_______________________________________________________________________ Special Devices
Figure 3-55. Effects of Forward-bias (N-channel MOSFET in
Enhancement Mode)
Figure 3-56. Induced Channel MOSFET Construction
3-94. The induced-channel MOSFET conducts from source to drain by the electric field
that is created when a voltage is applied to the gate. For example, assume that a negative
voltage is applied to the MOSFET in Figure 3-56. The effect of the negative voltage
modifies the conditions in the substrate material. As the gate builds a negative charge, free
electrons are repelled, forming a depletion region. Once a certain level of depletion has
occurred (determined by the composition of the substrate material), any additional gate
bias attracts positive holes to the surface of the substrate. When enough holes have
accumulated at the surface channel area, the channel changes from an N-type material to a
P-type material (since it now has more positive carriers than negative carriers). At this
point the channel is considered to be inverted and a P-type inversion layer or channel now
connects the two P-type regions at the source and the drain. As with the MOSFET, the gate
23 June 2005
TC 9-62
3-35
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