Posted by Ali on May 25, 2013, 1:05 pm
Hi, Question 4 from A4, 2000 asks you to find the density of electrons in the conduction band of an instrinsic semiconductor. It then asks you to calculate the band gap, Eg, of Silicon, given the Hall coefficients of undoped Silicon at two different temperatures, assuming only one type of carrier. This is straight forward, I got Eg=1.27eV, but the very last part of the question says: "The optical absorption of silicon as a function of photon energy exhibits large increases at 1.2 eV and 3.3 eV. Comment on your value for Eg in relation to these optical data." Not so sure how to interpret this. Maybe the 1.2eV absorption peak corresponds to the calculated value of the band gap, then what is the 3.3eV peak? From the way the question's phrased, I'm expecting some kind of discrepancy between the calculated and given value due to the fact that Silicon actually conducts via electrons and holes... but why these two absorption peaks? I guess the correct way to calculate Eg is to let the Hall coefficient be R=1/(n+p)e, whereas the question expects you to use R=1/ne [n, p density of electrons and holes respectively], but I can't link this with the given values. Any help appreciated, Ali 
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