Explain the following, giving reasons: (a) A doped semiconductor is electrically neutral. (b) In a p-n junctio

Question

Q. Explain the following, giving reasons:

(a) A doped semiconductor is electrically neutral.

(b) In a p-n junction under equilibrium, there is no net current.

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Concept understanding — P-N Junction Formation

P-N Junction Formation: From Intuition to Precision

Imagine two rooms connected by a door. One room is filled with people who have extra energy (they want to give it away), and the other room is filled with people who are missing energy (they want to take it). The moment you open the door, what happens? People rush from the high-energy room to the low-energy room until both rooms reach a balance. That rush, and the final balanced state, is the essence of a P-N junction.

In a semiconductor, the "people" are charge carriers: electrons (negative charge) and holes (the absence of an electron, which behaves like a positive charge). A P-type semiconductor has an excess of holes (positive carriers), and an N-type semiconductor has an excess of electrons (negative carriers). When you bring them together, they don't just sit still — they interact.

The Intuitive Picture

Take a P-type crystal and an N-type crystal. At the instant they touch, there is a huge concentration difference: lots of holes on the P-side, lots of electrons on the N-side. Nature hates steep gradients, so carriers begin to diffuse — they move from where they are abundant to where they are scarce.

Electrons from the N-side cross into the P-side.
Holes from the P-side cross into the N-side.

But here's the catch: when an electron from the N-side meets a hole on the P-side, they recombine — the electron fills the hole, and both disappear as free carriers. This recombination doesn't happen everywhere; it happens in a narrow region near the interface, called the depletion region (or space-charge region).

Why "depletion"? Because in that region, free electrons and free holes have been used up. All that remains are the fixed, immovable ions: positive donor ions on the N-side (which lost their electron) and negative acceptor ions on the P-side (which gained an electron). These fixed charges create an electric field that points from the N-side (positive ions) to the P-side (negative ions).

This electric field is crucial. It acts like a bouncer: it pushes electrons back toward the N-side and holes back toward the P-side. This drift motion opposes the initial diffusion. Eventually, the diffusion current (driven by concentration difference) exactly balances the drift current (driven by the electric field). The system reaches thermal equilibrium — no net current flows.

The Precise Statement

\text{A P-N junction is formed by bringing P-type and N-type semiconductors into intimate contact. At equilibrium, a depletion region of fixed ions creates a built-in electric field that prevents further net diffusion of carriers.}

More formally:

Diffusion: Majority carriers (electrons from N-side, holes from P-side) diffuse across the junction due to the concentration gradient.
Recombination: These carriers recombine near the interface, leaving behind fixed ionized impurities (donors on N-side, acceptors on P-side).
Depletion region: A region devoid of free carriers, containing only fixed charges, forms at the junction.
Built-in electric field: The fixed charges create an electric field ( $\vec{E}$ ) pointing from N to P.
Equilibrium: The drift current due to $\vec{E}$ exactly cancels the diffusion current. The net current is zero.

The width of the depletion region ( $W$ ) depends on the doping concentrations. For a one-sided junction (heavily doped on one side), the depletion region extends mostly into the lightly doped side. …

Why this formula?

Why the P-N Junction Forms: The Physics Behind the Barrier

A p-n junction isn't just two pieces of semiconductor stuck together. The key to understanding it is this: nature hates sharp gradients in carrier concentration. When you bring p-type (excess holes) and n-type (excess electrons) material into contact, carriers immediately begin to diffuse across the junction — holes from p to n, electrons from n to p.

This diffusion is the engine that drives everything else.

Step 1: Diffusion Creates a Depletion Region

As holes leave the p-side, they leave behind fixed, negatively charged acceptor ions ( $A^-$ ). As electrons leave the n-side, they leave behind fixed, positively charged donor ions ( $D^+$ ). These ions are immobile — they're locked in the crystal lattice.

The region near the junction that gets stripped of mobile carriers is called the depletion region (or space-charge region). It contains only fixed ions, creating an electric field that points from the n-side (positive ions) toward the p-side (negative ions).

Watch out

Do not confuse "depletion" with "no charge." The depletion region is highly charged — it's just that the charge is from fixed ions, not mobile carriers.

Step 2: The Electric Field Opposes Diffusion

The built-in electric field $E$ exerts a force on any mobile carrier that tries to cross:

Holes (positive) feel a force pushing them back toward the p-side.
Electrons (negative) feel a force pushing them back toward the n-side.

