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Questions and Answers
In a semiconductor material, what happens to photons with energy less than the bandgap energy?
In a semiconductor material, what happens to photons with energy less than the bandgap energy?
- They are completely absorbed, generating heat.
- They cause significant structural changes in the semiconductor's crystal lattice.
- They pass through the material with little interaction. (correct)
- They interact strongly with the semiconductor, creating many electron-hole pairs.
When photons with energy greater than the bandgap energy interact with a semiconductor, what is the primary effect?
When photons with energy greater than the bandgap energy interact with a semiconductor, what is the primary effect?
- Creating electron-hole pairs by breaking covalent bonds. (correct)
- Increasing the material's bandgap energy.
- Heating the semiconductor material evenly.
- Reflecting off the surface of the semiconductor.
How does the absorption depth of higher energy photons compare to that of lower energy photons in a semiconductor material?
How does the absorption depth of higher energy photons compare to that of lower energy photons in a semiconductor material?
- Higher energy photons penetrate deeper into the material.
- Energy level has no effect on the depth.
- Lower energy photons are absorbed closer to the surface.
- Higher energy photons are absorbed closer to the surface. (correct)
What does the generation rate (G) in the equation $G = \alpha Ne^{-\alpha x}$ represent in the context of semiconductor physics?
What does the generation rate (G) in the equation $G = \alpha Ne^{-\alpha x}$ represent in the context of semiconductor physics?
How is the photon flux (N) defined in the context of the equation for the electron-hole pair generation rate?
How is the photon flux (N) defined in the context of the equation for the electron-hole pair generation rate?
What is the significance of the absorption coefficient ($\alpha$) in the context of light absorption in semiconductors?
What is the significance of the absorption coefficient ($\alpha$) in the context of light absorption in semiconductors?
In the equation $G = \alpha Ne^{-\alpha x}$, what does 'x' represent?
In the equation $G = \alpha Ne^{-\alpha x}$, what does 'x' represent?
After light is switched off, what generally happens to the electron-hole pairs in a semiconductor?
After light is switched off, what generally happens to the electron-hole pairs in a semiconductor?
What role do defects or impurities play in the recombination process within a semiconductor?
What role do defects or impurities play in the recombination process within a semiconductor?
What is defined as the average time for recombination to occur after electron-hole generation?
What is defined as the average time for recombination to occur after electron-hole generation?
What does the 'carrier diffusion length' represent in a semiconductor material?
What does the 'carrier diffusion length' represent in a semiconductor material?
What is the typical carrier lifetime for silicon?
What is the typical carrier lifetime for silicon?
Which factor fundamentally enables a semiconductor to produce electrical power?
Which factor fundamentally enables a semiconductor to produce electrical power?
How are functional solar cells typically produced from semiconductor materials?
How are functional solar cells typically produced from semiconductor materials?
How do carrier lifetime and carrier diffusion length parameters serve as indicators?
How do carrier lifetime and carrier diffusion length parameters serve as indicators?
Flashcards
Absorption of Light in Semiconductors
Absorption of Light in Semiconductors
Light striking a semiconductor material can cause photons to interact with electrons.
Low-Energy Photon Interaction
Low-Energy Photon Interaction
Photons with energy less than the bandgap energy interact weakly and pass through the material.
High-Energy Photon Interaction
High-Energy Photon Interaction
Photons with energy greater than the bandgap energy interact with electrons, breaking covalent bonds and creating electron-hole pairs.
Generation Rate (G)
Generation Rate (G)
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Photon Flux (N)
Photon Flux (N)
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Absorption Coefficient (α)
Absorption Coefficient (α)
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Distance from Surface (x)
Distance from Surface (x)
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Absorption Depth vs. Energy
Absorption Depth vs. Energy
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Recombination After Light Removal
Recombination After Light Removal
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Carrier Lifetime
Carrier Lifetime
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Carrier Diffusion Length
Carrier Diffusion Length
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Directionality Requirement
Directionality Requirement
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Functional Solar Cells
Functional Solar Cells
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Study Notes
Absorption of Light
- Light interacts with semiconductor materials when it falls onto them.
- Photons with energy (Eph) less than the bandgap energy (Eg) interact weakly with the material and pass through it.
- Photons with energy greater than the bandgap energy (Eph > Eg) interact with electrons in covalent bonds.
- These photons expend their energy to break bonds and create electron-hole pairs, which then diffuse independently.
- Higher energy photons are absorbed closer to the surface of the semiconductor, compared to lower energy photons.
Generation Rate of Electron-Hole Pairs
- The generation rate (G) of electron-hole (e-h) pairs per unit volume can be calculated using the formula: G= αNe-αx
- N = photon flux (photons per unit area per second)
- α= absorption coefficient
- x = distance from the surface
Photon Flux Example
- Problem: Find the photon flux of a crystal with the absorption coefficient of 103 cm-1.
- The distance from the surface to the back is 0.2 cm, and the generation per unit volume is 400 cm3 per second.
- Solution: N = 2.89 x 10^86 / cm^2 s
Absorption Coefficient
- The absorption coefficient, α, of silicon depends on the vacuum wavelength of light at a certain temperature (300 K).
Light Switch Off
- When light is switched off, the system returns to a state of equilibrium, and the generated electron-hole pairs disappear.
- Electrons and holes wander around and eventually meet and recombine.
- Defects or impurities within or at the surface of the semiconductor material result in recombination.
Carrier Lifetime and Diffusion Length
- Carrier lifetime is the average duration before recombination happens post electron-hole generation.
- For silicon, the typical carrier lifetime is 1 µs.
- Carrier diffusion length is the average distance a carrier travels from its generation point until recombination.
- For silicon, the typical carrier diffusion length is 100–300 µm.
- Material quality and solar cell suitability can be determined using these two parameters.
- Directionality must be applied to the moving electrons for a semiconductor to produce any power.
- Functional solar cells are generally made from semiconductor material by adding a rectifying p-n junction.
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