How JFET transistor works
There is a lot more that we could do with field effect devices, but it
is probably time to move on to new topics. For one final point
however, we might just look at something called the JFET, or junction
field effect transistor. The JFET structure looks like
1. It consists of a piece of p-type silicon, into which two
n-type regions have been diffused. However, instead of being both on
the same surface, as with a MOSFET, the two regions are opposite one
another on either side of the crystal. In cross-section, the JFET
2. We also show the biasing here.
The two n-regions are connected together, and are reverse biased with
respect to the p-type substrate. A second battery,
Vds is used to pull current
out of the source, by applying a negative voltage between the drain
and the source. The reverse biased n-p junctions creates a depletion
region which extends into the p-type material through which the holes
travel as they go from source to drain (a channel?). By adjusting the
value of Vgs,
one can make the depletion region smaller or larger, thus increasing
or decreasing the drain current.
The observant student will also note that the polarity of the
battery makes it so that there is more reverse bias across the p-n
junctions at the drain end of the channel than at the source end.
Thus, a more accurate depiction of the JFET would be what is shown in
3. When the drain/source voltage gets large enough, the two
depletion regions will join together, and, just as with the MOSFET,
the channel pinches off, as shown in
Depletion region controls current
Surprising as it may seem, when you work out the equations which
describe how the depletion region extends with Vgs
and how the pinch-off mechanism changes ID,
you end up with behavior, and equations, which are quite similar to
those of a depletion-mode MOSFET.
Using JFETs is a little more cumbersome than a normal MOSFET. You must
make sure that the gate-substrate junction always remains reverse
biased, and since the JFET can only be a depletion-mode device, you
have to have a voltage on the gate if you want to turn the transistor
off. The JFET does have one advantage over the MOSFET however.
A while back we calculated the value for Cox
the oxide capacitance and found that it was on the order of
A typical MOSFET gate might be 1 μm long by 20 μm wide, and so it
would have a gate area of 20 μm2
the total gate capacitance is only about 10-14F.