Week 8 - Lecture 23 for EEE132 Electronic Device

For this lecture, we have covered two sub-sections, namely the example of GaAs for direct gap recombination (sub-section 4.3.2) and 4.3.2 Indirect Recombination based on Trapping

After the class, one student asked me to explain trapping as given in the textbook, which is on page 128. I did not explain to him clearly as I have looked at the wrong angles. I mentioned about recombination process for direct bandgap material such as GaAs, which is the transition of an electron from the conduction band to valence band; thereby, eliminating the EHP (TRUE!). Also, in the discussion I did try to relate the recombination center to the Fermi level of the extrinsic materials but I forgot that this is for the excitation process, which is related to optical absorption for both GaAs (direct) and Si (indirect). Hence, having the Fermi level of a an filled state near the conduction for n-type material is not the right description for recombination as this is really an excitation process. Similarly, for the p-type material, having empty states near the valence band is not the right description for hole capture. Due to this misconception, we will definitely think the arrow shown in Fig. 4-8 as wrong (NOT TRUE!).

We can definitely relate the recombination center, which is assumed to be filled with electrons at equilibrium, as the process of capturing holes and electrons and since the center is filled with electrons the first occurance should be the hole capture as shown by the "(a)" arrow in Fig. 4-8. We can follow the arrow as if the hole is flowing from the valence band to the recombination center, leaving the center with deficient of an electron. To complete the recombination process, therefore requires for the capturing of the opposite carrier, i.e. an electron moving from the conduction band to the recombination center. Another alternate view is to think of hole moving upward to the conduction band.