This field grows stronger as more carriers diffuse and more ions are uncovered. Eventually, the field becomes strong enough that the drift current (carriers swept by the field) exactly balances the diffusion current (carriers moving due to concentration gradient). At this point, the net current is zero — thermal equilibrium is reached.

Step 3: The Built-in Potential Barrier

Because the electric field exists over a distance, there is a potential difference across the depletion region. This is the built-in potential $V_0$ (also called $V_{bi}$ ). It represents the energy barrier that a majority carrier must overcome to cross to the other side.

V_0 = \frac{kT}{q} \ln\left(\frac{N_A N_D}{n_i^2}\right)

Where:

$k$ = Boltzmann constant
$T$ = absolute temperature
$q$ = electron charge magnitude
$N_A$ = acceptor doping concentration (p-side)
$N_D$ = donor doping concentration (n-side)
$n_i$ = intrinsic carrier concentration

Why This Formula Holds: The Derivation

The derivation comes from equating the Fermi levels on both sides. In equilibrium, the Fermi level must be constant throughout the entire structure.

On the p-side, the Fermi level $E_F$ lies close to the valence band. The position relative to the intrinsic Fermi level $E_i$ is:

E_F - E_i = kT \ln\left(\frac{N_A}{n_i}\right) \quad \text{(for p-type)}

On the n-side, the Fermi level lies close to the conduction band:

E_F - E_i = -kT \ln\left(\frac{N_D}{n_i}\right) \quad \text{(for n-type)}

The difference in $E_i$ between the two sides (which is the same as the difference in $E_F$ between the two sides before contact) must be accommodated by the built-in potential. The total band bending $qV_0$ equals this difference:

qV_0 = \left[ kT \ln\left(\frac{N_A}{n_i}\right) \right] - \left[ -kT \ln\left(\frac{N_D}{n_i}\right) \right]

$qV_0 = kT \left[ \ln\left(\frac{N_A}{n_i}\right) + \ln\left(\frac{N_D}{n_i}\right) \right]$ …

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A doped semiconductor is electrically neutral because dopant atoms replace host atoms and the charges of created carriers are balanced by immobile ionized dopants; in a p-n junction at equilibrium, diffusion and drift currents balance, resulting in no net current; and in a diode under reverse bias, the current is due to thermally generated minority carriers, whose number is largely independent of the applied voltage.

(a) A doped semiconductor is electrically neutral.

The electrical neutrality of a doped semiconductor stems from the fundamental principle of charge conservation during the doping process. Doping involves intentionally introducing impurity atoms into a pure (intrinsic) semiconductor crystal lattice.

Intrinsic Semiconductor Neutrality: An intrinsic semiconductor, like pure silicon, is electrically neutral. Each silicon atom has 14 protons in its nucleus and 14 electrons orbiting it, making each atom neutral. The crystal as a whole is a collection of these neutral atoms.
Dopant Atom Neutrality: When a dopant atom (e.g., phosphorus for n-type, boron for p-type) is introduced, it replaces a host semiconductor atom in the crystal lattice. Crucially, the dopant atom itself is electrically neutral, possessing an equal number of protons and electrons. No net charge is added to the crystal by introducing the dopant atom.
N-type Doping (Donor Impurities):
- Consider a pentavalent impurity like phosphorus (P) in silicon (Si). Phosphorus has 5 valence electrons. When it replaces a silicon atom, four of its valence electrons form covalent bonds with the surrounding silicon atoms.
- The fifth valence electron is very loosely bound to the phosphorus atom. At room temperature, this electron gains enough thermal energy to break free and move into the conduction band, becoming a free electron (a mobile negative charge carrier).
- When the phosphorus atom loses this electron, it becomes a fixed positive ion ( $\text{P}^+$ ) within the crystal lattice. This ion is immobile.
- The crystal now contains a mobile free electron and an immobile positive ion. The positive charge of the ion exactly balances the negative charge of the free electron. The overall crystal remains electrically neutral.
P-type Doping (Acceptor Impurities):
- Consider a trivalent impurity like boron (B) in silicon. Boron has 3 valence electrons. When it replaces a silicon atom, it forms three covalent bonds with surrounding silicon atoms, but one bond remains incomplete, creating a hole (a mobile positive charge carrier).
- An electron from a neighboring silicon atom can move to fill this hole, effectively making the hole appear to move.
- When the boron atom accepts an electron to complete its bonds, it becomes a fixed negative ion ( $\text{B}^-$ ) within the crystal lattice. This ion is immobile.
- The crystal now contains a mobile hole (which represents the absence of an electron, thus a positive charge) and an immobile negative ion. The negative charge of the ion exactly balances the positive charge of the hole. The overall crystal remains electrically neutral.

