3.1.3.1 Modulation-Doped FET

The typical layer structure of an n-S-MODFET [57] is shown in Fig. 3.3. It is grown by molecular beam epitaxy on a $p^{-}$-substrate with $\rho\geqslant1000\thickspace\Omega\cdot$cm starting with a relaxed SiGe buffer layer whose Ge content is linearly graded to 40-50%. The core of the layer structure is the 7-10 nm thick biaxially strained Si channel embedded in undoped $\textrm {Si}_{1-x}\textrm {Ge}_{x}$ spacers which separate the following carrier supply layers from the channel. Due to a Ge content of 40-50% in the SiGe layers, a high conduction band offset is achieved and the resultant quantum well enables sheet carrier densities up to $n_{s}=7\times 10^{12}$ cm$^{-2}$.

Figure 3.3: S-MODFET structure.

S. Smirnov: