Chapter 5

Optical Detector

Introduction

A detector’s function is to convert the received optical signal into an electrical signal, which is then amplified before further processing.

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ILight

a) High sensitivity at the operating wavelength.

b) High fidelity. To reproduce the received signal waveform with fidelity (Example: for analog transmission the response of the photodetector must be linear with regard to the optical signal over a wide range).

c) Large electrical response to the received optical signal. The photodetector should produce a maximum electrical signal for a given amount of optical power.

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Requirements:

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d) Short response time. (pn-µsec, PIN/APD-nsec)

e) Minimum noise.

f) Stability.

g) Small size

h) Low bias voltage.

i) High reliability

j) Low cost

Optical Detection Principles

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Operation of the p-n photodiode: (a) the structure of the reverse biased p-n junction illustrating carrier drift in the depletion region; (b) the energy band diagram of the reverse biased p-n junction showing photogeneration and the subsequent separation of an electron-hole pair.

Fig. 5.1

(a)

(b)

This device is reverse biased and the electric field develop across the p-n junction sweeps mobile carriers (holes and electrons) to their respective majority sides (p and n).

A photon incident in or near the depletion region of this device which has an energy greater than or equal to the bandgap energy Eg of the fabricating material (i.e. hf > Eg) will excite an electron from the valence band into the conduction band.

This process leaves an empty hole in the valance band and is known as the photogeneration of an electron-hole (carrier) pair.

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Carriers pairs so generated near the junction are separated and swept (drift) under the influence of the electric field to produce a displacement current in the external circuit in excess of any reverse leakage current (Fig 5.1 (a)).

Photogeneration and the separation of a carrier pair in the depletion region of this reverse biased p-n junction is illustrated in Fig. 5.1 (b).

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Absorption outside depletion region – diffusion current - reduces speed.

Absorption inside depletion region – drift current – fast due to the large electrical field.

The depletion region must be sufficiently thick to allow a large fraction of the incident light to be absorbed in order to achieve maximum carrier pair generation. (PN 1 to 3µm, PIN 20 to 50µm).

However, since long carrier drift times in the depletion region restrict the speed of operation of the photodiode it is necessary to limit its width.

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Photodetector Characteristics1. Responsivity Responsivity - ratio of current output to light input

varies with wavelength theoretical maximum resposivity: 1.05A/W at 1300nm typical responsivity: 0.8 - 0.9 A/W at 1300nm formula for theoretical maximum responsivity (quantum efficiency = 100%)

where:R = theoretical maximum responitivity in Amps/Watt = quantum efficiency = wavelength in nanometers

1240

R

R = ηeλ

hc

e=1.6e-19, h=6.63e-34, c=3e8

1 AWP

IR

o

p

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Quantum Efficiency - ratio of primary electron-hole pairs created by incident photons to the photons incident on the diode material

Figure 5.2 Typical Spectral Response of Various Detector Materials

(Illustration courtesy of Force, Inc.)

2. Quantum Efficiency

pr

re

number of electrons collectedNumber of incident photons

where rp is the incident photon rate (photon per second) and re

is the corresponding electron rate (electrons per second)

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3. Capacitance of a detector dependent upon the active

area of the device and the reverse voltage across the device.

A smaller active diameter makes it harder to align the fiber to the detector.

Also, only the center should be illuminated

photodiode response is slow at the edges

edge jitter

Figure 6.2 Capacitance versus Reverse Voltage

(Illustration courtesy of Force, Inc.)

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Response Time

Time needed for the photodiode to respond to optical input and produce an external current

Dependent on photodiode capacitance load resistance design of photodiode

Measured between 10% and 90% of amplitude

90%

10%

Vout

Time

Rise Time

Fall Time

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Response Time

Approximate -3 dB frequency formula:

where:R = Impedance that the detector operates intoC = Capacitance of the detector

Rise or fall time formula:

Formula for and f-3dB

RCf dB 2

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RC2.2

dBf 3

35.0

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Problems

Calculate the theoretical maximum responsivity of a detector at 1550nm.

Calculate the theoretical maximum responsivity of a detector at 820nm.

Calculate the -3dB frequency and rise time of a detector with a capacitance of 0.5pF operating into an impedance of 50.

Calculate the responsively of a detector with quantum efficiency of 10% at 800 nm.

Ans: 6.45 A/W A detector operating at 800 nm produces an output current of 80 A for an

incident light beam of power 800 W. Calculate the quantum efficiency and responsivity of the detector.

Ans: 0.1 A/W , 15.5%

Answers: 1.25 Amps/Watt, 0.661 Amps/Watt, 6.4 GHz

Semiconductor diodes can be classified into two categories

1. With internal gain

2. Without internal gain

Semiconductor photodiodes without internal gain generate a single electron hole pair per absorbed photon.

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Semiconductor Photodiodes

Semiconductor Photodiodes Without Internal Gain

a) P-N Photodiode as given in figure 5.3

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In the depletion region the carrier pairs separate and drift under the influence of the electric field, whereas outside this region the hole diffuses towards the depletion region in order to be collected .

