Motivation for high-mobility channel materials in MOSFETs

The progress of information processing relies on advancements in decreasing the switching time of transistors in logic circuits built on CMOS technology. Defining this quantity as the time it takes to move the transistor between its OFF and ON states, there are two device parameters of specific importance: the sub-threshold voltage swing and the transition time for charge carriers along the channel. The first property, discussed in Chapter 11, depends on the capacitive coupling between gate and channel and is connected with the prospect of realizing high-k gate oxides. As we saw in Section 11.4, the second quality depends on the carrier mobility of the channel material and on extrinsic scattering mechanisms originating from the influence of phonons or charge in the gate oxide or from surface roughness (Laux, 2007). For very short channels, below about 20 nm, additional charge interactions are expected for hot carriers and also between channel charge and carriers in the highly doped source and drain areas (Fischetti et al., 2007; Kuhn, 2011). These limitations have motivated the search for new channel materials with higher intrinsic mobility.

The first steps for such improvements were done by creating strain and compressive stress in silicon to change lattice properties, which modifies the semiconductor band structure and gives rise to lower effective mass and thus higher mobility for electrons in n-channel and holes in p-channel devices, the latter built on Ge/Si compounds (Sugii et al., 1999; Fischetti et al., 2002; Leadley et al., 2010). A more radical encroachment into silicon technology has been the introduction of III–V materials for n-channels to speed up carrier transfer. Not surprisingly, the advantage in one parameter earned by such a modification must be offset by a disadvantage in another. Increased mobility is connected with smaller effective masses, which gives lower density of states in the semiconductor energy bands. InGaAs is an interesting candidate in this connection and, as will be further developed below, this compound requires a larger semiconductor surface potential drop in order to achieve a high enough channel charge for acceptable drive current. Its lower density of states demands a larger gate voltage swing for switching the transistor and gives rise to an inferior sub-threshold slope, thus counteracting the improvement in switching time offered by the larger mobility.