PN Junction Diodes Quiz

PN Junction Diode

• P-type and N-type semiconductors, when used alone, have limited applications.
• A PN junction is formed by joining P-type and N-type semiconductor materials maintaining continuous crystal structure at the boundary.
• PN Junction diodes can function as rectifiers, amplifiers, and switches in electronic circuits.
• Simply joining two pieces of P and N materials does not create a PN junction; special fabrication techniques are needed.
• A PN junction is a device formed by joining p-type (doped with Boron, Aluminum) and n-type (doped with Phosphorus, Arsenic, Antimony) semiconductors.
• The junction is separated by a thin layer and is called a PN junction diode or junction diode.
• The triangle in the electronic symbol indicates the direction of current flow.
• In a PN junction diode, N is right and P is left.
• The majority carriers in the N region are electrons, and in the P region are holes.

Formation of Depletion Layer

• In a PN junction with no external connections, excess electrons from the N region cross the junction and combine with excess holes in the P region.
• The N region loses electrons and becomes positively charged.
• The P region gains electrons and becomes negatively charged.
• The migratory action stops at a point when the opposite charges formed create a region with no mobile charge carriers.
• This region is called the depletion layer.
• The depletion layer acts as an insulator between the two sides.
• The thickness of the depletion layer depends on the doping level.
• Heavy doping leads to a thin depletion layer, while light doping leads to a thick depletion layer.
• Additional electrons from the N region and holes from the P region are repelled by the net negative and positive charges, respectively, creating the depletion layer.
• The depletion layer has fixed, immobile ions.
• The thickness of the depletion layer is about 10⁻⁷ meters.

Potential Barrier

• Electrons in the N region need to overcome a potential barrier to reach the P region.
• Electrons trying to cross from the N to P region experience a retarding field from the battery and are repelled.
• Similarly, holes experience a retarding field when moving from the P to N region.
• The potential difference across the depletion layer is called the potential barrier.
• The potential barrier voltage for germanium (Ge) is 0.3V, and for silicon (Si) is 0.7V.

PN Junction Biasing

• A PN junction can work in two modes: forward bias and reverse bias, depending on how a battery is connected to the diode.
• Forward bias: The positive terminal of the battery is connected to the P-region, and the negative terminal is connected to the N-region.
• In forward bias, the majority charge carriers (electrons in N, holes in P) move across the junction, decreasing the width of the depletion layer.
• In forward bias, no current flows until the barrier voltage is overcome. When the barrier is overcome, the current increases linearly with increasing voltage. The slope corresponds to a current-voltage characteristic.
• Above 3 V, the majority carrier gain sufficient energy to knock valence electrons into the conduction band and the forward current sharply increases.
• Reverse bias: The negative terminal of the battery is connected to the P-region, and the positive terminal is connected to the N-region.
• In reverse bias, the potential barrier is increased and the width of the depletion layer grows, preventing current flow.
• In reverse bias, only minority carriers cross the junction to create a very small current (reverse saturation current).
• This current is in the order of microamperes.
• When the reverse voltage increases above a certain value (breakdown voltage), the reverse current increases sharply. The diode exhibits almost zero resistance, known as avalanche breakdown.
• Exceeding a certain reverse-bias voltage (typically 25 V for silicon) damages the PN junction permanently.

I-V Characteristics

• The voltage-current (V-I) characteristics of a PN junction diode plot the voltage across the junction (x-axis) versus the current through the circuit (y-axis).
• The graph shows the diode's DC behavior.
• In forward bias, there's a small voltage drop (barrier voltage) before current starts flowing. Above the barrier voltage, the current increases proportionally to the voltage increase, like an ordinary conductor.
• In reverse bias, there is virtually no current flowing until a breakdown voltage is reached, where the current abruptly increases.

Applications

• Rectifier: Converts AC to DC voltage.
• Switch: Controls current flow in circuits.
• Detector: Detects audio signals in radios.
• LED (Light-Emitting Diode): Emits light of different colors.

Advantages

• No filament needed.
• Occupies less space.
• Long lifespan.

Test your understanding of PN junction diodes with this quiz. Explore key concepts such as charge carriers, depletion layers, and semiconductor doping. Perfect for students learning about electronics and semiconductor physics.