Important

While doping creates mobile charge carriers (electrons or holes), these carriers are always balanced by an equal and opposite charge of immobile ionized donor or acceptor atoms fixed within the crystal lattice. Therefore, the semiconductor material as a whole remains electrically neutral.

(b) In a p-n junction under equilibrium, there is no net current.

A p-n junction is formed by joining p-type and n-type semiconductor materials. When this junction is in equilibrium, it means there is no external voltage applied, and the system is in a stable state where all opposing processes are perfectly balanced.

Initial Diffusion: When the p-type and n-type materials are brought into contact, there are high concentrations of holes in the p-region and electrons in the n-region. Due to this concentration gradient, holes from the p-side diffuse across the junction to the n-side, and electrons from the n-side diffuse across the junction to the p-side. This movement of majority carriers constitutes a diffusion current ( $I_{diff}$ ).
Formation of Depletion Region and Built-in Electric Field:
- As electrons diffuse from the n-side to the p-side, they leave behind immobile positively charged donor ions ( $\text{N_D}^+$ ) in the n-region near the junction.
- Similarly, as holes diffuse from the p-side to the n-side, they leave behind immobile negatively charged acceptor ions ( $\text{N_A}^-$ ) in the p-region near the junction.
- These immobile ions create a region devoid of mobile charge carriers, known as the depletion region.
- The fixed positive and negative charges in the depletion region establish an internal electric field ( $\vec{E}$ ) directed from the n-side (positive ions) to the p-side (negative ions). This electric field creates a potential difference, known as the built-in potential barrier ( $V_0$ ).
Drift Current: The built-in electric field opposes the further diffusion of majority carriers. However, it aids the movement of minority carriers.
- Minority electrons generated thermally in the p-region that reach the depletion region are swept by the electric field to the n-side.
- Minority holes generated thermally in the n-region that reach the depletion region are swept by the electric field to the p-side.
- This movement of minority carriers under the influence of the electric field constitutes a drift current ( $I_{drift}$ ).
Equilibrium Condition: As diffusion continues, the built-in electric field and potential barrier grow stronger. This increasing barrier increasingly opposes the diffusion current. Eventually, a point is reached where the rate at which majority carriers diffuse across the junction (diffusion current) is exactly balanced by the rate at which minority carriers drift across the junction due to the built-in electric field (drift current).

I_{diff} = I_{drift}

No Net Current: Since the diffusion current and the drift current are equal in magnitude and flow in opposite directions, the net current across the p-n junction at equilibrium is zero.

I_{net} = I_{diff} - I_{drift} = 0

Note

The equilibrium state is dynamic; individual charge carriers are constantly moving, but the net flow of charge is zero. This balance ensures the stability of the junction without external influence.

(c) In a diode, the reverse current is practically independent of the applied voltage.

When a p-n junction diode is reverse-biased, the p-side is connected to the negative terminal and the n-side to the positive terminal of an external voltage source. …

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Same / Similar Concept — real previous-year questions on the same or a closely similar concept, not this exact question.

Showing the 12 most recent of 18 on this concept.

CBSE 2026Set 55/1/11 markMCQ
$Q. In an unbiased p-n junction, at equilibrium, which of the following statements is true? (A) Diffusion current is zero but drift current exists. (B) Diffusion current exists but drift current is zero. (C) Diffusion and drift currents are equal and opposite. (D) Both the diffusion and drift currents exist but are unequal.$
›Reveal solutionSolution
At equilibrium in an unbiased p-n junction, the net current is zero because the diffusion current (due to carrier concentration gradients) is exactly balanced by the drift current (due to the built-in electric field). The correct option is (C).

The core concept: Why two currents must cancel

A p-n junction at equilibrium means no external voltage is applied. Yet, inside the junction, two opposing processes are always active:

Diffusion: Holes from the p-side (high concentration) diffuse into the n-side, and electrons from the n-side diffuse into the p-side. This creates a diffusion current in the direction of the concentration gradient.

Drift: The diffusion of carriers leaves behind ionized dopant atoms (negative on the p-side, positive on the n-side), creating a built-in electric field across the depletion region. This field pushes carriers in the opposite direction — holes back to the p-side, electrons back to the n-side — producing a drift current.

