Pin Photodetector

These slides give an extensive knowledge about the photo diode. It covers the circuit diagram and its energy band diagram. Also includes important information about noise factors and resoponsitivities.

1. PIN DETECTOR By Deekshitha S

2. PIN PHOTODETECTOR The high electric field present in the depletion region causes photo generated carriers to separate and be collected across the reverse-biased junction. This gives rise to a current flow in an external circuit, known as photocurrent.

3. PHOTOCURRENT Optical power absorbed, in the depletion region can be written in terms of incident optical power: P(x) = P (1-e -𝛂(λ)x ) Absorption coefficient strongly depends on wavelength. The upper wavelength cutoff for any semiconductor can be determined by its energy gap as follows: λ c (µm) = 1.24/E g (eV) Taking entrance face reflectivity into consideration, the absorbed power in the width of depletion region w, becomes: (1-R f )P(w) = P (1-e- 𝛂(λ)x )(1-R f )

4. RESPONSIVITY ( ℜ ) Quantum Efficiency ( η ) = number of e-h pairs generated / number of incident photons η = I p /q P /h𝒗 Responsivity measures the input-output gain of a detector system. In the specific case of a photodetector, responsivity measures the electrical output per optical input. ℜ = I p / P = ηq/h𝒗 mA/mW

5. When λ<<λ c absorption is low When λ>λ c no absorption λ c = hc/E g

6. LIGHT ABSORPTION COEFFICIENT The upper cutoff wavelength is determined by the band gap energy E g of the material. At lower-wavelength end, the photo response diminishes due to low absorption (very large values of 𝛂 s )

7. SPEED OF RESPONSE 1 The transit time of the photo-generated carriers through the depletion region 2 The electrical frequency response as determined by the RC time constant, which depends on the diode’s capacitance 3 The slow diffusion of carriers generated outside the depletion region

8. Rise Time Rise time is the time the output signal takes to rise from 10% to 90% of the peak value after the input is turned on instantaneously. Fall Time Fall time is the the time the output signal takes to drop from 90% to 10% of the peak value after the input is turned off abruptly.

9. NOISE SOURCES IN PHOTODETECTORS THERMAL NOISE This is the spontaneous fluctuation due to the thermal interaction between the electrons and the vibrating ions in a conduction medium and it is specially prevalent in resistors in room temperature . QUANTUM NOISE The quantum or shot noise arises from the random nature of the production and collection of photogenerated electron when an optical signal is incident on a photodiode. DARK CURRENT NOISE It is the current that continues to flow through the bias circuit in the absence of the light. This is the combination of bulk dark current, which is due to thermally generated electron and hole in the p-n junction.

10. SIGNAL TO NOISE RATIO In fiber optic communication systems, the photodiode is generally required to detect very weak optical signals. Detection of weak optical signals requires that the photodetector and it’s amplification circuitry be optimized to maintain a given signal-to-noise ratio. The power signal-to-noise ratio S/N at the output of an optical receiver is defined by: SNR can’t be improved by amplification

11. NOISE-EQUIVALENT POWER Noise-Equivalent Power (NEP) is the minimum input optical power to generate photocurrent, equal to the rms noise current in a 1 hertz bandwidth. This more directly measures the minimum detectable signal because it compares noise directly to optical power. NEP depends on the frequency of the modulated signal, the bandwidth over which noise is measured, the detector area, and the operating temperature.

12. Thank You