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Silver Mica capacitors in the radios I've been working on with an older friend have the same issues... they worked fine for sufficiently long that they survived

Silver Mica capacitors in the radios I've been working on with an older friend have the same issues... they worked fine for sufficiently long that they survived years of their first owner... and now 50 years later, they just cause "thunderstorm" noises in the audio output. (If you have such an old radio, take out all the tubes, but keep the B+, then measure the voltage across all the resistors.... the ones in the path to a leaky cap will have a voltage, because of the current flow, and the rest won't.) I'm amazed that any transistors subject to more than 50 C for a year survive.

adrian_b 3y ago
The ancient bipolar transistors (also the junction-FET transistors), where the active parts of the device were buried deep inside the semiconductor crystal, could easily achieve lifetimes of many decades, even at temperatures as high as 200 Celsius degrees (when designed for such temperatures). When the surface of a silicon crystal is properly passivated, its interior is extremely stable and impervious to external influences. With modern MOS transistors, where the active parts are extremely small and located on the surfaces of the devices, it is very difficult to achieve long lifetimes unless they are strongly derated, so they do not have to support high thermal or electrical stresses.
- RachelF 3y ago
  How were the active parts "buried deep inside the semiconductor crystal" of BJTs? Surely doping is a surface phenomenon?
  - adrian_b 3y ago
    Doping is not a surface phenomenon. Doping a semiconductor crystal means that some of the atoms throughout the volume of the semiconductor crystal are substituted with different kinds of atoms. The "field effect" of a MOS gate or the rectifying effect in a Schottky (metal-semiconductor) diode are examples of surface phenomena. The dopants are introduced through the surface (except when a crystal is grown from a doped melt or epitaxially), but they are driven by thermal diffusion to higher depths. In bipolar transistors and in JFETs the active parts are junctions inside the semiconductor crystal between regions of the same single crystal where different dopants predominate. These junctions can be microns away from the surface and in old devices or in power devices even up to hundreds of microns away from the surface of the crystal. In MOS transistors the active parts are at the interfaces between the semiconductor crystal and layers that are grown or deposited over it and moreover their properties are very sensitive to the presence or absence of various ions that can migrate during the lifetime of the device through those thin layers.
    - RachelF 3y ago
      Thank you for that detailed explanation!
fuzzfactor 3y ago
What part of the circuit are you finding silvered mica capacitors?

adrian_b 3y ago

The ancient bipolar transistors (also the junction-FET transistors), where the active parts of the device were buried deep inside the semiconductor crystal, could easily achieve lifetimes of many decades, even at temperatures as high as 200 Celsius degrees (when designed for such temperatures). When the surface of a silicon crystal is properly passivated, its interior is extremely stable and impervious to external influences. With modern MOS transistors, where the active parts are extremely small and located on the surfaces of the devices, it is very difficult to achieve long lifetimes unless they are strongly derated, so they do not have to support high thermal or electrical stresses.

RachelF 3y ago
How were the active parts "buried deep inside the semiconductor crystal" of BJTs? Surely doping is a surface phenomenon?
- adrian_b 3y ago
  Doping is not a surface phenomenon. Doping a semiconductor crystal means that some of the atoms throughout the volume of the semiconductor crystal are substituted with different kinds of atoms. The "field effect" of a MOS gate or the rectifying effect in a Schottky (metal-semiconductor) diode are examples of surface phenomena. The dopants are introduced through the surface (except when a crystal is grown from a doped melt or epitaxially), but they are driven by thermal diffusion to higher depths. In bipolar transistors and in JFETs the active parts are junctions inside the semiconductor crystal between regions of the same single crystal where different dopants predominate. These junctions can be microns away from the surface and in old devices or in power devices even up to hundreds of microns away from the surface of the crystal. In MOS transistors the active parts are at the interfaces between the semiconductor crystal and layers that are grown or deposited over it and moreover their properties are very sensitive to the presence or absence of various ions that can migrate during the lifetime of the device through those thin layers.
  - RachelF 3y ago
    Thank you for that detailed explanation!

fuzzfactor 3y ago

What part of the circuit are you finding silvered mica capacitors?