POWER MOSFET
Introduction:
Figure 5.19 IC model for Power
MOSFET
A metal-oxide-semiconductor field-effect
transistor(MOSFET) is developed by combining the areas of field-effect concept
and MOS technology.The Conventional planar MOSFET has the restriction of
handling the high power. In high power applications, the Double-diffused
vertical MOSFET or VMOS is used which is simply known as Power MOSFET.
Power MOSFET
The Power
MOSFET is the three terminal (Gate, Drain and Source), four layer (n+pnn+),Unipolar
( only majority carriers in conduction) semiconductor device.
·
The MOSFET is a majority carrier device, and as the
majority carriers have no recombination delays, the MOSFET achieves extremely
high bandwidths and switching times.
·
The gate is electrically isolated from the source,
and while this provides the MOSFET with its high input impedance, it also forms
a good capacitor.
·
MOSFETs do not have secondary breakdown area, their
drain to source resistance has a positive temperature coefficient, so they tend
to be self protective.
·
It has very low ON resistance and no junction
voltage drop when forward biased. These features make MOSFET an extremely
attractive power supply switching device.
Symbol
The
symbol for n-channel MOSFET is given below. The direction of the arrow on the
lead that goes to the body region indicates the direction of current flow. As
this is the symbol for n channel MOSFET, the arrow is inwards. For p-channel
MOSFET, the arrow will be towards outside.
Figure 5.20 Symbol of Power MOSFET
Structure
Figure 5.20 Structure of Power MOSFET
·
The Power MOSFET has a vertically oriented four
layer structure of alternating P and N type(n+pn-n+)
layers.
·
The P type middle layer is called as body of
MOSFET. In this region , the channel is formed between source and drain.
·
The n- layer is called as drift region,which
determines the breakdown voltage of the device. This n- region is present only
in Power MOSFETs not in signal level MOSFET.
·
The gate terminal is isolated from body by silicon
dioxide layer.
·
When the positive gate voltage is applied with
respect to source, the n-type channel is formed between source to drain. As
shown in the figure 5.20 there is a parasitic npn BJT between source and drain.
·
To avoid this BJT turns on, the p-type body region
is shorted to source region by overlapping the source metallization on to the p
type body. The result is a parasitic diode which is formed between drain to
source terminals. This integral diode plays an important role in half and full
bridge converter circuits.
Characteristics
The VI
characteristics of n-channel enhancement mode MOSFET.
Figure 5.21 V-I characteristics of n-channel
enhancement mode MOSFET
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