MOSFET: Output Characteristics
Following section contains the output characterstics of the MOSFET in brief.
The applied gate voltage VG is accounted for plus the voltage required to achieve flat band:
The induced charge Qs in the semiconductor is composed of mobile charge Qn and fixed charge in the depletion region Qd. Substituting Qn Qd for Qs,
we can solve for the mobile charge:
With a voltage VD applied, there is a voltage rise Vx from the source to each point x in the channel. Thus the potential Φs(x) is that required to achieve strong inversion (2ΦF) plus the voltage Vx:
If we neglect the variation of Qd(x) with bias Vx,can be simplified to:
This equation describes the mobile charge in the channel at point x in fig given below. The conductance of the differential element dx is μnQn(x)Z/dx, where Z is the width of the channel and μn is a surface electron mobility (indicating the mobility in a thin region near the surface is not the same as in the bulk materials