a. Principle of PN Junction Diode:
- A PN junction diode is a semiconductor device made of two layers, one with an excess of negatively charged electrons (N-type) and the other with a deficiency of electrons (P-type).
- When these two layers are brought together, the free electrons from the N-type layer move to the P-type layer, creating a depletion region with no free charge carriers.
- This forms a potential barrier that prevents further movement of electrons from the N-type layer to the P-type layer.
- When a voltage is applied across the diode in the forward direction, the barrier is lowered, allowing current to flow.
- In the reverse direction, the barrier is increased, preventing current flow.
| P-type Semiconductor | N-type Semiconductor |
|---|---|
| Contains impurities that create an excess of holes (positive charge carriers) | Contains impurities that create an excess of electrons (negative charge carriers) |
| Majority charge carriers are holes | Majority charge carriers are electrons |
| Has a higher concentration of positive charge carriers | Has a higher concentration of negative charge carriers |
| Electrons in the valence band can jump into holes in the p-type material, creating a depletion region | Electrons from the n-type material can jump into the holes in the p-type material, creating a depletion region |
| P-type material has a lower electron mobility | N-type material has a higher electron mobility |
| P-type material is more easily oxidized | N-type material is more easily reduced |
b. Forward Bias and Reverse Bias:
- Forward bias is when a voltage is applied across a diode in the direction that allows current to flow.
- This lowers the potential barrier, allowing current to flow easily. Reverse bias is when a voltage is applied across a diode in the direction that prevents current flow.
- This increases the potential barrier, preventing current from flowing.
c. Applications of PN Junction Diode:
- PN junction diodes have many applications in electronics, including as rectifiers, voltage regulators, oscillators, and signal limiters.
- They are also used in solar cells, light-emitting diodes (LEDs), and photodiodes.
d. Diode as Rectifier:
- A rectifier is a device that converts AC voltage to DC voltage. A PN junction diode can be used as a rectifier by connecting it in series with an AC voltage source and a load resistor.
- When the diode is forward biased, it allows current to flow through the load resistor, resulting in a positive half-cycle of the AC voltage.
- When the diode is reverse biased, it blocks current flow, resulting in a negative half-cycle of the AC voltage.
- This produces a pulsating DC voltage across the load resistor. A filter capacitor can be added to smooth out the pulsations and produce a steady DC voltage.