Chapter 6

Photodetectors

Content• Physical Principles of Photodiodes• pin, APD• Photodetectors characteristics (Quantum efficiency,

Responsivity, S/N)• Noise in Photodetector Circuits• Photodiode Response Time• Photodiodes structures

Introduction

• At the output end of an optical transmission line, there must be a receiving device which interprets the information contained in the optical signal.

• The first element of this receiver is a photodetector.

• The photodetector senses the luminescent power falling upon it and converts the variation of this optical power into a correspondingly varying electric current.

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 give rise to a current Flow in an external circuit, known as photocurrent.

w

Energy-Band diagram for a pin photodiode

Photocurrent• Optical power absorbed, in the depletion region can be written in terms

of incident optical power, :

• Absorption coefficient strongly depends on wavelength. The upper wavelength cutoff for any semiconductor can be determined by its energy gap as follows:

• Taking entrance face reflectivity into consideration, the absorbed power in the width of depletion region, w, becomes:

)1()( )(0

xsePxP [6-1]

)( s

)(xP0P

(eV) 24.1)m(

gc E

[6-2]

)1)(1()()1( )(0 f

wf RePwPR s

Optical Absorption Coefficient

Responsivity

• The primary photocurrent resulting from absorption is:

• Quantum Efficiency:

• Responsivity:

)1)(1( )(0 f

wp ReP

hqI s

[6-3]

hPqI P

//

photonsincident of #pairs atedphotogener hole-electron of #

0

[6-4]

[A/W] 0

hq

PI P [6-5]

Responsivity vs. wavelength

Avalanche Photodiodes

• Avalanche Photodiodes ( APDs ) are high sensitivity, high speed semi-conductor "light" sensors. Compared to regular PIN construction photodiodes.

• APDs, have an internal region where electron multiplication occurs, by application of an external reverse voltage, and the resultant "gain" in the output signal means that low light levels can be measured at high speed.

• An APD is operated under a reverse-bias voltage which is sufficient to enable avalanche multiplication (impact ionization) to take place.– vary large internal current gain– fast response to the light modulated at microwave frequencies– high sensitivity to low-level optical signals with about 1 ns

response time particular useful in fiber optic communication.• The avalanche action enables the gain of the diode to be increased

many times, providing a much greater level of sensitivity.

Avalanche Photodiode (APD)APDs internally multiply the

primary photocurrent before it enters to following circuitry.

In order to carrier multiplication take place, the photogenerated carriers must traverse along a high electric field region. In this region, photogenerated electrons and holes gain enough energy to ionize bound electrons in VB upon colliding with them. This multiplication is known as impact ionization. The newly created carriers in the presence of high electric field result in more ionization called avalanche effect.

Reach-Through APD structure (RAPD) showing the electric fields in depletion

region and multiplication region.

Optical radiation

Responsivity of APD

• The multiplication factor (current gain) M for all carriers generated in the photodiode is defined as:

• Where is the average value of the total multiplied output current & is the primary photocurrent.

• The responsivity of APD can be calculated by considering the current gain as:

p

M

II

M [6-6]

MI PI

MMhq

0APD

[6-7]

Current gain (M) vs. Voltage for different optical wavelengths

Photodetector Noise & S/N

• Detection of weak optical signal requires that the photodetector and its following amplification circuitry be optimized for a desired signal-to-noise ratio.

• 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. That is the optical power that generates a photocurrent with the amplitude equal to that of the total noise current (S/N=1)

power noiseamplifier power noisetor photodetecntphotocurre frompower signal

NS

Noise Sources in Photodetecors

• The principal noises associated with photodetectors are : 1- Quantum (Shot) noise: arises from statistical nature of the production

and collection of photo-generated electrons upon optical illumination. It has been shown that the statistics follow a Poisson process.

2- Dark current noise: 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 e and h in the pn junction, and the surface dark current, due to surface defects, bias voltage and surface area.

• In order to calculate the total noise presented in photodetector, we should sum up the root mean square of each noise current by assuming that those are uncorrelated.

• Total photodetector noise current=quantum noise current +bulk dark current noise + surface current noise

Comparisons between PIN and APD detectors• APDs have greater sensitivity for a given material

– detection of weaker signals over longer distances of fiber at a higher bit rate.• APDs have higher dark current.

– PIN diode may have greater sensitivity for long wavelength detection than an APD.• PIN diodes are easier to fabricate.

– Hence produce higher yield devices and array at lower cost.• PIN diodes are easier to integrate (with amplifiers)• APDs are very temperature sensitive for dark current and multiplication factor.

– Temperature stabilization circuitry is usually needed to increase their reliability.• APDs require a high (tens or hundreds of volts) operating voltage.

The use of APDs is thus presently limited to applications where high gain is of paramount important.– In an APD one has to maintain very stable voltage and temperature values which

males the system costly and somewhat unreliable especially if it is placed in a region that is difficult to access (like in undersea regenerators for long distant optical communications)

Comparison of various photodetectors

Comparison of photodetectors