Tham khảo tài liệu 'mosfet modeling for vlsi simulation - theory and practice episode 4', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 96 3 MOS Transistor Structure and Operation drain structures formed by using two donor type implants. The two most commonly used graded junctions are double diffused drain DDD 30 and lightly doped drain LDD 31 32 Figure . A DDD structure for n-channel MOSFET is formed by implanting phosphorous P and arsenic As into the source-drain region. A lightly doped n-region n is first formed using p and then a heavily doped n region using As remembering that p is lighter than As and therefore diffuses faster. Thus in a DDD structure a lightly doped n region encloses the n region as shown in Figure the doping level drops by 2-3 orders of magnitude from the n to n region. Note that in practice the source is also modified due to the symmetrical nature of the MOSFET although it is the drain side where the maximum field is to be reduced. The DDD structure though simple is normally used to reduce the hot-carrier effects for channel lengths down to pm devices. However this structure is not suitable for submicron devices due to the fact that it results in deeper junctions and hence increased shortchannel effects and more gate-to-source drain overlap capacitance. For submicron devices the most commonly used S D structure is the LDD. In this structure a lightly doped n-region n is first created by implanting low energy p or As and then oxide spacers are formed at the side wall of the polysilicon gate see Figure . The oxide spacers then serve as a mask for the standard n As implant. The n implants do not diffuse laterally under the gate but diffuse under the spacers to the edges of the gate. The lateral doping profile of the LDD structure is shown in Figure also shown Figure is a conventional nMOST with its doping profile 31 . By introducing an n region between the drain and channel the peak channel field is not only shifted towards the drain but is also 21 Fig. MOSFET cross-section and doping profile for a lightly doped drain LDD and b .
