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EE 2 HW 2 09Spring-PS2-Solns

EE 2 HW 2 09Spring-PS2-Solns - EE2 Problem Set#2 Solutions...

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EE2 Problem Set#2 Solutions Question #1: (5 points) Describe the difference between unipolar and bipolar conduction. Unipolar conduction involves only one type of charge carrier (electrons or holes), while bipolar conduction involves both electrons and holes. Question #2: (5 points) The density of states in the conduction band increases with the square root of energy. Why then, can we state that most of the electrons are near the band edge, E c ? The number of carriers is equal to the integral (over energy) of the density of states multiplied by the probability that the state is filled (Fermi-Dirac or Maxwell-Boltzmann statistics). The probability exponentially decreases with increasing energy, and the exponential decreases much more rapidly than the square root increases. Question #3: (15 points): (a) Show that the Fermi function is symmetric about E F , i.e., f E F + Δ E ( ) = 1 f E F − Δ E ( ) (b) Show that when E E F ( ) >> kT , f E ( ) e E E F kT .
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