TABLE OF CONTENTS The carrier inversion layer mobility ? s for electrons varies in the range 400-700cm 2/V-s while for the holes it is in the range 100-300cm 2/V s. These values are lower than the bulk mobility values (cf. section 2.4) because carriers in the channel undergo scattering by the charges at the surface boundary and by surface roughness, in addition to the scattering with the crystal lattice and ionized impurity atoms. [9] In fact, carrier mobility of a MOSFET is a strong function of the Si-SiO 2 interface and is strongly influenced by processing techniques.
While developing the drain current model, we assumed that the ? s is constant, independent of the gate or the drain voltage (assumption 5). In reality this is not true because when carriers in the channel under the influence of the normal electric field 
and the lateral electric field 
due to the gate voltage V gs and drain voltage V ds, respectively, they undergo increased scattering with increasing fields. The reason being that the normal field 
acts in a direction so as to accelerate the charge carriers towards the surface causing carriers to scatter more frequently than in the absence of the gate field. On the other hand, the lateral field 
causes charge carriers to move faster, so that at high enough V ds, the carriers become velocity saturated. Clearly ? s is not constant and depends...
