I'm designing a low residual phase noise buffer amplifier for a 10MHz clock that uses an NPN transistor as the gain element. The NPN ends up being the most significant contributor to the noise performance of this circuit. Using the noise analysis in the Art of Electronics, the input-referred noise ends up being determined by the base-spreading resistance, DC current gain, 1/f corner frequency, and transition frequency (ignoring other important parameters such as collector current, temperature and source impedance). The Art of Electronics provides a really nice table of low-noise BJTs. Several standouts (among others) are the Sanyo 2SC3601 and 2SC3955, which have very low base-spreading resistance and good high-frequency operation (low input capacitance and high transition frequency). Unfortunately, these, along with many other good transistors are obsolete.
Why did OnSemi discontinue these? Same question probably applies to other companies and transistors. I've noticed a similar trend for good RF transistors. Is the market for high-quality discrete transistors too small? Do companies not use discrete transistors for low-noise applications anymore? Why is this? Is it because they can build their own ICs with the transistors they want, or are happy with the good, but not quite as good, performance of modern opamps? Any other reasons? I know a lot of these are through-hole parts, but presumably it isn't hard to place the die in a SMT package if the problem is that people aren't buying through-hole anymore.
I'm sure I'm not the only one who feels this way, but this is incredibly frustrating as a designer without the capabilities to fabricate custom transistors. Also, I know this isn't a particularly good reason, but I like being able to use discrete transistors in designs, rather than having a lot of the interesting work done for me with opamps.
I should also mention that the ZTX618 and ZTX851 are often used as replacements in applications where low rbb is desired, but they have pretty horrible input capacitance, so the high-frequency performance is significantly worse than the Sanyo NPNs mentioned above.