Two terminal diode devices

Question 1

What determines the energy of a photon emitted when an electron jumps a potential barrier?

Answer 1

Larger barrier: Higher photon energy, smaller wavelength.
Smaller barrier: Lower photon energy, larger wavelength.

Question 2

What is the structure of an LED?

Answer 2

LEDs have metal contacts on both sides to allow voltage and current application.
When the voltage exceeds the forward bias drop, the LED lights up.

Question 3

What makes LEDs efficient?

Answer 3

Electron-hole recombination:
Direct energy gap.
High-quality materials.
Proximity of electrons and holes:
Using materials with different bandgaps brings carriers closer together.

Question 4

What are direct and indirect energy bands?

Answer 4

Direct: Maximum of valence band aligns with minimum of conduction band.
Indirect: Maximum of valence band does not align with minimum of conduction band.
Electrons fall to lowest energy states, holes rise to highest states.

Question 5

What determines if a material has direct or indirect energy bands?

Answer 5

Decided by energy band structure.
Compounds are more likely to be direct.
Alloying elements can widen the bandgap range (e.g., InGaN with a bandgap of 0.7–3.4 eV).

Question 6

What are good and bad characteristics of LED materials?

Answer 6

Bad:
Unsuitable materials have carriers with small momentum, making recombination unlikely.
Indirect bands make it hard for electrons and holes to meet.
Good:
Direct band materials conserve energy and momentum.
Alloying narrows the active region, increasing recombination likelihood.

Question 7

How do photons get detected?

Answer 7

Photons create electron-hole pairs if their energy exceeds the material’s bandgap.
Separation of pairs via an electric field prevents recombination, creating a photocurrent.

Question 8

How do PN junctions help in photon detection?

Answer 8

Built-in electric field separates electron-hole pairs.
Reverse-bias increases the field, enhancing photocurrent.
Solar panels utilize large-area PN junctions to generate light-based current.

Question 9

What is rectification?

Answer 9

Rectification is the conversion of AC to DC.
Types:
Half-wave rectifier: Converts negative wave of AC to zero, leaving only the positive wave.
Full-wave rectifier: Uses four diodes in a bridge, turning the AC output into an absolute sine wave.

Question 10

How does a full-wave rectifier smooth the DC output?

Answer 10

A capacitor is added to reduce ripples in the output, creating a smoother DC current.