A Necessary Device for the Modern IC
instead of the reverse-biased
junction that was used in the
JFET. This one is sometimes
called an “IGFET,” because of
the insulated gate. It is a normal- 5
ly off device, which has to be 20 4
turned on by some sort of action,
and therefore it is referred to as C 3
an “enhancement mode” device.
(The IGFET can also be made in 2
a depletion mode configuration.) 1 B
In the figure, if the pot is
turned down to zero voltage,
then the battery current tending
to go through both the lightbulb
and the transistor will be
stopped by one of the PN junctions. In this diagram, it is the
upper one, which is reverse-biased. (Initially, the dashed
line and the N region in the middle are not present.)
If the potentiometer arrow is raised, and a positive
potential is now applied to the gate, holes in the P-type silicon are repelled, causing this region to become N-type (as
indicated by the N in parenthesis). Now there is no PN
junction directly in the path between the upper and lower
N-type regions, because it is all one continuous N-type
ID = DRAIN CURRENT, ma
VDS = DRAIN-TO-SOURCE VOLTAGE
Figure 4. Characteristic curves for a 2N7000
MOSFET, with a load line.
Figure 5. CMOS transistor pair.
The current is extremely low when
there is no input signal.
region (drawn as a vertical bar, with the dashed-line as one
edge). This transistor is also N-channel, because the electricity goes through N-type silicon when it is turned on.
If the reader wishes to get some experience with the
MOSFET, an ammeter can be placed as in Figure 3, to show
that no measurable current flows into the gate, even when the
bulb is lit. In this diagram, the multimeter has been switched
to measure current, and it is moved to the gate lead. (This cir-