MOBILITY ANISOTROPY AND PIEZORESISTANCE IN SILICON P-TYPE INVERSION LAYERS

The Hall mobility of holes in silicon p-type inversion layers has been measured as a function of gate voltage (perpendicular electric field), inversion layer orientation [(100), (110), and (111) surfaces], direction of current flow within an inversion layer, and temperature. It has been shown that hole mobility in silicon inversion layers depends not only on the crystalline orientation of the inverted surface, but also on the azimuthal direction of current flow within the inversion layer. Thus, on the (110) silicon surface at room temperature, the inversion-layer hole mobility is 40% higher in the[l̄10] direction than in the [001] direction. Room-temperature piezoresistance tensors have been experimentally determined for inversion layers on the (100), (110), and (111) surfaces of silicon. It is found that, in general, the piezoresistance coefficients are not the same as those for the same directions in bulk silicon and that they depend on the orientation of the surface. The existence of these anomalous effects can be understood in terms of quantization of the carrier wavefunction in the surface channel, which is narrow compared with the carrier wavelength in bulk silicon. This quantization tends to depopulate that part of the Brillouin zone within which k⊥, the component of wavevector perpendicular to the surface, is small. If the dependence of energy on k is not quadratic, as is true for the valence band of silicon, the effective masses for conduction in the plane of the surface can be complicated functions of k⊥. The mass anisotropies estimated from the cyclotron resonance parameters for the valence band of bulk silicon are in qualitative agreement with the experimental mobility values. © 1968 The American Institute of Physics.