fotodiodos 2

Question 1

Noise in photodetectors

Answer 1

It is the noise current which determines the minimum optical power level that can be detected. This minimum detectable optical power defines the sensitivity of photodetector
-SNR Can NOT be improved by amplification

Question 2

Noise sources in photodetectors

Answer 2

-Photon noise
-Photoelectron noise
-Gain noise
-Receiver circuit noise

Question 3

noise sources associated with photodetectors

Answer 3

-Quantum (Shot) noise: arises from statistical nature of the production and collection of photo-generated electrons upon optical illumination.
- Dark current noise: is the current that continues to flow through the bias circuit in the absence of the light

Question 4

Surface dark current

Answer 4

-Surface dark current is also known as surface leakage current.
-It depends on surface defects, cleanliness, bias voltage and surface area.
-The surface current can be reduced by using the guard rings so that the surface current should not flow through the load resistor

Question 5

Dark current noise

Answer 5

-When there is no optical power incident on the photodetector a small reverse leakage current still flows from the device terminals.
-Dark-current noise results from random electron-hole pairs generated thermally (or by tunneling).
-This dark current contributes to the total system noise

Question 6

Thermal noise

Answer 6

-Thermal noise results from random thermal motions of the electrons in a conductor

Question 7

Response time depends on…

Answer 7

1. The transit time of the photocarriers in the depletion region.
2. The diffusion time of the photocarriers generated outside the depletion region.
3. The RC time constant of the photodiode and its associated circuit.

Question 8

Parameters responsible for (transient time,diffusion time, RC time constant) :

Answer 8

1. Absorption coefficient, α
2. Depletion region width
3. Photodiode junction and package capacitance
4. Amplifier capacitance
5. Detector load resistor
6. Amplifier input resistance
7. Photodiode series resistance

Question 9

diffusion processes

Answer 9

The diffusion processes are slow compared with the drift of carriers
To have a high speed photodiode,
-Photocarriers should be generated in the depletion region or close to the depletion region.
-Diffusion times should be less than or equal to the carrier drift times.

Question 10

Fast and slow carriers

Answer 10

-Charge carriers produced in the depletion region are separated and collected quickly.
-Electron hole pairs generated in the n and p regions must slowly diffuse to the depletion region before they can be separated and collected.

Question 11

Photodetector response

Answer 11

The risetime of the square-pulse response is determined by the RC circuit-limited bandwidth of the
photodetecto

Answer 12

To achieve a high quantum efficiency, the depletion layer width must be larger than alpha^-1
, so that most of the light will be absorbed.
-At the same time with large width, the capacitance is small and RC time constant getting smaller, leading to faster response, but wide width results in larger transit time in the depletion region. Therefore there is a trade-off between width and QE.

Question 14

Uneven rise and fall times depending on

Answer 14

1.Absorption coefficient
2. Junction Capacitance C