Standard switching transistors: 2N3904 and copies

A project log for Evaluating Transistors for Bipolar Logic (RTL)

Experiments on optimizing discrete logic gates based on bipolar transistors

timTim 04/01/2020 at 13:220 Comments

The 2N3904 is one of the most widely spread switching transistors. There are countless SMT variants available from at least 15+ different vendors in all kinds of packages. Mouser lists more than 50 variants, LCSC more than 80.
I picked four devices in SOT23 package at random, trying to balance between more established manufacturers and lesser known chinese sources. Ironically the Nexperia part was the cheapest.
The table above lists the four devices and their respective datasheet values. Siko admits to having a slightly slower device while the other datasheets look like they were copied from each other.

Ring oscillator results are shown above. Minimum tpd is 20ns for the Nexperia device. The others achieve around 40ns, while Toshiba is dead last with >200ns at typical operating conditions.

The extreme spread of these results is quite irritating - it shows how devices can easily differ by factors despite being specified the same. The only consolation is that Nexperias device is again the fastest, confirming that it may be a good idea to stick to better known vendors.

On the other hand, the performance of the Toshiba candidate is utterly irritating, as it will hardly be able to meet datasheet specs. I double checked the package marking. The only sane explanation could be that this is a counterfeit device.

In general, the PMBT3904 also seems to be a good candidate for bipolar logic. It is about 2-3x slower than the PMBT2369, but also 4x cheaper and available in smaller packages. More attention needs to be spent on correct biasing though, since the device is far less forgiving than the PMBT2369 when driven into saturation.