Is a large hFE value a sign of damage?

Question

I have been testing a couple of BFY50 transistors that I salvaged using my multimeter. One of them reports an hFE of ~109 (this looks consistent with the data sheet value of 112). The multimeter reports a value of ~470 for the other. Is this an indication that the transistor is damaged?

Answer 1

EDIT: this particular transistor is planar epitaxial, so the aging is not affecting it as bad as older technology transistors. But the age of device is still a major factor.

There is observation, that old transistors, made by diffusion-alloy technology get some abnormally high hFe after several decades of shelf life. The doping material slowly moved inside silicon (or germanium) and caused change of hFe.

This does not necessarily mean, that device is damaged by age. But it means that other characteristics changed as well. The device does not correspond to its info from datasheet anymore and in some sense each such specimen is something unique and undocumented. If you need predictable results, do not use ones made with diffusion-alloy technology (approx pre 1970 ?)

For hoobyist, very old semiconductors can be valuable as voltage references or temperature/radiation sensors. The value is in stability, since every expected drift already happened and device characteristics are not changing as fast as in freshly manufactured devices. There is for example a reserved stock of ovenized voltage references specifically produced in extra quantities few decades ago to maintain supply of 8.5 digit voltmeters for few centuries ahead.

Answer 2

If the gain reported is way above the maximum value in the datasheet, then it's quite likely the transistor is damaged. Test the resistance from collector to emitter with no power applied, if it's damaged then you will probably see a lowish value (it should be very high >> 1 MΩ)