Experiment to find the bond gap of a semiconductor material by transmittance using a spectrophotometer

Determine Energy Band Gap (Eg) and Absorption Coefficient (α) from Transmittance vs Wavelength.

🎯 Objective

To determine the optical energy band gap (Eg) and absorption coefficient (α) of a semiconductor sample using a spectrophotometer.

⚙️ Apparatus

• Spectrophotometer device
• Semiconductor sample on glass substrate
• Reference glass slide
• Power supply
• Recording sheet

🧭 Theory

The energy band gap (Eg) of a semiconductor separates the valence and conduction bands. When light of various wavelengths passes through the material, photons with energy greater than Eg are absorbed, while those with lower energy are transmitted. The relationship between energy and wavelength is given by E = 1240/λ (in eV·nm). By measuring transmittance versus wavelength, Eg can be found from the critical wavelength (λc) where transmittance rises sharply.

🧩 Procedure

1. Place the reference and test samples in the spectrophotometer.
2. Adjust to 100% transmittance using the reference at 400 nm.
3. Measure the transmittance of the sample from 400–900 nm in 20 nm steps.
4. Record the readings in the interactive table below.
5. Analyze the data to plot Transmittance vs Wavelength.
6. Identify the sharp rise and determine λc.
7. Calculate Eg = 1240 / λc and α = (1/x)·ln(1/T).

Sample Thickness (nm)

Wavelength (nm)	Transmittance (%)	Action

Scientific Interpretation:

Results

Energy Band Gap (Eg)

—

Absorption Coefficient (α)

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Critical Wavelength (λc)

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nm