Semiconductor Diode

A semiconductor diode is a two-terminal p-n junction device that allows current to pass easily in one direction and offers high resistance in the opposite direction.

1. P-type semiconductor contains a large number of holes.
2. An n-type semiconductor contains a large number of electrons.
3. When the two sides are joined, electrons diffuse from the n-side to the p-side, and holes diffuse from the p-side to the n-side.
4. Near the junction, recombination occurs.
5. Fixed ions are left behind, creating a depletion layer.
6. This region develops an internal electric field, creating a potential barrier that opposes further diffusion.

Biasing means connecting the diode to an external battery so that its behaviour can be controlled.

Forward Bias

A diode is said to be in forward bias when:

• p-side is connected to the positive terminal of the battery.
• n-side is connected to the negative terminal of the battery.

Effect of forward bias

• The barrier potential decreases.
• The depletion region becomes thinner.
• Current starts increasing significantly after a certain minimum voltage.

Cut-in / Knee Voltage

• For a germanium diode, the cut-in voltage is about 0.3 V.
• For a silicon diode, the cut-in voltage is about 0.7 V.
• In forward bias, diode resistance becomes low after the threshold voltage is crossed.

Everyday analogy

A forward-biased diode acts like a door pushed in the correct direction, so it opens easily after a small push.

Reverse Bias

A diode is said to be in reverse bias when:

• p-side is connected to the negative terminal of the battery.
• n-side is connected to the positive terminal of the battery.

Effect of reverse bias

• The barrier potential increases.
• The depletion region widens.
• Only a very small current flows due to minority charge carriers.

The graph between applied voltage and resulting current is called the V-I characteristic curve of the diode.

In forward bias

• The current is very small at first.
• After the cut-in voltage, the current rises rapidly.

In reverse bias

• A very small reverse saturation current flows.
• At the breakdown voltage, the reverse current increases suddenly.

The opposition offered by a diode changes with the operating region, so diode resistance is not always constant.

• In forward bias after the cut-in voltage, the effective resistance becomes low.
• In reverse bias before breakdown, effective resistance is very high.

(a) Forward-Biased Resistance

1. From the V–I graph:
   At I = 10 mA, V = 0.7 V
   At I = 20 mA, V = 0.8 V
2. The forward resistance is the dynamic (AC) resistance, calculated using the change in voltage and current:
   r_fb = \[\frac {ΔV}{ΔI}\]
3. Substitute the values:
   \[r_{fb}=\frac{0.8-0.7}{20-10\mathrm{~mA}}=\frac{0.1}{10\times10^{-3}}=10\mathrm{~}\]
4. Therefore, the forward resistance is 10 Ω

(b) Reverse-Biased Resistance

1. From the graph, at V = −10 V, I = −1 μA
2. Apply Ohm's law: r_rb = \[\frac {V}{I}\]
3. Substitute the values:
   \[r_{rb}=\frac{10}{1\times10^{-6}}=1.0\times10^7\Omega\]
4. Therefore, the reverse resistance is 1.0 × 107 Ω

Conclusion

• In forward bias, the diode offers low resistance (10 Ω), so current flows easily.
• In reverse bias, the diode offers very high resistance (1.0 × 10⁷ Ω), so only a tiny leakage current flows.