ON THE UNIVERSALITY OF INVERSION LAYER MOBILITY IN SI MOSFETS .2. EFFECTS OF SURFACE ORIENTATION

This paper reports the studies of the inversion layer mobilities in n-channel MOSFET's fabricated on Si wafers with three surface orientations ((100), (110), and (111)) from the viewpoint of the universal relationship against the effective field, E(eff)(= q(N-dpl + eta . N-s)/epsilon Si). It is found that the universality does hold for the electron mobilities on (110) and (111), when the value of eta is taken to be 1/3, different from the electron mobility on (100), where eta is 1/2. Also, the E(eff) dependence of the electron mobility is found to differ among (100), (110), and (111) surfaces, This is attributed to the differences in the E(eff) dependence of the mobility limited by surface roughness scattering among the orientations. The origins of E(eff) and eta are discussed on the basis of the relaxation time approximation for a 2DEG (2-dimensional electron gas), While the surface orientation dependence of eta in phonon scattering can be understood in terms of the subband occupation, it is found that the theoretical formulation of surface roughness scattering, used currently, needs to be refined in order to explain the differences in E(eff) dependence and the value of eta among the three orientations.