At equilibrium, these two currents must exactly cancel. If they didn’t, there would be a net flow of charge, which would change the electric field until balance is restored. This is a fundamental consequence of the condition that the Fermi level is constant throughout the junction.

Watch out

A common mistake is to think that at equilibrium, no currents exist at all. In reality, both diffusion and drift currents are present — they are just equal and opposite, so the net current is zero.

Step-by-step reasoning

Identify the equilibrium condition

An unbiased p-n junction has no external voltage source. The system is in thermodynamic equilibrium, meaning the Fermi level is flat (constant) across the entire device. This implies that the net current through the junction must be zero.

Recognize the two current components

The total current $I$ across the junction is the sum of the diffusion current $I_{\text{diff}}$ and the drift current $I_{\text{drift}}$ :

$I = I_{\text{diff}} + I_{\text{drift}}$
Both are non-zero because there is a concentration gradient (driving diffusion) and a built-in electric field (driving drift).

Apply the equilibrium constraint At equilibrium, $I = 0$ . Therefore: …
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CBSE 2026Set DS1 markMCQ
$p$
i) only electrons
ii) only holes
iii) both electrons and holes
iv) neither electrons nor holes
›Reveal solutionSolution

The depletion region is depleted of mobile charge carriers — it contains only fixed ions.

Concept. When a $p$ – $n$ junction forms, electrons from the $n$ -side diffuse across and recombine with holes on the $p$ -side (and vice-versa). This leaves behind, near the junction, a thin region containing only the immobile positive donor ions (on the $n$ -side) and immobile negative acceptor ions (on the $p$ -side). Since the free electrons and holes have recombined/been swept away, this region h …

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CBSE 2026Set ANNUAL1 markMCQ
$Q. The value of barrier potential for Si semiconductor is:$
(a) 0.3 V
(b) 1.1 V
(c) 0.7 eV
(d) 0.7 V
›Reveal solutionSolution

The depletion-layer potential barrier that must be overcome to forward-bias a silicon diode is about 0.7 volt (compare germanium's ~0.3 V).

When a p-n junction forms, diffusion of majority carriers across the junction sets up a depletion region and an internal electric field, which corresponds to a potential difference — the barrier potential — that opposes further diffusion. For silicon this barrier potential is characteristically about $0.7\ \text{V}$ (for germani …

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CBSE 2026Set ANNUAL1 markMCQ
$Q. In P-n junction diode the width of depletion layer is :$
(a) 1 m
(b) 1 μm
(c) 1 nm
(d) 1 pm
›Reveal solutionSolution

The depletion layer of a p-n junction is about 1 μm (≈10⁻⁶ m) wide.

When a p-n junction is formed, electrons and holes diffuse across the junction and recombine, leaving behind immobile ionised donor and acceptor atoms. This creates a region devoid of free charge carriers — the depletion layer.

…

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CBSE 2025Set IMPROVEMENT1 markMCQ
$Q. The depletion layer of a p-n junction diode consists of:$
(a) Only holes
(b) Only electrons
(c) Both electrons and holes
(d) Neither electron nor hole
›Reveal solutionSolution

The depletion layer is depleted of free charge carriers; it contains only fixed (immobile) ionised donor and acceptor atoms.

When a p-n junction is formed, free electrons from the n-side diffuse into the p-side and recombine with holes, and free holes from the p-side diffuse into the n-side and recombine with electrons, near the junction. This leaves behind immobile positively charged donor ions on the n-side and immobile negatively charged acceptor ions on the p-side, forming a narrow region called the depletion layer. This region is depleted of mobile …

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CBSE 2025Set ANNUAL1 mark
$Q. A depletion layer consists of ................ . (fill in the blank)$
›Reveal solutionSolution

The depletion layer at a p-n junction is a thin region stripped of mobile carriers, containing only fixed ionised donor/acceptor atoms.

When a p-n junction forms, free electrons near the junction on the n-side diffuse across and recombine with holes on the p-side (and vice-versa). This leaves behind, on either side of the junction, a narrow region of immobile ions: positively charged (uncompensated) donor ions on the n-side and negatively charged (uncompensated) acceptor ions on the p-side. This region is depleted of free (mobile) charge carriers — …

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CBSE 2025Set ANNUAL1 markMCQ
$Q. Two statements are given below : one labelled as Assertion (A) and other labelled as Reason (R). Read the statements carefully and choose the correct option. Assertion (A) : The thickness of the depletion layer is fixed in all semi-conductor devices. Reason (R) : Free carriers are available in depletion layer.$
(a) Both Assertion (A) and Reason (R) are true and the Reason (R) is the correct explanation of Assertion (A).
(b) Both Assertion (A) and Reason (R) are true, but the Reason (R) is the incorrect explanation of Assertion (A).
(c) Assertion (A) is true, but Reason (R) is false.
(d) Assertion (A) and Reason (R) both are false.
›Reveal solutionSolution

The depletion layer's thickness is not fixed (it changes with applied bias), and it contains no free carriers at all — both statements are wrong.

