JFET2.122000/08/042008/05/28 17:01:18.353 GMT-5BillWilsonwlw@madriver.netBillWilsonwlw@madriver.netElizabethGregoryelizabeth.gregory@gmail.comJeffreyMSilvermanJSilverman@astro.berkeley.eduGerardWysockigerardw@rice.eduScottWKravitzswkravitz@gmail.comJFETtransistorsHow JFET transistors work.
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 . 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 looks
like . We also show the biasing here.
JFET
Biasing a JFET
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
Vds 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 . 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
Pinch-Off
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
10-7Fcm2. A typical MOSFET gate might be 1 μm long by 20
μm wide, and so it would have a gate area of 20
μm2
or 210-7cm2.
Thus, the total gate capacitance is only about
10-14F.