589Microwave Transistors
14.3.1.7 Limitations of Transistors
Johnson studied the limitations of transistors in view of the following
assumptions:
• The maximum possible velocity of carriers in a semiconductor is the
saturated drift velocity (
υ
s
), which is on the order of 6 × 10
6
cm/s for
electrons and holes in silicon and germanium.
• The maximum electric eld (E
m
) sustained in a semiconductor
without dielectric breakdown is about 10
5
V/cm in germanium and
2 × 10
5
V/cm in silicon.
• The maximum current of a microwave power transistor is limited
by its base width.
In view of these assumptions, the four derived relations specify (i) voltage–
frequency limitation, (ii) current–frequency limitation, (iii) power–frequency
limitation ...