Ok so this is the question along with its four option given in my text book

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I can't figure out what the answer should be as it is not stated that is the semiconductor forward biased or reverse biased. I assumed it to be forward biased so according to that i assume the answer should be $C$. Please tell am i correct or wrong and if i am wrong what is the correct solution to it.

Thanks Akash

  • \$\begingroup\$ how can you please explain it a little bit \$\endgroup\$
    – Deiknymi
    Apr 15 '14 at 2:45
  • 3
    \$\begingroup\$ There is no junction to be forward- or reverse-biased. It's just a chunk of semiconductor. \$\endgroup\$
    – Dave Tweed
    Apr 15 '14 at 4:47

Either this question is poorly phrased, or there is additional information related to it.

We know that increasing the voltage will increase the current through both the conductor and the semiconductor, therefore the question here is: which of the currents will increase more?

We know that the conductor's current increases linearly with voltage, but what behavior will semiconductor show?

This is the point where we can't say anything without additional information:

1) Are metal-semiconductor contacts ohmic or rectifying?

2) Is semiconductor intrinsic or extrinsic (is it doped)?

3) Should we consider second order effects (like conductivity change due to heating, mobility degradation under heavy bias, etc.)?

Assuming that there is no additional information provided with the question, I would guess that the contacts are ohmic and their resistance may be neglected, the semiconductor bulk is intrinsic and we should not consider second order effects. In this case, since the current is equal for the same bias, the resistances of semiconductor and conductor are equal, and the current-voltage characteristics will be the same, and the correct answer will be C.

  • \$\begingroup\$ exactly what i am saying it lacks information thanks for strengthening my assumption \$\endgroup\$
    – Deiknymi
    Apr 16 '14 at 17:10

I see the only difference is that conductor have positive thermoresistance coefficient and semiconductor have negative one. So, unless in thermostabilized environment, you should get more current on semiconductor (a) as they both heated up by current.

UPDATE (based on comments) That could be true only with several assumptions, like:

  • Conductor is a typical electronic conductor, like most metals. This excludes electrolytes, being ionic conductors. Typical electronic semiconductor have valence and conduction bands overlapped, so its resistance primarily determined by thermal motion of crystalline grid, and thus rises with temperature.

  • Semiconductor is a typical semiconductor, like silicon or germanium, and is not doped. Typical semiconductor have small band-gap, and its resistance primarily determined by density of conduction electrons and holes, which rises with temperature causing resistance to fall.

  • Circuit is not in superconductivity state.

  • Current does not cause phase transitions, like material melting.

  • Semiconductor have ohmic, rather than rectifying contacts.

  • ...and so on.

These assumptions are most common, but in case of textbook context may matter.

  • \$\begingroup\$ So are you saying that all materials that can conduct current have a positive temperature coefficient of resistance, except the semiconductor materials? \$\endgroup\$
    – Joe Hass
    Apr 15 '14 at 13:29
  • \$\begingroup\$ Doped silicon normally has a positive temperature coefficient, which tends to be higher than that of elemental metals (but I'm not sure this is always true, it may depend on doping level or something else). This principle is used in some temperature sensors. You can also look at ancient datasheets for UJT "interbase resistance" (just a bar of doped Si) tempco if you have doubts. I recall numbers like +0.9%/K vs. about +0.385%/K for Cu. \$\endgroup\$ Apr 15 '14 at 14:31
  • \$\begingroup\$ @JoeHass, No, he means that metals generally have a positive tempco, while intrinsic semiconductors have a negative tempco; that this is the historical distinction between them we are normally taught in a fudnamental materials science course, and that this is probably the context in which the question was asked. On the other hand I think we all agree the question is not well posed to stand on its own outside that context. \$\endgroup\$
    – The Photon
    Apr 15 '14 at 16:27
  • \$\begingroup\$ @ThePhoton My point is that the problem never stated that the conductor was made of metal. \$\endgroup\$
    – Joe Hass
    Apr 15 '14 at 16:44
  • \$\begingroup\$ @JoeHass, I agree it's a poorly stated question. Maybe it's translated from another language where the common words for "conductor" and "metal" are the same? \$\endgroup\$
    – The Photon
    Apr 15 '14 at 16:48

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