IGBT Modules - Technologies, Driver and Application (Second Edition) - page 49

weak inversion. As soon as the concentration of the minority carriers exceeds
that of themajority carriers in deeper zones of the substrate, it is called strong
inversion. In strong inversion, the external voltage has exceeded the threshold
Fig. 1.26
Accumulation, depletion and inversion (not to scale)
Since the gate is insulated from the other MOSFET layers by a layer of silicon dioxide,
the MOSFET only needs a current charging the gate at turn-on and discharging it at
turn-off. Disregarding parasitic influences, the MOSFET does not need any control
power when it is stationary, unlikeaBJT.
As described in the introduction, the vertical MOSFET structure is the preferred layer
design for power applications. This is because the channel can bemade shorter than it
does for a lateral structure. For the same voltage rating of the component it has a lower
channel bulk resistance, which leads to less forward loss. While in a lateral design the
blocking capability is essentially determined by the length of the channel, this can be
determined by the substrate in a vertical design, considerably increasing the cross-
An increase in dielectric strength also increases the vertical size of the MOSFET. The
bulk resistance R
between the drain and source terminals is determined by the
gauge and doping when theMOSFET is turned on. In applications that require blocking
voltages U
of several 100V and high currents at the same time, the increased bulk
resistance in comparison to a BJT has a negative effect on the conduction loss, which
increases to the squareof the current.
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