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I'm trying to understand how the P-N junction works. I understand how it behaves, but I'm not sure if that is all there is to know, because I'm not sure what the doped semiconductors are on their own. They are always mentioned in context of P-N junctions.

But what are their properties on their own? Is it that on their own, they are basically just semiconductors? And doping them doesn't change how they are on their own, but only how they behave when used in P-N junctions?

Would it make sense to think of doping as of adding some new property that wasn't there before? Kind of like magnetising, which also doesn't just tune properties, it basically creates a new property by changing the composition of the material.

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  • $\begingroup$ There are other junctions like the p-i-n junction. Doped semiconductors behave like a more conductive version of their original material. $\endgroup$ – NaOH Nov 25 '16 at 3:37
  • $\begingroup$ Right, so the question becomes more general: are doped semiconductors used on their own, or always in relation to some differing material? $\endgroup$ – psycho brm Nov 25 '16 at 3:48
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    $\begingroup$ That is a hard question to answer, because a device would have to be placed in relation to something else to make it useful. There are examples such as MOS (metal-oxide-semiconductor) capacitors which use only one kind of doping for the semiconductor, and used in CCDs. In the past, doped polysilicon materials were used as gate material. They aren't terribly interesting on their own, just a more conductive semiconductor. $\endgroup$ – NaOH Nov 25 '16 at 10:53
  • $\begingroup$ Great, MOS capacitors seem to show that doping creates a new property in semiconductor, that isn't terribly interesting on their own, and their advantage is in how they now behave in relationship with their neighboring materials. $\endgroup$ – psycho brm Nov 25 '16 at 22:51
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Doped semiconductors on their own can be used in Hall-effect sensors. The concentration of charge carriers (electrons or holes) is given by the dopant concentration, quite independent of temperature. So if there is a current through the semiconductor, one can know the drift velocity of the charge carriers. It will be many orders of magnitude higher than in metals, because the concentration of charge carriers is so much lower. In a magnetic field, the Lorentz force will be higher because of the high drift velocity in semiconductors, and this will give a larger Hall voltage.

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Doping a semiconductor like silicon means that you introduce into the crystal lattice an atomic species that has a surplus of 1 electron or a deficiency of one electron compared to the four valence electrons (and bonds)of the host lattice. This has the effect that the semiconductor(region) has either a preponderance of electrons in the conduction band or a preponderance of holes in the valence band. Thus you call the semiconductor (region) n-type or p-type, respectively. The other semiconductor properties stay, in essence, the same. Junction between n- and p-type regions are essential for creating semiconductor devices which can be used for many different electronic functions, like bipolar transistors (npn or pnp structures) that can be used as amplifiers and switches, diodes (pn-junctions) as rectifiers, or thyristor (npnp-structures) as power switches, metal-oxide-semiconductor transistors as amplifiers and switches and in highly integrated integrated circuits and memories.

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  • $\begingroup$ If the other properties stay the same, does that mean that doping adds new property that is only useful in relation to different semiconductors? $\endgroup$ – psycho brm Nov 25 '16 at 4:14
  • $\begingroup$ The doping changes only the dominant carrier type and the combination of n- and p-type regions produces internal energy barriers which enable structures producing special electronic functions for device applications. $\endgroup$ – freecharly Nov 25 '16 at 4:19
  • $\begingroup$ And those structures require cooperation between our doped semiconductor and something else, right? Because on its own it's doping is irrelevant? $\endgroup$ – psycho brm Nov 25 '16 at 4:48
  • $\begingroup$ The doping is not irrelevant. It produces regions of higher or lower potential (electrochemical potential) so that at the junctions potential barriers are formed for the charge carriers that are essential for the creation of different electronic device functions. $\endgroup$ – freecharly Nov 25 '16 at 5:26
  • $\begingroup$ Yes, at junctions. So it seems like doping is only useful for junctions. $\endgroup$ – psycho brm Nov 25 '16 at 22:49

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