The bipolar and the MOSFET transistors exploit
the same operating principle. Fundamentally,
both type of transistors are charge controlled
devices which means that their output current is
proportional to the charge established in the
semiconductor by the control electrode. When
these devices are used as switches, both must be
driven from a low impedance source capable of
sourcing and sinking sufficient current to provide
for fast insertion and extraction of the controlling
charge. From this point of view, the MOSFETs
have to be driven just as “hard” during turn-on
and turn-off as a bipolar transistor to achieve
comparable switching speeds. Theoretically, the
switching speeds of the bipolar and MOSFET
devices are close to identical, determined by the
time required for the charge carriers to travel
across the semiconductor region. Typical values
in power devices are approximately 20 to 200
picoseconds depending on the size of the device.
The popularity and proliferation of MOSFET
technology for digital and power applications is
driven by two of their major advantages over the
bipolar junction transistors. One of these benefits
is the ease of use of the MOSFET devices in high
frequency switching applications. The MOSFET
transistors are simpler to drive because their
control electrode is isolated from the current
conducting silicon, therefore a continuous ON
current is not required. Once the MOSFET
transistors are turned-on, their drive current is
practically zero. Also, the controlling charge and
accordingly the storage time in the MOSFET
transistors is greatly reduced. This basically
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eliminates the design trade-off between on state
voltage drop – which is inversely proportional to
excess control charge – and turn-off time. As a
result, MOSFET technology promises to use
much simpler and more efficient drive circuits
with significant economic benefits compared to
bipolar devices.
Furthermore, it is important to highlight
especially for power applications, that MOSFETs
have a resistive nature. The voltage drop across
the drain source terminals of a MOSFET is a
linear function of the current flowing in the
semiconductor. This linear relationship is
characterized by the RDS(on) of the MOSFET and
known as the on-resistance. On-resistance is
constant for a given gate-to-source voltage and
temperature of the device. As opposed to the
-2.2mV/°C temperature coefficient of a p-n
junction, the MOSFETs exhibit a positive
temperature coefficient of approximately
0.7%/°C to 1%/°C. This positive temperature
coefficient of the MOSFET makes it an ideal
candidate for parallel operation in higher power
applications where using a single device would
not be practical or possible. Due to the positive
TC of the channel resistance, parallel connected
MOSFETs tend to share the current evenly
among themselves. This current sharing works
automatically in MOSFETs since the positive TC
acts as a slow negative feedback system. The
device carrying a higher current will heat up
more – don’t forget that the drain to source
voltages are equal – and the higher temperature
will increase its RDS(on) value. The increasing
resistance will cause the current to decrease,
therefore the temperature to drop. Eventually, an
equilibrium is reached where the parallel
connected devices carry similar current levels.
Initial tolerance in RDS(on) values and different
junction to ambient thermal resistances can cause
significant – up to 30% – error in current
distribution.