Forward and Reverse Bias — Revision Notes

⚡ 30-Second Revision

Forward Bias — P-side to positive, N-side to negative.

- Depletion region: Narrows. - Potential barrier: Decreases ($V_0 - V_F$). - Current: Large, exponential (majority carriers). - Diode: Low resistance, 'ON'.

Reverse Bias — P-side to negative, N-side to positive.

- Depletion region: Widens. - Potential barrier: Increases ($V_0 + V_R$). - Current: Very small (minority carriers, $I_0$). - Diode: High resistance, 'OFF'.

Cut-in Voltage ($V_{knee}$) — $approx 0.7,\text{V}$ (Si), $approx 0.3,\text{V}$ (Ge).
Reverse Saturation Current ($I_0$) — Doubles for every $10^circ\text{C}$ rise.
Breakdown Voltage ($V_{BR}$) — Sudden current increase in reverse bias.

2-Minute Revision

Forward and reverse bias are crucial for understanding p-n junction diodes. In forward bias, the p-type is connected to the positive terminal and the n-type to the negative terminal of a voltage source.

This configuration reduces the internal potential barrier and narrows the depletion region, allowing a large current to flow due to majority carriers once the applied voltage exceeds the cut-in voltage (e.

g., $0.7,\text{V}$ for silicon). The current increases exponentially. Conversely, in reverse bias, the p-type is connected to the negative terminal and the n-type to the positive terminal. This increases the potential barrier and widens the depletion region, blocking majority carrier flow.

Only a very small, temperature-dependent reverse saturation current (due to minority carriers) flows, which is almost constant until the breakdown voltage is reached. At breakdown, the current sharply increases.

These distinct behaviors give the diode its rectifying property, essential for converting AC to DC and forming the basis for many electronic devices.

5-Minute Revision

The operational modes of a p-n junction diode are defined by how an external voltage, or bias, is applied. When a p-n junction is forward biased, the positive terminal of the external source is connected to the p-type material (anode) and the negative terminal to the n-type material (cathode).

This connection causes the external electric field to oppose the internal electric field of the depletion region. Consequently, the effective potential barrier across the junction is reduced (from $V_0$ to $V_0 - V_F$), and the depletion region narrows.

Once the applied forward voltage ($V_F$) exceeds the diode's cut-in voltage (e.g., $0.7,\text{V}$ for silicon, $0.3,\text{V}$ for germanium), majority carriers (holes from p-side, electrons from n-side) gain enough energy to overcome the reduced barrier, resulting in a large, exponentially increasing forward current.

In contrast, when the p-n junction is reverse biased, the negative terminal of the external source is connected to the p-type material and the positive terminal to the n-type material. Here, the external electric field reinforces the internal electric field of the depletion region.

This leads to an increase in the effective potential barrier (to $V_0 + V_R$) and a significant widening of the depletion region. Majority carriers are pulled away from the junction, making it extremely difficult for them to cross.

As a result, only a very small current, known as the reverse saturation current ($I_0$), flows. This current is primarily due to the drift of thermally generated minority carriers and is largely independent of the applied reverse voltage until a critical point called the breakdown voltage ($V_{BR}$) is reached.

Beyond $V_{BR}$, the current increases sharply due to Zener or avalanche breakdown. The I-V characteristic curve clearly illustrates these behaviors: exponential current rise in forward bias (first quadrant) and a small, constant current followed by breakdown in reverse bias (third quadrant).

These principles are fundamental to rectifiers, LEDs, and Zener diodes.

Prelims Revision Notes

Forward Bias (P-type to +, N-type to -)

Connection — Positive terminal of battery to p-side (anode), negative terminal to n-side (cathode).
External Field — Opposes internal electric field of depletion region.
Potential Barrier — Decreases. Effective barrier potential becomes $(V_0 - V_F)$.
Depletion Region — Narrows significantly.
Majority Carriers — Pushed towards the junction, easily cross.
Current Flow — Large current flows (due to majority carriers). Increases exponentially with $V_F$ after cut-in voltage.
Cut-in Voltage ($V_{knee}$ or $V_T$) — Minimum $V_F$ for significant current. $approx 0.7,\text{V}$ for Si, $approx 0.3,\text{V}$ for Ge.
Diode Behavior — Low resistance, acts as a closed switch.
I-V Curve — First quadrant, exponential rise after $V_{knee}$.

Reverse Bias (P-type to -, N-type to +)

Connection — Negative terminal of battery to p-side (anode), positive terminal to n-side (cathode).
External Field — Reinforces internal electric field of depletion region.
Potential Barrier — Increases. Effective barrier potential becomes $(V_0 + V_R)$.
Depletion Region — Widens significantly.
Majority Carriers — Pulled away from the junction, cannot cross.
Current Flow — Very small current flows (reverse saturation current, $I_0$). Primarily due to minority carriers (thermally generated). Almost constant with $V_R$ until breakdown.
Reverse Saturation Current ($I_0$) — Highly temperature-dependent. Doubles for every $10^circ\text{C}$ rise in temperature.
Breakdown Voltage ($V_{BR}$) — Critical reverse voltage where current sharply increases due to Zener or avalanche breakdown.
Diode Behavior — High resistance, acts as an open switch.
I-V Curve — Third quadrant, small constant current followed by sharp increase at $V_{BR}$.

Key Formulas/Concepts

Effective barrier in forward bias: $V_{eff} = V_0 - V_F$
Effective barrier in reverse bias: $V_{eff} = V_0 + V_R$
Diode current (Shockley equation, qualitative for NEET): $I = I_0 (e^{eV/nkT} - 1)$
Current in series circuit with diode: $I = (V_{source} - V_{knee}) / R$
Temperature dependence of $I_0$: $I_0(T_2) = I_0(T_1) \times 2^{(T_2-T_1)/10^circ\text{C}}$

Vyyuha Quick Recall

Forward Bias: For Big Current, Narrow Depletion, Lower Barrier. (P to +, N to -) Reverse Bias: Rare Bit of Current, Wide Depletion, Higher Barrier. (P to -, N to +)