Semiconductor Diode — 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.
Potential Barrier ($V_B$): — Electric potential across depletion region. Si $approx 0.7,\text{V}$, Ge $approx 0.3,\text{V}$.
Forward Bias: — Positive to p, negative to n. $V_{ext} > V_B \rightarrow$ current flows, depletion region narrows.
Reverse Bias: — Negative to p, positive to n. Depletion region widens, very small reverse saturation current ($I_S$).
I-V Characteristics: — Exponential current in forward bias after $V_B$. Constant $I_S$ in reverse bias until breakdown.
Diode Equation: — I = I_S left( e^{\frac{V}{eta V_T}} - 1 \right).
Breakdown Voltage ($V_{BR}$): — Reverse voltage where current sharply increases (Zener/Avalanche).
Ideal Diode: — $0,\text{V}$ drop in forward, infinite resistance in reverse.

2-Minute Revision

The semiconductor diode is a p-n junction, a fundamental electronic component that allows current flow primarily in one direction. This unidirectional property arises from the formation of a depletion region at the junction, which creates an internal potential barrier (e.

g., $0.7,\text{V}$ for silicon, $0.3,\text{V}$ for germanium). When the diode is forward biased (positive voltage to p-side, negative to n-side), the external voltage opposes this barrier. If the applied voltage exceeds the cut-in voltage, the depletion region narrows, and a large, exponentially increasing current flows.

In reverse bias (negative voltage to p-side, positive to n-side), the external voltage adds to the barrier, widening the depletion region and blocking majority carrier flow. Only a tiny reverse saturation current ($I_S$), due to minority carriers, flows.

This $I_S$ is highly temperature-dependent. If the reverse voltage becomes too high, it reaches the breakdown voltage ($V_{BR}$), where current surges due to Zener or Avalanche effects. For NEET, remember the I-V curve shape, the cut-in voltages for Si/Ge, and how to apply the ideal diode model (short circuit in forward, open in reverse) or practical diode model (constant $0.

7, ext{V}$ drop) in simple circuits.

5-Minute Revision

A semiconductor diode is essentially a p-n junction, formed by joining p-type and n-type semiconductor materials. The key to its operation is the depletion region that forms at the interface. This region is depleted of mobile charge carriers but contains immobile charged ions, creating an internal electric field and a potential barrier ($V_B$). For silicon, $V_B approx 0.7,\text{V}$, and for germanium, $V_B approx 0.3,\text{V}$.

When the diode is forward biased (positive terminal to p-side, negative to n-side), the external voltage reduces the effective potential barrier. Once the applied voltage exceeds $V_B$ (the cut-in voltage), the depletion region narrows, and majority carriers can easily cross the junction, leading to a large, exponentially increasing current.

The diode acts like a low-resistance path. For example, if a $5,\text{V}$ battery is connected to a silicon diode and a $100,Omega$ resistor in series, the voltage across the resistor would be $5, ext{V} - 0.

7, ext{V} = 4.3, ext{V}$, and the current would be$4.3, ext{V} / 100,Omega = 43, ext{mA}$.

When reverse biased (negative terminal to p-side, positive to n-side), the external voltage adds to the potential barrier, widening the depletion region. This effectively blocks the flow of majority carriers.

A very small, almost constant reverse saturation current ($I_S$) flows, primarily due to thermally generated minority carriers. This current is highly sensitive to temperature. If the reverse voltage continues to increase, it eventually reaches the breakdown voltage ($V_{BR}$), where the current suddenly increases sharply due to Zener or Avalanche breakdown.

This region is typically avoided in normal operation.

I-V Characteristics: The graph shows negligible current below $V_B$ in forward bias, followed by an exponential rise. In reverse bias, a small, constant $I_S$ flows until $V_{BR}$ is reached, after which current increases dramatically.

For circuit analysis, remember the ideal diode model: acts as a short circuit (0V drop) in forward bias and an open circuit (0A current) in reverse bias. For practical diodes, use the constant voltage drop model ($0.

7, ext{V}$for Si,$0.3, ext{V}$ for Ge) in forward bias and assume an open circuit in reverse bias (unless breakdown is considered).

Prelims Revision Notes

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Semiconductor Basics: — Doping creates p-type (holes majority, trivalent impurities like Boron) and n-type (electrons majority, pentavalent impurities like Phosphorus) semiconductors. Intrinsic semiconductors have equal electron and hole concentrations.
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p-n Junction Formation: — When p and n types are joined, majority carriers diffuse across the junction. Electrons from n to p, holes from p to n. This leaves behind immobile positive donor ions on the n-side and negative acceptor ions on the p-side.
3
Depletion Region: — The region around the junction depleted of mobile charge carriers. Its width is inversely proportional to doping concentration. It contains immobile ions.
4
Potential Barrier ($V_B$): — The electric field across the depletion region creates a potential difference. For Silicon (Si), $V_B approx 0.7,\text{V}$. For Germanium (Ge), $V_B approx 0.3,\text{V}$.
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Biasing:

* Forward Bias: Positive terminal of battery to p-side, negative to n-side. External voltage opposes $V_B$. Depletion region narrows. Current flows significantly when external voltage $> V_B$. Diode acts as low resistance. * Reverse Bias: Negative terminal of battery to p-side, positive to n-side. External voltage adds to $V_B$. Depletion region widens. Very small **reverse saturation current ($I_S$)** flows due to minority carriers. Diode acts as high resistance.

1
I-V Characteristics:

* Forward: Current is negligible until $V_B$ (cut-in/knee voltage), then rises exponentially. $I = I_S (e^{V/eta V_T} - 1)$. * Reverse: Current is nearly constant $I_S$ until **breakdown voltage ($V_{BR}$)**. Then current increases sharply.

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Breakdown Mechanisms:

* Zener Breakdown: Occurs in heavily doped junctions (narrow depletion region) at lower $V_{BR}$. Due to quantum mechanical tunneling. * Avalanche Breakdown: Occurs in lightly doped junctions (wider depletion region) at higher $V_{BR}$. Due to high-energy minority carriers colliding with lattice atoms, creating more electron-hole pairs.

1
Temperature Effects:

* $V_B$ (cut-in voltage) decreases by approximately $2,\text{mV/}^circ\text{C}$ for both Si and Ge. * $I_S$ (reverse saturation current) doubles for every $10^circ\text{C}$ rise in temperature.

1
Diode Models:

* Ideal Diode: Short circuit in forward bias (0V drop), open circuit in reverse bias (0A current). * Practical Diode (Constant Voltage Drop Model): $0.7,\text{V}$ drop for Si (or $0.3,\text{V}$ for Ge) in forward bias, open circuit in reverse bias.

1
Applications: — Rectification (AC to DC), switching, voltage regulation (Zener diodes), clipping, clamping.

Vyyuha Quick Recall

Positive to P, Negative to N = Forward Bias (Current Flows). Reverse is the other way, current Rarely flows.