PN Junction Diodes and Semi-conductor Diodes

Introduction to PN Junction Diodes and Semi-conductor Diodes

• A PN junction diode, also known as a semi-conductor diode, forms when P-type and N-type semi-conductor materials are joined.
• The junction is the meeting point of these materials.

Semi-conductor Materials

• Semi-conductor materials exist in intrinsic (pure) and extrinsic (impure) forms.
• Extrinsic semi-conductors are P-type or N-type, depending on the added impurity.

P-Type Semi-conductor

• P-type semi-conductors include negatively charged acceptor ions that readily accept electrons.
• Holes are the majority carriers (positive charge), and electrons are the minority carriers (negative charge) in P-type materials.

N-Type Semi-conductor

• N-type semi-conductors contain positively charged donor ions, which can donate electrons.
• Electrons are the majority carriers (negative charge), and holes are the minority carriers (positive charge) in N-type materials.

PN Junction Formation

• When P-type and N-type materials connect, majority carriers attract each other.
• Electrons from the N-side diffuse to the P-side, recombining with holes.
• Holes from the P-side diffuse to the N-side.
• Diffusion continues until equilibrium.

Diffusion and Charge Distribution

• Electrons diffusing from the N-side to the P-side carry a negative charge, accumulating negative charge on the P-side.
• Simultaneously, holes diffusing from the P-side to the N-side create positive charge build-up on the N-side.

Depletion Layer Formation

• Diffusion ceases upon reaching a balance, and the depletion layer forms at the junction.
• The depletion layer lacks free charge carriers (electrons and holes).
• The depletion layer contains immobile ions.

Unbiased Conditions

• An unbiased diode has no external voltage applied.
• Diffusion of charge carriers (electrons and holes) occurs without external voltage.
• Recombination of charge carriers takes place near the junction.
• Diffusion leads to the creation of the depletion region.
• The depletion region is depleted of mobile charge carriers.
• Diffusion stops once the depletion region is established.

Barrier Potential

• Charge separation in the depletion layer leads to the development of a barrier potential at the junction.
• The barrier potential opposes further diffusion of charge carriers across the junction.
• Barrier potential values vary depending on the semi-conductor material and is approximately 0.3V for Germanium and approximately 0.7V for Silicon.
• The barrier potential needs to be overcome for the diode to conduct electricity.
• The barrier developed at the junction is also known as the cut-in voltage.

Biasing a Diode

• "Biasing" refers to applying DC voltage to a circuit.

Forward Biasing

• In forward biasing, the P-side connects to the positive terminal and the N-side to the negative terminal of a battery.
• Holes in the P-side are repelled by the positive terminal and move towards the junction.
• Electrons are repelled by the negative terminal and move towards the junction.
• The width of the depletion region decreases.
• As voltage increases and the depletion region decreases, more electrons cross the junction.

Effects of Forward Biasing

• Majority carriers move toward the junction.
• Minority carriers move away from the junction.
• The width of the depletion region decreases.
• Majority charge carriers cause a net current flow across the junction from P to N side.

Current Flow in Diode in Forward Biasing

• Electrons moving from the N-side to the P-side result in P to N current flow.
• A minimum positive voltage, the cut-in voltage, is needed to start conduction.

Reverse Biasing

• In reverse biasing, the P-side connects to the negative terminal, and the N-side connects to the positive terminal of a battery and interchanging the polarities has an affect.
• The depletion region gradually increases.
• When voltage is applied, minority charge carriers cross, creating a small flow.

Effects of Reverse Biasing

• Majority carriers move away from the junction.
• Minority carriers move toward the junction.
• The width of the depletion region increases.
• Minority carriers cause a small current to flow across the junction, from N to P side.
• The depletion region increases because electrons are repelled away from the junction.

Reverse Saturation Current

• The small, constant current in reverse bias is the reverse saturation current or leakage current.
• This current becomes constant at very low reverse bias voltages.
• The diode breaks down if the reverse voltage increases further.

Additional Notes

• VI Characteristics of a PN Junction Diode will be reviewed, and breakdowns will be discussed in the next recording.