The 2N3904 (NPN) and the complementary 2N3906 (PNP) are classic jellybean transistors, approaching 60 years of age and still going strong. Without exaggeration, DigiKey has over 10 million of these devices and variants in stock. See for yourself by searching “3904 BJT” and “3906 BJT”.
They don’t get more ubiquitous than that!
In relative terms, the 2N3904/06 aren’t the most powerful or highest performing transistors. In many respects, the jellybean 2N2222 and its complementary 2N2907 are better devices. However, there is a “horses for courses” consideration where the lower-cost 2N3904 family members continue to perform admirably in their market niche. One reason for the continued momentum is the all-plastic TO-92 packaging while the 2N2222/2907 were originally offered in the older TO-18 metal can. This cost advantage certainly led to its early adoption.
Figure 1: Image of type 2N3906 TO-92 packaged transistors.
History
Transistors are numbered consecutively according to the Joint Electron Device Engineering Council (JEDEC). This gives us a convenient method to approximate when a particular transistor was produced, as we can compare numbers across multiple manufacturers. For example, if the largest number seen in 1965 is 2N3818, and then we see the 2N3904 in a 1966 handbook, we can be reasonably confident that the 2N3904 was released in 1966.
The earliest references I could find to the 2N3904 family are located in Motorola’s 1966 Semiconductor Data Book. The 2N3903 and 2N3904 are described as general-purpose NPN silicon annular transistors. The 2N2905 and 2N3906 appear several pages later. Note that the 2N3905 and 2N3906 are complementary with the 2N3903 and 2N3904, respectively.
When compared to the 2N3904, the 2N3903 is almost identical, but has lower input impedance, lower gain, and lower high-frequency response. I’m led to believe, but cannot verify, that the 2N3903 and 2N3905 are lower spec versions of the 2N3904 and 2N3906. Perhaps Motorola performed a binning process to select the premium parts. With improved manufacturing processes, this binning would be unnecessary. Today the 2N3903 and 2N3905 are rare with minimal to no variants.
State of electronics in 1968
Let’s go back to 1968 and explore state-of-the-art electronics. This was an exciting time as solid-state transistors and JFETs were in the process of replacing the older vacuum tube electronics. Amateur radio enthusiasts, affectionately called Hams, were producing cutting-edge design. A publication called QST published by the American Radio Relay League perfectly captures the situation. The April 1968 edition of QST is contained in this folder and here from worldradiohistory.com.
DigiKey’s History
The April 1968 edition of QST holds historical significance for DigiKey. On page 22 we find an article describing “An Integrated-Circuit Electronic Keyer.” This little circuit offered cost savings to Hams communicating with Morse code – especially the “speed demons” who wanted something faster than a straight key. Instead of using an expensive mechanical vibrating key, they could use simple low-cost paddles and the new Digital Keyer. It’s a familiar story of an electronics circuit replacing a mechanical assembly, although some Hams prefer the look and feel of the mechanical keyer.
The DigiKey was a popular device. Dr. Ron Stordahl (K0UXQ) responded by providing kits (Figure 3), and so began DigiKey as a mail order company in 1972, leading the DigiKey of today.
With regards to the 2N3904 family, we are close. The Digi Keyer used a 2N4888, which is described in the article as “a new silicon p-n-p made by Fairchild Semiconductor.” The 2N3906 could have been used, though with the whirlwind of change, it would have been desirable to use the latest offerings. After all, QST projects represent the cutting edge of technology.
History has a funny way of changing things. Today the 2N4888 is a $15 part occasionally available in online auctions, while the 2N3906 variant cost pennies.
Figure 3: Early DigiKey advertisement from a page 162 of the Dec 1968 edition of QST.
Historical parallels
Exploring the 1968 QST magazines, we find the 2N3906 used in several projects along with the 2N3904. Fast forward 55 years and my 100th anniversary of the ARRL handbook has the same transistors. It’s as if history has frozen – or better yet, those jellybean parts have become interwoven into the fabric of time. Same story for other classic parts such as the 1N4148 and 2N2222.
Where do I find the 2N3904 and 2N3906 transistors?
The DigiKey tools are designed to help you find a needle in a haystack. You could search directly for the 2N3904 or the 2N3906. You will quickly locate a few parts. However, that’s not the way to search for the Jellybean parts, instead search using a root number and type keyword such as “3904 BJT” or “3906 BJT”. This will return a large number of components including the original TO-92 parts plus a number of surface mount derivatives including the latest devices such as the OnSemi NST3904MX2T5G with the X2DFN3 packaging as shown in Figure 3.
Figure 3: Image of the OnSemi NST3904MX2T5G transistor in a X2DFN3 package.
Precautions for the 2N3904 and 2N3906
These devices are robust and long-lived if used within their rated specifications. Be sure to operate well within the design maximum specifications. For example, the ABSOLUTE MAXIMUM collector current is 200 mA. I recommend a wide safety margin with no more than 150 mA. If the enclosure is hot, consider derating this to 100 mA. The same applies for power dissipation. The surface-mount device is rated for 350 mW with a clown-shoe-sized 1.6-inch by 1.6-inch pad. A dissipation of 75 mW would be more appropriate. See this link for information about surface mount heatsinks. One additional consideration is flyback voltage. This article explores some of the device considerations.
Parting thoughts
Without question, the 2N3904 and 2N3906 are popular. DigiKey has over 10 million of these devices and variants in stock. Chances are high that you will be able to find what you need today and many decades into the future. There is something to be said about cost, as there is stiff competition in this penny market. There is also something to be said about providing robust solutions against supply chain disruptions.
For these reasons and more, the jellybean 2N3904 and 2N3906 are highly desirable components.
Please share your comments and questions in the comments below. Do these lessons resonate with the 2020 supply chain crunch?
Best wishes,
APDahlen
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About this author
Aaron Dahlen, LCDR USCG (Ret.), serves as an application engineer at DigiKey. He has a unique electronics and automation foundation built over a 27-year military career as a technician and engineer which was further enhanced by 12 years of teaching (interwoven). With an MSEE degree from Minnesota State University, Mankato, Dahlen has taught in an ABET-accredited EE program, served as the program coordinator for an EET program, and taught component-level repair to military electronics technicians. Dahlen has returned to his Northern Minnesota home and thoroughly enjoys researching and writing articles such as this.