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How were the active parts "buried deep inside the semiconductor crystal" of BJTs? Surely doping is a surface phenomenon?
by RachelF 2y ago
How were the active parts "buried deep inside the semiconductor crystal" of BJTs?
Surely doping is a surface phenomenon?
- adrian_b 2y agoDoping 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 2y agoThank you for that detailed explanation!