Advanced MOS Device Physics by Norman G. Einspruch

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First, the implant also increases the doping level under the p s o u r c e / d r a i n diffusions, increasing the junction capacitance; this is dis­ cussed further in the following section. The second drawback is that addi­ tional phosphorus makes the /7-channel transistors more strongly enhance­ ment mode. For example, the devices used for the measurements shown in Fig. 8 had conventional n-doped polysilicon gates, and this resulted in thresholds of nearly - 2 V with the highest doses. This is too negative for high-performance C M O S applications, since the high absolute values of threshold would result in low drive current and hence low circuit speed.

S U M M A R Y The benefits and the drawbacks of device scaling cannot be properly visualized without a coherent accurate physics-based M O S transistor model. In this chapter, the essential features of one such model are presented. In drain current modeling, the formulation is greatly simplified by incor­ porating only the dominant scaling effects, namely mobility degradation due to the oxide field and velocity saturation, without sacrificing accuracy. Based on the modeling and experimental results, a device is more "longchannel like" if it has a long channel length or thinner gate oxide.

One method to make /7-channel thresholds less strongly enhancement mode is to use a gate material with a higher work function, such as p-doped polysilicon [21]. However, the incorporation of p polysilicon into a C M O S process presents several difficulties in terms of process integration. First, boron from a heavily d o p e d polysilicon gate can more easily penetrate a thin gate oxides than can p h o s p h o r u s , leading to potentially u n c o n t r o l l e d threshold shifts. Second, if n-doped polysilicon is still to be used with the n-channel devices, then a means of achieving good ohmic contacts between the two gate materials must be found.