6 Inversion

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This preset is basically the same as the preceding, but with the gate voltage a little more positive. While you watch the densities graph, increase the Vapplied in small increments by clicking in the triangle at the end of the slider box. The increasingly positive gate voltage gives a field that must be shielded by an increasing amount of negative charge in the semiconductor if the field in the bulk semiconductor is to be zero. Describe how the nature of that shielding changes abruptly as the gate voltage goes above 1.3 V.

We speak of the sample being in inversion: at the surface, the shielding of the perpendicular gate field and conduction induced by a field parallel to the surface (e.g., by a source-drain voltage) is by electrons, though the material is doped p-type. In a MOSFET transistor, two ohmic contacts are made to this conducting layer, the channel. The resistance between the ohmic contacts can be modulated by using the gate voltage to vary the electron concentration in the channel.