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This is for my AP Chemistry course and the question is:

Which of the following elements is important in the semiconductor industry to improve the conductivity of Ge?

  • K
  • Ga
  • Al
  • Mg

It seems like any of the above could be a correct answer. The answer given was Ga. What about gallium would make it the most appropriate answer?

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    $\begingroup$ It is a dopant. But then, Al is also. So, bad question by the AP folks... $\endgroup$ – Jon Custer Jan 5 '16 at 2:12
  • $\begingroup$ I've heard that $\ce {Na}$ poisons any semiconductor by, loosely speaking, giving the outermost electron away too easily to far away sites, countering p-doping. This would even more be true to $\ce K$. I would relay like to see some answers that are more substantial than my hearsay . $\endgroup$ – Gyro Gearloose Jan 5 '16 at 20:16
  • $\begingroup$ Most likely the key factor favoring Ga is just a similar size, electronegativity valence shell structure, which the other choices do not have. To get the band structure you want you just want to modify the electron count without geometric distortions or giving up good covalent overlap in the diamond-like structure. $\endgroup$ – Oscar Lanzi Sep 6 '18 at 19:45
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This questions has two parts if you look closely:

Which of the following elements is important in the semiconductor industry to improve the conductivity of Ge?

All of the above will improve the conductivity of $\ce{Ge}$. This is because germanium is a semiconductor which conducts electricity via the movement of holes and electrons. Since germanium is tetravalent and gallium is trivalent, doping germanium with gallium creates a p-type semiconductor which is exponentially rich in holes, thus increasing the conductivity. This mechanism holds true for aluminum, potassium and magnesium meaning all of them can increase the conductivity of germanium

Which of the following elements is important in the semiconductor industry to improve the conductivity of Ge?

This part is what makes gallium special. Gallium and germanium are sequential elements in atomic number. They have very similar atomic radii. Gallium is also very much less reactive than the other elements mentioned. These characteristics allow gallium to dope the germanium to a higher concentration without forming a new undesired solid phase. Sodium and magnesium form germanides with germanium and will not dope as shown by the phase diagram. Aluminum will dope, but its smaller atomic radius can alter and strain the crystal structure of germanium much more than gallium which can affect band gap.

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  • $\begingroup$ Thank you for the good answer to the question, but I see that you know a whole lot more about it than was asked in this question. Where can I (and others) learn more about that topic? $\endgroup$ – Gyro Gearloose Jan 5 '16 at 21:24

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