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ECE331 Homework #4 Solution
1.
Explain the concept of electron effective mass. At what scenario the effective
mass is the actual electron mass? Use equations if necessary.
±
Effective mass is the term used to describe the total effect of forces outside
and inside of a crystal lattice. In reference to the band diagram, effective
mass is inversely proportional to the band curvature. That is, for high band
curvature the effective mass is low, while for low curvature the effective
mass is high. Also, at the band minima the effective mass is positive, and at
the band maxima (the top of the valence band for example) the effective
mass is negative. Negative effective electron mass is the positive effective
mass of a hole.
±
Effective mass will equal the electron mass only if there are no interior
crystal forces, thus only in free space the effective mass is equal to the free
electron mass. One equation for effective mass is:
k
E
m
∂
∂
=
2
2
*
}
2.
Explain the concept of direct and indirect semiconductors. Use EK diagram
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 Spring '09
 RAJAN
 0k, 0 K, effective mass

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