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