (m th ) was set to be 1.71m 0 , being based on the calibration with a planar 27nm PtSi S/D SB-MOSFET[4], which is close to the theoretical value [5]. For electron, m te =0.19m 0 was used according to theoretical analysis [5]. P-SB-NWTs with various structural parameters were also performed, including DC and AC characteristics. 3. Results and discussion As shown in Fig.3, there is a “convert point” in the transfer characteristics, which separates the thermal emission region and field emission region. The band structure near the convert point is illustrated in Fig.4. Convert point is reached at different V g with different hole barrier (Fig5 and Fig.6). As a result, the threshold voltage should not be simply defined by a single mechanism: it should be defined through the interaction of both thermal emission, which is approximately the same with SNWTs, and field emission, which dominates the on-state current of SB-NWTs. Moreover, the extraction of V t by the ‘constant drain current’ method fluctuates a lot and SS is also affected by this phenomenon. The band structure of 10nm and 30nm p-SB-NWTs are shown in Fig.7~Fig.10. For each channel length, the band structure near the channel surface (0.5nm under the channel/oxide interface) and at the center (in the middle of the channel) are given. The difference of band structure between the surface and the center when V g =V t is shown in Fig.11, which indicates that the current can conducts both at the surface and at the center. This can explain why the threshold voltage shifts only a little with different channel diameter (Fig.12). The larger SS can be attributed to the different mechanisms between SNWTs and SB-NWTs. The tunneling mass of hole (m th ) is one of the most important factors in p-SB-NWTs that determines the 978-1-4244-2186-2/08/$25.00 ©2008 IEEE