Page 96 - Rashid, Power Electronics Handbook

82 I. Batarseh

GATE SOURCE MOSFET and, unlike the BJT, MOSFET devices do not suffer
from second breakdown voltages and sharing current in
Metal
parallel devices is possible.
SiO
2
n + n + 6.5.2 Internal Body Diode
P P The modern power MOSFET has an internal diode called a
body diode connected between the source and the drain as
n -
shown in Fig. 6.8a. This diode provides a reverse direction for
n + the drain current, allowing a bidirectional switch implementa-
tion. Even though the MOSFET body diode has adequate
current and switching speed ratings, in some power electronic
applications that require the use of ultra-fast diodes, an
DRAIN
(a) external fast recovery diode is added in antiparallel fashion
GATE after blocking the body diode by a slow recovery diode as
shown in Fig. 6.8b.
SOURCE
6.5.3 Internal Capacitors
SiO 2
Another important parameter that affect the MOSFET switch-
n n
ing behavior are the parasitic capacitances between the device's
p p
three terminals, namely, gate-to-source, C , gate-to-drain C gd
gs
and drain-to-source (C ) capacitances as shown in Fig. 6.9a.
ds
n
The values of these capacitances are nonlinear and a function
n
D
Body diode
DRAIN
(b)
FIGURE 6.7 (a) Vertical cross-sectional view for a power MOSFET; G
and (b) simpli®ed representation.

The value of R can be signi®cant and varies between tens
DSðonÞ
of milliohms and a few ohms for low-voltage and high-voltage S
MOSFETS, respectively. The on-state resistance is an impor- (a)
tant data sheet parameter, because it determines the forward
voltage drop across the device and its total power losses.
Unlike the current-controlled bipolar device, which requires
base current to allow the current to ¯ow in the collector, the
power MOSFET device is a voltage-controlled unipolar device
and requires only a small amount of input (gate) current. As a
result, it requires less drive power than the BJT. However, it is
a nonlatching current like the BJT, that is, a gate source voltage
must be maintained. Moreover, as only majority carriers
contribute to the current ¯ow, MOSFETs surpass all other
devices in switching speed, which switching speeds can exceed
a few megahertz. Comparing the BJT and the MOSFET, the
BJT has greater power handling capabilities and smaller
switching speed, while the MOSFET device has less power
handling capabilities and relatively fast switching speed. The
MOSFET device has a higher on-state resistor than the bipolar
transistor. Another difference is that the BJT parameters are FIGURE 6.8 (a) MOSFET internal body diode; and (b) implementa-
more sensitive to junction temperature when compared to the tion of a fast body diode.