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Transistors
4.10
Power MOSFET (N/P)
The double-diffused or DMOS transistor is an example of a power MOSFET. This device is
fabricated on a lightly doped n-type substrate with a heavily doped region at the bottom for
drain contact. Two diffusions are used, one to create the p-type body region and another to
create the n-type source region.
The DMOS device is operated by applying a positive gate voltage, vGS, greater than the
threshold voltage Vt, which induces a lateral n channel in the p-type body region underneath
the gate oxide. Current is conducted through the resulting short channel to the substrate and
then vertically down the substrate to the drain.
The DMOS transistor can have a breakdown voltage as high as 600 V and a current capability
as high as 50 A is possible.
Power MOSFETs have threshold voltages in the range of 2 to 4 V. In comparison with BJTs,
power MOSFETs do not suffer second breakdown, nor do they require the large base-drive
currents of power BJTS. They also have a higher speed of operation than the power BJTs.
These advantages make power MOSFETs suited to switching applications, such as in motorcontrol circuits.
4.11
Power MOSFET Complementary
These DMOS dual N- and P-channel enhancement mode power field effect transistors
minimize on-state resistance, provide superior switching performance, and withstand high
energy pulses in the avalanche and commutation modes. These devices are particularly suited
for low voltage applications such as notebook computer power management and other battery
powered circuits where fast switching, low in-line power loss, and resistance to transients are
needed.
Multisim Component Reference Guide
4-20
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