Exercise 9.2-1

Quick Questions to

	Here are some quick questions
		Describe some measures necessary of you want to produce a high-efficiency LED. Use hand drawings to illustrate (at least) three major points.
		How can you best define a recombination volume?
		Draw the band diagram of an Np junction in equilibrium with the bandgap of the N-type semiconductor about twice the size of the p-type material. Discuss choice you make if necessary.
		Discuss advantages an problems of hetero junctions for light emitting devices
		Describe the working principle of MBE and what it means in terms of realization.
		Describe the mechanisms of fundamental absorption (FA) and stimulated emission (SE) with the aid of a band diagram. What kind of relation between the rates R_Se and R_FA, i.e. the number of events per second (and cm²) must you have if amplification of light is to take place?
		What is the meaning of "inversion" in the context of a semiconductor Laser?
		What is "pumping" in the context of a Laser and why is a semiconductor very well suited for efficient "pumping"?
		How can you turn a light amplifier into a Laser? What does it mean technically for processing your semiconductor?

Here is a somewhat "longer" question:

The picture shows a schematic "to scale" drawing of a simple blue Laser diode and the relevant material parameters in the "master" diagram.

What kind of approximate band gap energy can you assign to the various layers? Draw a schematic band diagram of this kind of situation.

Which layer is the light-producing one?

What is the function of the three relatively thin layers in the center region?

What could be the function of the two thicker pure GaN layers?

Why is the whole structure on an Al₂O₃ substrate (= sapphire) and what is the electrical problem encountered?

What might be the problem necessitating a "buffer" layer between the Al₂O₃ substrate and the stack of functional layers?

Where are the mirrors necessary for a Laser?

What is obviously used for n- or p-doping? Make a guess as to why the very thin central layer contains Mg and Si as dopants