Chapter 4

Photodetectors

Content

• Physical Principles of Photodiodes

• pin, APD

• Photodetectors characteristics (Quantum efficiency,

Responsivity)

• Photodiode Response Time

Introduction

• Light detection is a process that converts incident light

into an electrical photocurrent.

• A light detection device is used at the front end of every

optical receiver to generate a photocurrent proportional

to the incident light intensity.

• Two main types (both semiconductors):

– Photoconductors – their conductivity increases when

the intensity of the incident light increases.

– Photodiodes – absorb photons and generate

photocurrent and are the primary type used in

communication systems for their high response

speeds. Two types – PIN diodes and APDs.

Absorption Coefficient

• One common characteristics of every light detection device is its

light absorption ability.

• Light absorption is mainly determined by the absorption coefficient α

of the detection device and the wave length of the light.

Cont’d

Cont’d

• For an absorption layer of thickness d and

absorption coefficient α, the fraction of

light power absorbed is

• When the energy of the incident photons is

smaller than the energy gap of the

material, i.e, hf < Eg or

λ>λc=(hc/Eg[ev])[μm], photons can not be

absorbed to excite EHPs. α drops sharply

when the wavelength exceeds λc.

)1(d

e

Responsivity

• To quantify the photon absorption ability, a

parameter called the quantum efficiency (η) is

used.

• It is the ratio of the no of excited EHPs to the

total number of incident photons. It is given by

• Where R = r2 is the reflectivity at the front

material. The equation assumes that a larger

fraction of the incident power is absorbed. η can

be increased by having a smaller R, a larger

thickness or a larger α

)1)(1( deR

Cont’d

• Once the quantum efficiency of a light detection

device is known, the corresponding responsivity

of the device is defined as

• The responsivity of a light detection device is the

ratio of the output photocurrent to the incident

light power when the current gain is unity. It

increases with λ but it has a sudden drop due to

the wavelength cutoff condition.

WAhf

q/

24.1

Photoconductors

• Two main types of Photoconductions

• Intrinsic – is an intrinsic semiconductor.

• Extrinsic – a semiconductor with either N-type or P-type

doping. Its conductivity increases when electrons (or

holes) are excited from the N-type (or P-type) impurity

level.

• Because intrinsic photoconductors require photons of

much higher energies, they exhibit a strong long-

wavelength cutoff effect.

• Extrinsic semiconductors have free carriers, so they

have low resistance. This is undesirable from the thermal

noise consideration.

Photodiodes

• Two types – PIN and APDs.

• The structure of a typical PIN diode is shown below.

Cont’d

• A photon with sufficient energy (hf) can excite an

electron-hole pair. If the pair is in the presence

of a large electric field, the electron and hole will

be separated and move quickly in opposite

direction, resulting in a photocurrent.

• If the pair is in the presence of a small or zero

electric field, they move slowly and may even

recombine and generate heat.

• Therefore, a strong electric field in the depletion

region is essential.

Cont’d

• Because one absorbed photon generates

one EHP in PINs, the photocurrent is a

linear function of the input power Pin.

• At zero input power, the reverse bias

current is called the dark current. The total

current is thus

inininph PPPhf

qI

24.1

phdtot III

Cont’d

• Unlike LEDs and LDs, photodiodes are generally

operated at reverse bias for detection in optical

communications. Some reasons:

– Photodiodes have large resistance at reverse bias.

Low resistance is undesirable from thermal noise

consideration.

– The electric field in the absorption layer is large at

reverse bias – carriers move quickly to the external

ckt – fast response – larger BW.

– The width of the depletion region is large at reverse

bias – results in small junction capacitance – small

RC time constant – fast response.

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

)( s

)(xP

0P

(eV)

24.1)m(

g

cE

)1)(1()()1()(

0 f

w

f RePwPR s

Responsivity

• The primary photocurrent resulting from absorption is:

• Quantum Efficiency:

• Responsivity:

)1)(1()(

0 f

w

p RePh

qI s

hP

qI P

/

/

photonsincident of #

pairs atedphotogener hole-electron of #

0

[A/W] 0

h

q

P

I P

Responsivity vs. wavelength

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 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

I

IM

MIPI

MMh

q0APD

Photodetector Response Time

• The response time of a photodetector with its output circuit depends mainly

on the following three factors:

1- The transit time of the photocarriers in the depletion region. The transit

time depends on the carrier drift velocity and the depletion layer

width w, and is given by:

2- Diffusion time of photocarriers outside depletion region.

3- RC time constant of the circuit. The circuit after the photodetector acts

like RC low pass filter with a passband given by:

dt dv

d

dv

wt

TT CRB

2

1

Photodiode response to optical pulse

Typical response time of the

photodiode that is not fully depleted

QUESTIONS ??

23