Page 79 - Troubleshooting Analog Circuits

66 6. Understanding Diodes and Their Problems

lWmA
10 mA

1nA
1WY
0 0.2 0.4 0.8 0.8 1 .o 1.2 1 A 1.6 1.8 2.0 2.2
VFORWARD
Figure 6. I. The diode made up of a transistor’s emitter has high conductance over a wide range of cur-
rents. All the other diodes you can buy have inferior conductances, and they are just about all
different.. . . surprise. For details, see Appendix E.

rectifiers are also available; they have been designed for fast switching-regulator and
other high-frequency applications. They don’t have quite as low VFS as Schottky
diodes and are not quite as fast, but they are available with high reverse-voltage rat-
ings and thus are useful for certain switch-mode circuit topologies that impress large
flyback voltages on diodes.
When you reverse-bias these various diodes, ah, that is where you start to see even
more wild dissimilarities. For example, the guaranteed reverse-current specification,
I,,, for many types of diodes is 25 nA max at 25 “C. When you measure them,
many of these devices actually have merely 50 or 100 pA of leakage. But the popular
1N914 and its close cousin, the 1N4148, actually do have about 10 or 15 nA of
leakage at room temperature because of their gold doping. So although these diodes
are inexpensive and popular, it’s wrong to use them in low-leakage circuits since
they’re much leakier than other diodes with the same leakage specs.
Why, then, do some low-leakage diodes have the same mediocre 25-nA leakage
spec as the 1N914? Diode manufacturers set the test and price at the level most people
want to pay, because automatic test equipment can test at the 25-nA level-but no
lower-without slowing down. If you want a diode characterized and tested for 100
pA or better, you have to pay extra for the slow-speed testing. Of course, high-con-
ductance diodes such as Schottkys, germaniums, and large rectifiers have much larger
reverse leakage currents than do signal diodes, but that’s not normally a problem.
If you want a very-low-leakage diode, use a transistor’s collector-base junction
instead of a discrete diode (Ref. 1). The popular 2N930 or 2N3707 have low leakage,
typically. Some 2N3904s do, too, but some of these are gold-doped and are leakier.
The plastic-packaged parts are at least as good as the TO-18 hermetic ones. You can
easily find such “diodes” having less than 1 pA leakage even at 7 V, or 10 pA at 50
V. Although this low leakage is not guaranteed, it’s usually quite consistent.
However, this c-b diode generally doesn’t turn ON or OFF very quickly.
Another source of ultra-low-leakage diodes are the 2N4117A and the PN4117A,
-18 A, and -19 A. These devices are JFETS with very small junctions, so leakages
well below 0.1 pA are standard with 1 .O pA max, guaranteed-not bad for a $0.40
part. The capacitances are small, too.