Many electronic devices rely on so-called p-n junctions, where p-type and n-typedoped regions of a semiconductor are placed in contact with one another. One com-mon example is the light emitting diode (LED). Here are some data for the Il-V semiconductor GaN that you will need to answer thisquestion. (a) What are the ionisation energies (in eV) for donor and acceptor dopants in GaN? (b) When n-type and p-type material are placed in direct contact, the Fermi levelsin the two regions must become equal. How large will the relative shift betweenthe bandgaps the p-type and n-type GaN regions (in eV) be to allow this to takeplace at a temperature of 300 K?[5] (c) To operate an LED, a voltage must be applied to overcome this offset. Once thatis done, if an electron from the conduction band of the n-type region recombineswith a hole from the valence band of the p-type region in this GaN p-n junction,in what part of the electromagnetic spectrum will the emitted photon lie?[2] (d) Why can't a junction between an electron-like metal (e.g. Na) and a hole-likemetal (e.g. Al) be used to make an LED?[4]

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Many electronic devices rely on so-called p-n junctions, where p-type and n-typedoped regions of a semiconductor are placed in contact with one another. One com-mon example is the light emitting diode (LED). Here are some data for the Il-V semiconductor GaN that you will need to answer thisquestion. (a) What are the