I need to replace a (panasonic) transistor and read the datasheet to possibly replace it with something else. However in the datasheet they call this transistor a "Silicon NPN Epitaxial Planar Type". What is "Epitaxial Planar Type", is this a special type with special characteristics? The specifications doesn't look very special to me. Can I replace it with a more generic transistor?


Update 10/07/2022

Thank you all for all explanations received. Very nice. To find a more common replacement I found this site. I need to replace 3 types of transistors.

  • 2SD1423
  • 2SC3311
  • 2SA1309

By entering the modelnumbers in the search box I found an equivalent of these ancient transistors: replacement table

Very helpful!

  • 2
    \$\begingroup\$ It’s just a manufacturing method. If the electrical parameters are within your specifications, how they got there does not matter. \$\endgroup\$
    – winny
    Oct 6, 2022 at 21:54
  • 7
    \$\begingroup\$ "Epitaxial Planar Type" means "It's 1965, and we have really modern transistors to sell you". \$\endgroup\$
    – TimWescott
    Oct 6, 2022 at 22:03
  • \$\begingroup\$ This was exciting new technology in the 60s. Today it's hard to find any that aren't made that way. \$\endgroup\$
    – Hearth
    Oct 7, 2022 at 0:37
  • \$\begingroup\$ In those were the days, Fairchild was the only Exitaxial Planer guy, making 2N2222 etc. Nowadays, everybody is so. References: (1) Epitaxy - Wikipedia en.wikipedia.org/wiki/Epitaxy, (2) Fairchild Planar process (1959) - Wikipedia en.wikipedia.org/wiki/Planar_process \$\endgroup\$
    – tlfong01
    Oct 7, 2022 at 6:34
  • \$\begingroup\$ @TimWescott The device I need to repair was made in the 90s. But okay, I get the point you want to make. ;-) \$\endgroup\$
    – Codebeat
    Oct 7, 2022 at 21:07

4 Answers 4


It's not special these days, no; epitaxy is a common process. It's a description of how it's made. Compare with double and triple-diffused types, which form the junctions by diffusing dopants into the die; epitaxy means growing silicon directly on top of the die, which can include dopants itself, giving different shapes and doping profiles. Diffusion can also be done afterward to get a hybrid between the two.

The particular things to look for in substitution, is to match the pinout, meet or exceed all voltage, current, power and temperature ratings, and get similar hFE, fT, noise figure, etc. Some of these parameters are optional (you don't always see fT, or switching transistors will rate their speed in terms of switching times instead; and you rarely see noise figure or related parameters), and the choice of a device specifying them may indicate requirements for them (like transistors at the input of an amplifier will generally prefer a noise spec).


Time for a history lesson!

The first transistors that would be recognised as bipolar junction transistors are the grown-junction transistors.¹ These were made by growing a germanium (later silicon) crystal while changing the composition of the melt it's grown out of by adding dopants. This works, but the transistors produced have poor characteristics by modern standards and they're quite expensive.

Later, alloy-junction transistors would be made. These were produced by taking a piece of doped semiconductor and fusing beads of dopant metals of the opposite type onto it, where the alloyed areas would act as the opposite-type semiconductor to form a PNP or NPN structure. This allowed much better performance than the grown-junction transistors, but it was still an inherently slow process as each transistor had to be made individually.

Next came diffused-base transistors. They started with a substrate of appropriately doped semiconductor and then exposed it to a hot gaseous dopant material in an otherwise evacuated chamber, creating a region of opposite type on the surface. One of the two output terminals, either the emitter or collector, was still applied like an alloy-junction transistor, but the other was not.

The double-diffused transistor was produced shortly after, when it was realized that the diffusion process could be done with two dopants at once of opposite types, relying on the different diffusion rates of dopants of different atomic mass to get things to line up properly internally. This produced very good transistors for the time, and similar processes are still in use for certain things today.

The early double-diffused transistors were known as mesa transistors, because the transistor itself was formed on a raised part of the substrate² (Or possibly parts of the substrate were etched away after the transistor was formed; I'm having trouble finding clear sources). This caused some reliability problems, which were largely solved by the invention of the planar transistor in 1959.

The planar transistor was a major breakthrough, though not particularly due to its planar shape. In previous transistors, the oxide that formed during processing (due to heat and exposure to oxygen) was removed out of a concern that it could contaminate the transistor. This actually proved counterproductive (at least in silicon transistors, which were soon to take over from germanium entirely); the planar process, in which the oxide is just left there, proved to be much more reliable as the oxide actually protects the junction from outside contaminants. Planar transistors were the first ones that didn't need to be hermetically sealed in metal can packages (though it would be a while before plastic packages became the norm).

Planar transistors are still the type of BJT used most often today, but there's one more step before we get to the devices you're asking about:

Separate from the other developments here, a process called epitaxy was invented in the early 50s and perfected (or at least brought to a commercially viable state) in 1960. Rather than starting from a uniform semiconductor, you can grow a very thin layer of differently-doped semiconductor crystal on top of your existing semiconductor wafer. This may sound similar to the grown-junction transistor mentioned earlier, but that used a process that couldn't be controlled very precisely--epitaxy involves growing the crystal through chemical vapor deposition, and produces more uniform and thinner films than the grown-junction process ever could. This was combined with the planar process to produce epitaxial planar transistors, which had the much better reliability of planar transistors combined with excellent performance (both improved switching speed and better blocking voltage) from the epitaxial process.

Epitaxial planar BJTs are still common today, some sixty years after their invention. Most discrete transistors you buy today will be either epitaxial planar BJTs or a completely different type of transistor called a VDMOS, a type of MOSFET. The processes for making integrated circuits also developed out of the epitaxial planar process (though they changed quite a lot over time).


¹ The first transistors ever made were point-contact transistors, but they're somewhat different from BJTs in ways I personally don't fully understand, so I'm not going into any detail here.

² I'm not quite sure why it was like this; none of the sources I can find say. It may have been a way to isolate adjacent transistors from each other so that they could make many on the same wafer, keeping any sawing to the areas between transistors instead of trying to cut one transistor into two (which could cause problems on the cut edges). That is only an educated guess, however.

Sources used: Primarily the Computer History Museum's timeline of semiconductor breakthroughs and the various articles linked in the History section of the Wikipedia page on BJTs and their citations.

  • \$\begingroup\$ Impressive, thanks! \$\endgroup\$
    – Codebeat
    Oct 7, 2022 at 20:53
  • \$\begingroup\$ @Codebeat You're welcome! The history of electronics is fascinating, as are all the bizarre devices people came up with en route to modern ones (and continue to come up with, in the hope of making them even better). \$\endgroup\$
    – Hearth
    Oct 8, 2022 at 0:44

It's a very standard modern transistor construction, nothing to be concerned with.

For substitution, pay attention to the voltage and current ratings (which are modest) ft of 150MHz (also modest but too high ft in a replacement could lead to issues) and the hFE rank:

Q 160 ~ 260

R 210 ~ 340

S 290 ~ 460

Make sure your replacement has hFE in the appropriate range compared to the original.

Also note the SC-72 package and the Japanese standard E-C-B pin order.


These days "epitaxial planar" is the same as "bog-standard". It results in homogenous structures and good frequency and noise characteristics. The larger homogeneity makes modern issues of the ancient RCA 2N3055 NPN workhorse more susceptible to the "Secondary Breakdown" thermal runaway condition of local thermal hotspots focusing current density to destruction.

For things like old power amps, that may end up a drawback. And current-issue 2N3055 all use epitaxy. It's not commercially viable to depend on other kinds of wafers these days.


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