B.TECH - Semester 7 opto electronic devices Question Paper 2019 (jun)
Practice authentic previous year university questions for better exam preparation.
Sample Questions
- Draw band to band recombination in direct band gap and indirect band gap Semiconductors.
- Give an account on Franz-Keldysh effect with a simple illustration.
- Calculate the photocurrent density of a Si p-i-n photodiode with $8 \mu \mathrm{~m}$ i-region when $0.87 \mu \mathrm{~m}$ light power density $0.5 \mathrm{~W} / \mathrm{cm}^{2}$ is incident upon it. It is assumed the top illuminated surface is coated...
- Draw the equivalent circuit of a PIN diode and mention what each circuit element Indicates.
- With mathematical support, explain the effects of bias dependence on barrier heights in triangular barrier diode.
- Compute the output power from a GaAs LED with an electron current of 1 mA and efficiency $\eta_{0}=50 \%$. Wavelength of emission is $0.87 \mu \mathrm{~m}$.
- The total efficiency of an injection laser with a GaAs active region is $18 \%$. The voltage applied to the device is 2.5 V and the bandgap energy for GaAs is 1.43 eV . Calculate external power efficiency of the device.
- Determine the relative population of the two states in a ruby laser that produces a light beam of wavelength 694.3 nm at 300 K and 500 K .
- What are the possible processes which contribute to the total loss in a two level lasing system?
- Explain axial and transverse modes existing in a Fabry Petrot cavity. Answer any two questions from each Module. Each question carries 10 marks.
- Explain different types of absorption of photons in semiconductors with neat diagrams.
- (a) Derive an expression for radiative life time of carriers in a direct bandgap semiconductor.
- (b) A germanium p-i-n diode with active dimensions $100 \times 50 \mu \mathrm{~m}$ has a quantum efficiency $55 \%$ when operating at wavelength of $1.3 \mu \mathrm{~m}$. The measured dark current at this wavelength is 8 nA . Compute NEP and specific...
- With neat sketches explain photoluminescence measurement system and set up for time resolved photoluminescence measurement.
- (a) Explain the principle of MSM photodiode with its energy band diagram.
- (b) A $5 \mathrm{~cm}^{2}$ Ge solar cell with a dark reverse current of 2 nA has radiation incident upon it, producing $4 \times 10^{17}$ electron-hole pairs per second. The electron and hole diffusion lengths shall be assumed as $5 \mu \mathrm{~m}$....
- (a) Explain the principle and operation of an edge emitting LED. ..... 5
- (b) A planar LED is fabricated from GaAs with R.I. of 3.6. Calculate the optical power emitted into the air as a percentage of internal optical power for the device when the transmission factor at crystal-air interface is 0.68 ..... 5
- Write notes on(a) Wavelength Selective Detection(b) Coherent Detection.10 Module - III
- (a) Derive the expression for gain in a two level lasing medium. ..... 5
- (b) Calculate the number of modes of an AIGaAs laser supported by the gain spectrum which has a bandwidth of 6 nm . The cavity length of the laser is $200 \mu \mathrm{~m}$ and emission wavelength is 800 nm . 5
- (a) Derive Einsteins Relation for a two level system having population densitiesN 1 and N 2 respectively.
- (b) Calculate the ratio of spontaneous to stimulated emission rates in a tungsten lamp that radiates an average frequency of $5 \times 10^{14} \mathrm{~Hz}$ at an operating temperature of 1300 K .
- Write notes on Surface Emitting Lasers and Rare Earth Doped Lasers 10 $$ \text { ( } 6 \times 10=60 \text { Marks) } $$