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Which of the options is correct for a Si diode?

a) It conducts if the voltage applied to its anode is greater than that applied to its cathode

b) If the voltage applied to the cathode is greater than that applied to the anode, it conducts

c) Conducts if the anode voltage is 0.7 V more positive than the cathode voltage

d) Allows current to flow even if the anode and cathode voltages are equal

In the face of such a question, I think that option a is correct because if the voltage applied to the anode (positive terminal) is greater than the voltage applied to the cathode (negative terminal), the diode conducts. because we know that current flows from anode to cathode. Is this correct? Is there a point I am missing?

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    \$\begingroup\$ It depends on which model/simplifications you are using. \$\endgroup\$
    – vir
    Commented Jul 6 at 14:49
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    \$\begingroup\$ What diode model(s) have been introduced so far? Might one of these answers be more accurate than the others? \$\endgroup\$
    – Tim Williams
    Commented Jul 6 at 14:49
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    \$\begingroup\$ Any of the above could be true depending on operating conditions and test instrumentation used. \$\endgroup\$
    – John Doty
    Commented Jul 6 at 15:31
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    \$\begingroup\$ @Neil_UK Shine light on it, a current will flow. Thermal currents will flow in the dark. \$\endgroup\$
    – John Doty
    Commented Jul 6 at 15:37
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    \$\begingroup\$ The correct answers are what you have being taught on the course about these subjects that these questions are asked. For a real world diode, all the claims are true. For the diode taught in your course, we can't know. \$\endgroup\$
    – Justme
    Commented Jul 7 at 16:39

2 Answers 2

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"a" is correct for an "ideal" diode, but given that the question specifically says "Si diode", which is non-ideal in the sense that there is a forward voltage drop (and internal resistance), "c" is probably the expected answer.

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“a”, “b” and “c” are all true for a Si diode. It conducts with all forward voltages, the current is exponentially dependent on voltage. It also conducts in reverse - a little bit, per the reverse saturation current. A Si diode conducts as predicted at -0.2V, +0.2V and +0.7V.

The whole problem is that the word “conducts” is so fuzzy. Conducts how? If they mean “conducts according to Shockley’s model, then “a”, “b” and “c” are true.

If they mean “conducts enough you can detect it with a light bulb in series with the diode”, then “c” is correct at room temperature. If the diode is very, very cold, say -50°C or less, it may not conduct enough at 0.7V to light a larger light bulb.

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