03-04-2012, 01:18 PM
Power MOSFET Basics
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Power MOSFET Basics
Discrete power MOSFETs
employ semiconductor
processing techniques that are
similar to those of today's VLSI
circuits, although the device
geometry, voltage and current
levels are significantly different
from the design used in VLSI
devices. The metal oxide
semiconductor field effect
transistor (MOSFET) is based
on the original field-effect
transistor introduced in the
70s. Figure 1 shows the
device schematic, transfer
characteristics and device
symbol for a MOSFET. The
invention of the power
MOSFET was partly driven by
the limitations of bipolar power
junction transistors (BJTs)
which, until recently, was the
device of choice in power
electronics applications.
BREAKDOWN VOLTAGE
Breakdown voltage,
BVDSS, is the voltage at
which the reverse-biased
body-drift diode breaks
down and significant
current starts to flow
between the source and
drain by the avalanche
multiplication process,
while the gate and
source are shorted
together. Current-voltage
characteristics of a
power MOSFET are
shown in Figure 6.
BVDSS is normally
measured at 250mA drain
current. For drain
voltages below BVDSS
and with no bias on the
gate, no channel is
formed under the gate at
the surface and the drain
voltage is entirely
supported by the
reverse-biased body-drift
p-n junction. Two related
phenomena can occur in
poorly designed and
processed devices:
punch-through and
reach-through. Punchthrough
is observed
when the depletion
region on the source side
of the body-drift p-n
junction reaches the
source region at drain
voltages below the rated
avalanche voltage of the
device.
GATE CHARGE
Although input capacitance
values are useful, they do not
provide accurate results when
comparing the switching
performances of two devices
from different manufacturers.
Effects of device size and
transconductance make such
comparisons more difficult. A
more useful parameter from the
circuit design point of view is
the gate charge rather than
capacitance.