The diffusion process is very slow compared to the drift process and thus limits the response of the photodiode.

Figure 5.3

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p-n photodiode showing depletion and diffusion regions.

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It is therefore important that the photons are absorbed in the depletion region.

Thus it is made as long as possible by decreasing the doping in the n type material.

The depletion region width in a p-n photodiode is normally 1-3µm and is optimized for the efficient detection of light at a given wavelength.

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Typical output characteristics for the reverse-biased p-n photodiode is illustrate in Fig 5.4.

The different operating conditions may be noted moving from no light input to a high light level.

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Figure 5.4

In order to allow operation at longer wavelengths where the light penetrates more deeply into the semiconductor material a wider depletion region is necessary.

To achieved this the n-type material is doped so lightly that it can be considered intrinsic, and to make a low resistance contact a highly doped n-type (n+) layer is added.

This creates a p-i-n (or PIN) structure as may be seen in Fig. 5.4 where almost all the absorption takes place in the depletion region.

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b) p-i-n Photodiode

Figure 5.4

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p-i-n photodiode showing combined absorption and depletion region.

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Germanium p-i-n photodiodes which span the entire wavelength range of interest are also commercially available, but the dark current is relatively high.

Dark current arises from surface leakage currents as well as generation- recombination currents in the depletion region in the absence of illumination.

Semiconductor Photodiode with Internal Gain - Avalanche Photodiode (APD)

The APD has more sophisticated structure than the p-i-n photodiode in order to create an extremely high electric field region.

Therefore, as well as the depletion region where most of the photons are absorbed and the primary carrier pairs generated there is a high field region in which holes and electrons can acquire sufficient energy to excite new electron-hole pairs.

The process is known as impact ionization and is the phenomenon that leads to avalanche breakdown.

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It requires very high reverse bias voltage (100-400 V) in order that the new carriers created by impact ionization can themselves produce additional carriers by the same mechanism as shown in Fig. 5.5 (b).

Carrier multiplication factors as great as 105 may be obtained using defect free materials to ensure uniformity of carrier multiplication over the entire photosensitive area.

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High reverse voltage. This accelerates electrons and holes thereupon acquires high energy. They strike neutral atoms and generates more free charge carriers. These secondary charges then ionize other carriers.

Primary generated electrons strike bonded electrons at the VB and excite them to the CB. Known as Impact Ionization.

The main advantage compared to p-i-n photodiode is the multiplication or gain factor, M.

Figure 5.5

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(a) (b)

(a) Avalanche photodiode showing high electric field (gain) region. (b) Carrier pair multiplication in the gain region.

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Often an asymmetric pulse shape is obtained from the APD which results from a relatively fast rise time as the electrons are collected and a fall time dictated by the transit time of the holes travelling at a slower speed.

Hence, although the use of suitable materials and structures may gives rise times between 150 and 200 ps, fall times of a 1 ns or more are quite common which limit the overall response of the device.

Drawbacks of The Avalanche Photodiode

1. Fabrication difficulties due to their more complex structure and hence increased cost.

2. The random nature of the gain mechanism which gives an additional noise contribution.

3. The high bias voltages required (100-400 V).

4. The variation of the gain with temperature.

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Multiplication Factor

The multiplication factor M is a measure of the internal gain provided by the APD.

It is define as:

where I is the total output current at the operating voltage and IP is the initial or primary photocurrent.

The gain M, increases with the reverse bias voltage, Vd.

where n=constant and VBR is the breakdown voltage of the detector which is usually around 20 to 500V. 31

Bandwidth: Maximum frequency or bit rate that a photodiode can detect. Determined by the response time.

The response time limited by three factors.

1) The transit time of the carriers across the absorption region, =d/Vsat

2) The RC time constant incurred by the junction capacitance (Cj) of the diode and its load. Cj =A/d. is the permittivity of the semiconductor and A is the active area of the photodiode.

3) The time taken by the carriers to perform the avalanche multiplication process (for APD).

Vsat=saturation velocity

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Comparison between PIN and APD

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Material Structure Risetime λ(µm) R(A/W) Dark Current (nA)

Gain

Silicon PIN 0.5 0.3-1.1 0.5 1 1

Germanium PIN 0.1 0.5-1.8 0.7 200 1

InGaAs PIN 0.3 0.9-1.7 0.6 10 1

Silicon APD 0.5 0.4-1.0 75 15 150

Germanium APD 1.0 1.0-1.6 35 700 50

InGaAs APD 0.25 1.0-1.7 12 100 20

Selection Chart

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CHOICES 0.6-0.8 µm 0.8-0.9 µm 1.2-1.7 µm

SOURCE LED LED LED LED(1.3µm)

LASER VCSEL VCSEL LD

FIBER GLASS 3 dB/km <1 dB/km

MM GRIN MM GRIN MM GRIN

SMF SMF

PLASTIC 160 dB/km

SI SI

DETECTOR MATERIAL Si Si InGaAs

Ge Ge Ge

PIN PIN PIN PIN

APD APD

Q1: Compare the important properties between a PIN photodiode and an

Avalanche Photodiode (APD).