Slide Chapter 4

7/31/2019 Slide Chapter 4

1/19

Optical Communications

Chapter 4: Photodiode


2/19

Content

Working principles

Optical characteristics

Modulation characteristic

PIN and APD

Noise


3/19


4/19

PN Junction

Reverse biased:increases depletionregion, preventscarriers from

moving across thejunction

Forward biased:

decreases depletion

region, diffusescarriers across thejunction


5/19

Working principles

Working principles


6/19

Working principles

Working principles


7/19




8/19




9/19




10/19

Modulation characteristics


Shunt Resistance, RSHthe slope of the I-V curve at V=0 Series Resistance, RSarises from the resistance of the contacts and

the resistance of the undepleted silicon

Junction Capacitance, CJthe boundaries of the depletion region act asthe plates of a parallel plate capacitor


11/19




12/19

Modulation characteristics tDRIFT, the charge collection time of the carriers in the depleted

region of the photodiode.

tDIFFUSED, the charge collection time of the carriers in the undepletedregion of the photodiode.

tRC, the RC time constant of the diode-circuit combination.

Response time

Modulationbandwidth


13/19

PIN

PIN photodiode


14/19

APD

APD photodiode


15/19

PIN vs APD


16/19

PIN vs APD High reverse voltage (~10 to 100V), sometimes just below breakdown. M depends strongly on the reverse voltage.

The excess noise factor increases with M. Therefore, the reverse voltage isoften chosen such that the multiplication noise approximately equals thenoise of the electronic amplifier

The detection bandwidth can be very high, because operation with a smallershunt resistor is acceptable.

InGaAs APDs are significantly more expensive than Ge APD, but exhibit

superior noise performance and a higher detection bandwidth.


17/19

Noise

Noise sources

Quantum (shot noise)

Bulk dark current

Surface dark current

Thermal noise

2 2 2

2 2 2

2 2

2 2

2 ( )

2 ( )

2

4

Q Q p

DB DB D

DS DS L

BT T

L

i qI BM F M

i qI BM F M

i qI B

k TBi

R

< >= =

< >= =

< >= =

< >= =

B: bandwidth

M: APD gainF(M): exceed noise factor


18/19

Noise

Signal to Noise ratio


19/19

Conclusion

Photodiodes use reverse-bias PN junction to

convert photons to electrons Due to the semiconductor material, the optical

bandwidth of photodiode is limited

Due to the working principles and structure of aphotodiode, the modulation bandwidth is limited

PIN photodiode is cheaper but has a lower gain

APD photodiode has better gain but increasesnoise

Slide Chapter 4

Documents