Assertion: False. The thickness of the depletion layer is not fixed — it changes with the applied voltage. Forward bias narrows the depletion layer (reduces the barrier), while reverse bias widens it (increases the barrier). It also depends on doping concentration.

…

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CBSE 2025Set ANNUAL1 markMCQ
$Q. Assertion: Thickness of depletion layer is fixed in all semiconductor devices. Reason: No immobile charged carrier ions are available in depletion layer.$
(a) If both assertion and reason are true and the reason is the correct explanation of the assertion.
(b) If both assertion and reason are true but reason is not a correct explanation of the assertion.
(c) Assertion is true but reason is false.
(d) Both assertion and reason are false.
›Reveal solutionSolution

The depletion layer's thickness is NOT fixed — it changes with applied bias — and the depletion layer DOES contain immobile ionised dopant ions, so both statements are false.

Assertion check: In a p-n junction, the depletion-region width depends on the applied voltage. Under forward bias, the width of the depletion layer decreases; under reverse bias, it increases; it also depends on doping concentration. So the assertion, that the thickness is 'fixed in all semiconductor devices', is FALSE.

…

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CBSE 2025Set ANNUAL1 mark
$Q. What is the effect on the width of a depletion region of a p-n junction diode if the doping concentration is increased?$
›Reveal solutionSolution

More dopant ions per unit volume means equilibrium (same barrier potential) is reached over a narrower depletion width.

The depletion region forms when majority carriers diffuse across the junction and leave behind fixed ionized dopant atoms. As doping concentration increases, there are more ionized dopant atoms per unit volume available near the junction, so the same total (equilibrium) amount of fixed charge — and hence the same barrier potential — is reached over a narrower region. So the width of the depletion region decreases as doping conc …

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CBSE 2024Set IMPROVEMENT1 mark
$Q. Why do holes diffuse from p-region to n-region in an unbiased p-n junction?$
›Reveal solutionSolution

Diffusion always moves majority carriers from a region of high concentration to low concentration.

In the p-region, holes are the majority carriers and are present in high concentration; in the n-region, holes are minority carriers, present in very low concentration. Whenever a p-n junction is formed, this large concentration gradient of holes across the junction drives them to diffuse from the p-region (high hole concentration) into the n-region (low hole concentration), exactly as any species diffuses from higher to lower concentration. (Similarly, electrons diffuse from the n-region into the …

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CBSE 2024Set A1 markMCQ
$Q. In depletion layer of p-n junction diode, there are -$
(a) Electron
(b) Protons
(c) Mobile ions
(d) Immobile ions
›Reveal solutionSolution

The depletion region is depleted of free charge carriers and contains only fixed ionised dopant atoms.

When a p-n junction forms, mobile electrons from the n-side diffuse into the p-side and mobile holes from the p-side diffuse into the n-side; these recombine near the junction. This leaves behind a thin region on either side of the junction that has no free (mobile) charge carriers, only the immobile ionised dopant atoms — positive donor ions on the n-side and negative acceptor ion …

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CBSE 2024Set ANNUAL1 markMCQ
$Q. The potential barrier in the depletion layer is due to -$
(a) ions
(b) holes
(c) electrons
(d) forbidden gap
›Reveal solutionSolution

The depletion layer's potential barrier arises from the fixed, immobile ions left behind after diffusion of mobile carriers, not from the carriers themselves.

At a p-n junction, electrons diffuse from the n-side to the p-side and holes diffuse from the p-side to the n-side. This leaves behind fixed ionized donor atoms (positive) on the n-side and fixed ionized acceptor atoms (negative) on the p-side, near the junction — the depletion region has no mobile carriers, only these immobile ions. The resulting space-charge regio …

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Q.Explain the following, giving reasons:

(a) A doped semiconductor is electrically neutral.

(b) In a p-n junction under equilibrium, there is no net current.

(c) In a diode, the reverse current is practically independent of the applied voltage.

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