# VE311 Electronic Circuits Homework 02 solved

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1. Pure aluminum has a resistivity of 2.83×10−6Ωcm. Based on its resistivity, should
aluminum be classified as an insulator, semiconductor, or conductor?
2. The maximum drift velocities of electrons and holes in silicon are approximately
107
cm/s. What are the electron and hole current densities if n = 1018/cm3 and
p = 102 /cm3
? What is the total current density (J)?
3. Calculate the intrinsic carrier densities in silicon and germanium at (a) 77 K, (b)
300 K, and (c) 500 K. Use the information according to Figure 1:
Figure 1: Intrinsec carrier densities for several materials
4. At what temperature will intrinsic silicon become an insulator, based on the
definitions in Figure 2? Assume that µn = 2000 cm2 /V·s and µp=750 cm2/Vs.
Figure 2: Material properties for some materials
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5. Suppose a semiconductor has ND = 1016 /cm3
, NA = 5×1016/cm3
, and ni =
1011/cm3
. What are the electron and hole concentrations?
6. InP is composed of equal atoms of indium and phosphorus in a lattice similar to
that of silicon. (a) Suppose a germanium atom replaces an indium atom in the
lattice. Do you expect the germanium atom to behave as a donor or acceptor
impurity? Why? (b) Suppose a germanium atom replaces a phosphorus atom in
the lattice. Do you expect the germanium atom to behave as a donor or acceptor
impurity? Explain.
7. Silicon is doped with 1016 boron atoms/cm3
. How many boron atoms will be in
a silicon region that is 0.5 µm long, 5 µm wide and 0.5 µm deep?
8. It is conceptually possible to produce extrinsic silicon with a higher resistivity
than that of intrinsic silicon. How would this occur?
9. Make a table of the values of thermal voltage VT forT = 50 K,75 K,100 K,150
K,200 K,250 K, 300 K, 350 K, and 400 K.
10. The electron concentration in a region of silicon is shown in Fig. 3. If the electron
mobility is 350 cm2/V · s and the width WB = 0.5 µm, determine the electron
diffusion current density. Assume room temperature.
Figure 3: Electron density movement ish
11. Figure 4 gives the electron and hole concentrations in a 2-µm-wide region of silicon. In addition, there is a constant electric field of 20 V/cm present in the
sample. What is the total current density at x = 0? What are the individual
drift and diffusion components of the hole and electron current densities at x =
1.0 µm?
Assume that the electron and hole mobilities are 350 and 150 cm2/V · s, respectively.
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Figure 4: Electron density movement ish
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