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UNIVERSITY OF CALIFORNIA
Department of Electrical Engineering and Computer Sciences EE130 Fall 2004 Prof. Subramanian Test #
- Consider the following elements:
Fe: Atomic number: 26, Atomic weight: 56 S: Atomic number: 16, Atomic weight: 32
a) Write the electronic configurations (1s 2 , 2s 2 , …) for both elements
i) Fe: 1s 2 2s 2 2p^6 3s 2 3p^6 4s^2 3d^6
ii) S: 1s 2 2s 2 2p^6 3s 2 3p^4
b) Iron is among the most abundant materials on the planet. Would you expect the following iron-based materials to be semiconductors? Give reasons.
i) Iron Pyrites (FeS 2 , also known as “Fool’s Gold”): Based on non-hybridized model above, you wouldn’t expect this to be a semiconductor. However, it turns out that this actually is a semiconductor due to hybridization.
ii) Ferric Chloride (FeCl 3 ): This will be strongly ionic, and therefore is not a semiconductor.
iii) Ferrous Chloride (FeCl 2 ): This will be strongly ionic, and therefore is not a semiconductor.
c) TiO 2 is in fact a semiconductor. Why? Give your answer in terms of the expected orbital configurations.
Ti is 1s 2 2s^2 2p 6 3s 2 3p^6 4s 2 3d^2 , while O is 1s 2 2s^2 2p^4_. To form a “group-IV-like” configuration, based on what we have learned in class (the reality is a little more complicated), Ti could contribute the 4s and 3d electrons to the two O atoms to form a fully stable configuration_
d) TiO 2 has a bandgap of approximately 3eV. All else, being equal, would you expect it to be more or less conductive at room temperature than (give reasons):
i) Silicon Probably less conductive, since it should have less intrinsic electrons and holes due to the larger bandgap
ii) Diamond Probably more conductive, since it should have more intrinsic carriers due to the smaller EG
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- In this question, we are going to investigate the effect of temperature on various material and device parameters.
a) Suppose I take a piece of silicon doped with 10^17 cm-3^ Phosphorus, and heat it from 300K to 400K. Which carrier concentration shows the greater % change – electrons or holes? Why? The minority carrier always shows the greater change in carrier concentrations as temperature is increased, due to greater intrinsic carrier generation. In other words, holes will show the larger increase.
b) What is the effect of the above increase in temperature on carrier mobility, i.e., does it increase or decrease? Why? Mobility will decrease due to increased phonon scattering.
c) Based on your analysis above, would you generally expect the carrier lifetime to increase or decrease? Give reasons. Since there is greater vibration, we would generally expect a smaller carrier lifetime due to more frequent scattering.
d) What will be the impact on diffusion coefficient? Again, give reasons. From the graphs in the notes, mobility decreases from ~700cm^2 /V-s to ~400, while T increases by a smaller fraction. Therefore, we would generally expect D to decrease slightly in this case.
- You are given a random piece of silicon with a resistivity of >10^4 ohm-cm and an electron mobility of ~300cm^2 /V-s.
a) Is the material n-type, p-type or essentially intrinsic? Why?
The material is essentially intrinsic, as evidenced by the large resistivity.
b) What is the concentration of donors and acceptors? Give reasons for your answers.
The total doping is probably ~10^18 (from the mobility vs. doping plot). Since the material is intrinsic, the donor and acceptor concentration are equal, and in that ballpark.
c) Suppose I were to cool the silicon down to 150K. Would the mobility increase or decrease? Why?
From the mobility vs. temperature curve, for a doping of 10^18 , the mobility would actually decrease going down to 150K due to increased impurity scattering.
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