In a p-n junction diode 1. In reverse bias mode the bulk resistance is dominant.
2. The junction impedance is variable.
Statements 1 and 2 are correct.
Statements 1 and 2 are correct — option (A), as recorded in the supplied answer key.
This item must be flagged. On the physics, statement 1 is wrong, and a candidate reasoning correctly would answer option (C). The keyed option is given here because it is the key; the correct account follows so the concept is not learnt wrongly.
Statement 2 is certainly correct. The junction impedance of a diode is not a constant — it varies with the operating point, and dramatically so. In forward bias the dynamic resistance falls as current rises,
\(r_{d}=\dfrac{\eta V_{T}}{I_{D}}\)
so at 1 mA it is a few tens of ohms and at 10 mA a few ohms. The junction capacitance varies too, and in opposite senses on the two sides :
| Bias | Dominant capacitance | Behaviour |
|---|---|---|
| Forward | Diffusion capacitance | Rises with current — stored minority charge |
| Reverse | Transition (depletion) capacitance | Falls as reverse voltage rises, \(C_{T}\propto V_{R}^{-1/2}\) for an abrupt junction — the varactor effect |
Why statement 1 is wrong. It has the two bias conditions the wrong way round :
| Bias | Which resistance dominates | Order of magnitude |
|---|---|---|
| Forward | Bulk (body) resistance of the neutral p and n regions | A few ohms — once the junction barrier is overcome, the junction contributes little and the leftover series resistance sets the slope of the I-V curve |
| Reverse | Junction resistance | Megohms — the depletion region is wide and carrier-free, so it swamps the few ohms of bulk material entirely |
The reasoning is simply that resistances in series are dominated by the largest. In reverse bias the junction is the largest by five or six orders of magnitude, so it is the junction, not the bulk, that dominates. Bulk resistance becomes the limiting factor only in heavy forward conduction, which is why a real diode's forward characteristic straightens into a line of slope \(1/r_{B}\) at high current.
Note on the recorded answer. The answer stored here follows the supplied key. On the physics the defensible answer is option (C) — statement 1 wrong, statement 2 correct.
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Important Questions from PN Junction
Diode junction breakdowns above 5 V are caused by:
The leakage current in a pn junction is of the order of:
For a PN junction, we have
A. Width of depletion layer
B. Junction barrier voltage
C. Reverse leakage current
Which of the above parameters will decrease when the temperature of the junction rises?
The depletion region consists of:
In the P-N junction, the barrier voltage
