Are doped seminconductors used for anything else than P-N junctions? 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.
 A: 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.
A: 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.
