Power Electronics Handbook

Thyristors

37

frequency of operation (f

max

) is given by equation (1.14), where W

c

is the

length of the channel in micrometres and V

c

is the carrier velocity in cm/s:

y

c

/max =

2^W~

C

(1

*

14)

Figure

1.21(b)

shows a short-channel arrangement which is formed

by

p

and n diffusions, hence it is often called a double-diffused (DMOS)

transistor. As the drain-source voltage is increased the depletion layer

spreads into the drain drift region since it is lightly doped. Since the drain

drift region is increased the breakdown voltage is higher, but the

drain-source resistance also increases. This can be reduced by using several

parallel cells on the semiconducto ...