Page 98 - Troubleshooting Analog Circuits

Fabrication Structures Make a Difference 85

F
EMITTER EMllTER

P-EPT N-EPI

N-EPI

N++SUBSTR*TE
NIISUBSTRATE
I I I I I17 I I I I I I I I I I I I I I I I I I I
COLLECTOR CONNECTIONS
(a) (b)
Figure 7.5. The characteristics of power transistors depend on their fabrication structure. The epitaxial-
base structure (a) takes advantage of the properties of several different epitaxial layers to
achieve good beta, good speed, low saturation, small die size, and low cost. This structure
involves mesa etching, which accounts for the slopes at the die edges. Planar power transis-
tors (b) can achieve very small geometries, small base-widths, and high-frequency responses,
but they’re less rugged than epitaxial-base types, in terms of Forward-Biassed SOA.

wrote all about how these devices had even more Safe Operating Area than the epi-
base device, so you might want to order these if you wanted a “really gutsy” tran-
sistor for driving inductive loads. Unfortunately, these transistors were obsolescent
and obsolete; they were slow (perhaps 0.5 MHz of fa), had a large die area, and were
expensive. For example, although these transistors required only one diffusion, in
some cases this diffusion had to run 20 hours. Because of all these technical reasons,
sales shrank until, in the last 2 years, all the single-diffused power transistors have
been discontinued.
So it’s kind of academic to talk about the old single-diffused parts, (see Figure 7.6)
but I included a mention here just for historical interest. Also, 1 included it because if
you looked in my old EDN write-up and then tried to buy the devices I recommended.
you would meet with incredulity. You might begin to question the sanity of yourself,
or the salesman, or of Pease. When I inquired into the availability of these parts, I
talked to many sales people who had no idea what I was talking about. Finally when I
was able to talk to technical people, they explained why these transistors were not
available-they admitted that I was not dreaming, but that the parts had been discon-
tinued recently. These engineers at some of the major power-transistor manufacturers
were quite helpful as they explained that newer geometries helped planar power tran-
sistors approach the safe area of the other older types without sacrificing the planar
advantages of speed. Also, power MOSFETs had even wider amounts of SOA, and
their prices have been dropping, and they were able to take over many new tasks
where the planars did not have enough SOA. So the puzzle all fits together.
There is still one tricky problem. Originally the old 2N3771 was a single-diffused
part. If you wanted to buy an epi-base part, that was the MJ377 1. But now if you
order a 2N3771, you get the epi-base part, which does meet and exceed the JEDEC
2N3771 specs. It just exceeds them a lot more than you would expect-like, the
current-gain-bandwidth is 10 or 20 X higher. So, if you try to replace an old
2N377 1 with a new 2N377 1, please be aware that they are probably not very similar
at all.