p-n Junction — Revision Notes

⚡ 30-Second Revision

p-n Junction: — Interface of p-type and n-type semiconductors.
Depletion Region: — Region near junction devoid of mobile carriers, contains immobile ions.
Barrier Potential ($V_B$): — Internal potential difference across depletion region.

- Si: $V_B approx 0.7,\text{V}$ - Ge: $V_B approx 0.3,\text{V}$

Forward Bias: — p-side to positive, n-side to negative.

- $V_{applied}$ opposes $V_B$. - Depletion width decreases. - $V_{eff} = V_B - V_{applied}$. - Large current due to majority carriers.

Reverse Bias: — p-side to negative, n-side to positive.

- $V_{applied}$ adds to $V_B$. - Depletion width increases. - $V_{eff} = V_B + V_{applied}$. - Small reverse saturation current ($I_0$) due to minority carriers.

Breakdown: — Sudden current increase in reverse bias.

- Zener: Heavily doped, narrow depletion, direct bond breaking, reversible. - Avalanche: Lightly doped, wide depletion, collision ionization, potentially destructive.

Temperature Effect: — $V_B$ decreases with $T$; $I_0$ increases with $T$ (doubles every $10^circ C$).

2-Minute Revision

The p-n junction is formed by joining p-type and n-type semiconductors. At the junction, majority carriers (electrons from n, holes from p) diffuse and recombine, creating a 'depletion region' that is empty of mobile carriers but contains immobile charged ions. This charge separation establishes an internal electric field and a 'barrier potential' ($0.7,\text{V}$ for Si, $0.3,\text{V}$ for Ge) that opposes further diffusion.

When forward biased (p-side to positive, n-side to negative), the external voltage reduces this barrier, narrows the depletion region, and allows a large current to flow due to majority carriers. The current increases exponentially after the knee voltage.

When reverse biased (p-side to negative, n-side to positive), the external voltage adds to the barrier, widens the depletion region, and blocks majority carrier flow. Only a tiny 'reverse saturation current' flows, caused by the drift of minority carriers.

If the reverse voltage is too high, breakdown occurs: Zener breakdown (heavily doped, direct bond breaking, reversible) or Avalanche breakdown (lightly doped, collision ionization, potentially destructive).

Temperature increases the reverse saturation current and decreases the barrier potential.

5-Minute Revision

The p-n junction is the fundamental interface between a p-type and an n-type semiconductor. Its formation involves several key steps: initially, due to concentration gradients, majority carriers (electrons from the n-side, holes from the p-side) diffuse across the junction.

This diffusion leads to recombination, leaving behind immobile positive donor ions on the n-side and negative acceptor ions on the p-side. This region, now 'depleted' of mobile charge carriers, is called the depletion region.

The immobile ions create an internal electric field, which in turn establishes a potential difference known as the barrier potential ($V_B$). For silicon, $V_B approx 0.7,\text{V}$, and for germanium, $V_B approx 0.

3, ext{V}$. This barrier opposes further majority carrier diffusion, leading to an equilibrium where diffusion current is balanced by a small drift current of minority carriers.

Biasing controls the junction's behavior:

1
Forward Bias: — Connect the p-side to the positive terminal and the n-side to the negative terminal of an external voltage source. The external voltage opposes the barrier potential, effectively reducing it ($V_{eff} = V_B - V_{applied}$). This reduction allows majority carriers to easily cross, leading to a significant forward current. The depletion region narrows. For example, if a silicon diode ($V_B = 0.7,\text{V}$) is forward biased with $0.6,\text{V}$, the effective barrier is $0.1,\text{V}$, allowing substantial current.
2
Reverse Bias: — Connect the p-side to the negative terminal and the n-side to the positive terminal. The external voltage adds to the barrier potential ($V_{eff} = V_B + V_{applied}$). This increased barrier prevents majority carriers from crossing. The depletion region widens. A very small 'reverse saturation current' flows, caused by the drift of minority carriers (electrons from p-side, holes from n-side) swept across by the strong electric field. This current is highly temperature-dependent.

Breakdown occurs when the reverse bias voltage becomes too high, causing a sudden, sharp increase in current. Zener breakdown happens in heavily doped junctions (narrow depletion) where the strong electric field directly pulls electrons from covalent bonds.

It's reversible. Avalanche breakdown occurs in lightly doped junctions (wider depletion) where minority carriers gain enough energy to collide with atoms, creating an 'avalanche' of new carriers. This can be destructive if current is not limited.

Understanding the I-V characteristics curve, identifying the knee voltage (forward bias) and breakdown voltage (reverse bias), is crucial for NEET.

Prelims Revision Notes

1
p-n Junction Formation: — Formed by joining p-type (majority holes, immobile negative acceptors) and n-type (majority electrons, immobile positive donors) semiconductors.
2
Depletion Region: — A region at the junction depleted of mobile charge carriers. Contains immobile positive ions on the n-side and negative ions on the p-side.
3
Barrier Potential ($V_B$): — An internal electric field and potential difference developed across the depletion region. Opposes majority carrier diffusion.

* Silicon (Si): $V_B approx 0.7,\text{V}$ * Germanium (Ge): $V_B approx 0.3,\text{V}$

1
Forward Bias:

* Connection: p-side to positive terminal, n-side to negative terminal. * **Effect on $V_B$:** Reduces effective barrier potential ($V_{eff} = V_B - V_{applied}$). External voltage opposes internal field. * Effect on Depletion Width: Decreases. * Current: Large current due to majority carriers (diffusion current). Increases exponentially after knee voltage. * Resistance: Low.

1
Reverse Bias:

* Connection: p-side to negative terminal, n-side to positive terminal. * **Effect on $V_B$:** Increases effective barrier potential ($V_{eff} = V_B + V_{applied}$). External voltage adds to internal field. * Effect on Depletion Width: Increases. * Current: Very small, almost constant reverse saturation current ($I_0$) due to minority carriers (drift current). * Resistance: High.

1
Breakdown Voltage: — The reverse voltage at which current sharply increases.

* Zener Breakdown: Occurs in heavily doped junctions (narrow depletion). Strong electric field directly breaks covalent bonds. Reversible. Used in voltage regulation. * Avalanche Breakdown: Occurs in lightly doped junctions (wider depletion). Minority carriers gain energy, collide with atoms, creating more carriers (avalanche effect). Potentially destructive.

1
Temperature Effects:

* Barrier potential ($V_B$) decreases with increasing temperature. * Reverse saturation current ($I_0$) increases significantly with increasing temperature (approx. doubles for every $10^circ C$ rise for Si) due to increased minority carrier generation.

1
I-V Characteristics: — Understand the graph showing exponential rise in forward current after knee voltage and small, constant reverse current until breakdown.

Vyyuha Quick Recall

Positive Negative, Forward Bias, Decreased Width, Large Current. Positive Negative, Reverse Bias, Increased Width, Small Current. (PN-FB-DW-LC, PN-RB-IW-SC) - Helps remember the effects of biasing on depletion width and current.