Chapter 6 PhotodetectorsChapter 6 Photodetectors -- OutlineOutline

6.1 Physical Principles of Photodiodes6.1 Physical Principles of Photodiodes-- The pin photodetectorThe pin photodetector-- Avalanche PhotodiodesAvalanche Photodiodes

6.2 Photodetector Noise6.2 Photodetector Noise-- Noise SourcesNoise Sources-- SignalSignal--toto--noise Rationoise Ratio

6.3 6.3 Detector response timeDetector response time

Chapter 6 PhotodetectorsChapter 6 Photodetectors

Prefer to have: High response or sensitivity in emission wavelengthMinimum addition of noiseFast response or sufficient bandwidth to handle the desirable date rate

o Insensitive to variations in temperature

Of the semiconductor-based photodetectors, the photodiode is used almost exclusively for fiber optic systems because of its small size, suitable material, high sensitivity, and fast response time.

PIN photodetectorAvalanche photodetector (APD)

6.1 Physical Principle of Photodetectors6.1 Physical Principle of Photodetectors6.1.1 The PIN photodetectors6.1.1 The PIN photodetectors

PIN: p region, intrinsic region, n regionApply reverse-bias voltageWhen a photon with an energy greater than Eg is incident, it become absorbed to generate a free electron-hole pair (EHP). The field E in the depletion layer separates EHP and drifts them in opposite directionsThe drifting carriers generate a current, called photocurrent I ph.

Depletion region

EW

6.1 Physical Principle of Photodetectors6.1 Physical Principle of Photodetectors

Upper cut-off wavelength : 1.24(μm) , (6 2)( )c

g g

hcE E eV

λ = = −

/number of electron-hole paris generated , (6 5)number of incident photons /

p

o

I qP h

ηγ

= = −Quantum efficiency :

Responsivity :

Example 6-2In a 100-ns pulse, 6x106 photons at a wavelength of 1300 nm fall on an InGaAs photodetector. On the average, 5.4x106 electron-hole pairs are generated. Find the quantum efficiency.

Example 6-3Photons of energy 1.53x10-19 J are incident on a photodiode which has a responsivity of 0.65 A/W. If the optical power level is 10 μ W, find photocurrent.

Example 6-4As shown in Fig.6.4, for the wavelength range 1300 nm < λ < 1600 nm, the quantum efficiency for InGaAs is around 90%. Find responsivity at 1300 nm.

, (6 6)p

o

I qP h

ην

= = −RIp: photocurrent; Po: optical power

ParametersParameters

6.1 Physical Principle of Photodetectors6.1 Physical Principle of Photodetectors

6.1.2 Avalanche Photodiodes6.1.2 Avalanche Photodiodes

Avalanche photodiodes (APDs) internally multiply the primary signal photocurrent before it enters the input circuitry of the following amplifier.A commonly used structure is the reach-through construction : n+ : p : i (π) : p+

High-field regionIn high-field region, photon-generated EHP can gain sufficient kinetic to impact-ionizes bound electrons in valence band and releases EHPsThese generated EHPs can also be accelerated in the high-field region to sufficiently large kinetic energies to further cause impact-ionization and release more EHPs, which leads to an avalanche of impact ionization processes.

6.1 Physical Principle of Photodetectors6.1 Physical Principle of Photodetectors

6.1.2 Avalanche Photodiodes6.1.2 Avalanche Photodiodes

Avalanche magnification :

Multiplied photocurrent , (6 7)Primary unmultipled photocurrent

M

p

IMI

= = −

, (6 8)q Mhη

γ= −RAPD

Responsivity :

Example 6-5A given silicon avalanche photodiode has a quantum efficiency of 65%at a wavelength of 900 nm. Suppose 0.5 μ W of optical power produces a multiplied photocurrent of 10 μ A. Find the multiplication M.

6.2 Photodetector Noise6.2 Photodetector Noise

6.2.1 Noise Source6.2.1 Noise Source

Noise power :- Quantum noise - Dark current ( bulk dark current; Surface dark current)- Thermal noise due to load resistor

Signal powerNoise power

SN

=Signal-to-noise ratio S/N :

Signal power :

0q P

hη

νIp : Photocurrent ; P0 : Optical powerIp = R P0 =

- Bulk dark current : arises from thermal EHPs in the pn junction

6.2 Photodetector Noise6.2 Photodetector Noise

6.2.1 Noise Source6.2.1 Noise Source

Dark Current : It is the current that continues to flow through the bias circuit of the device when no light is incident on the photodiode.

2 2 2 ; for both PIN & APD (6 15)DS DS Li qI Bσ= = −IL: Surface leaking current

- Surface dark current : Surface leakage current

2 2 2 2 2PIN: 2 ; APD: 2 ( ), (6 14)DB DB D DB DB Di qI B i qI BM F Mσ σ= = = = −ID: primary upmultiplied detector bulk dark current; B: receiver bandwidth;

Thermal noise due to load resistor RL :

2 2 4 , (6 17)BT T

L

k Ti BR

σ= = −kB: Boltzmann’s constantT: absolute temperatureRL: resistance of load resistor

Quantum noise : The discrete nature of photons means that there is an unavoidable random fluctuation in the rate of arrival of photons. The quantum nature of the photon therefore gives rise to a statistical randomness in the EHP photogeneration process.

2 2 2 2 2PIN : 2 ; APD: 2 ( ); (6 13)Q Q p Q Q pi qI B i qI BM F Mσ σ= = = = −Ip: Average value of photocurrent; M: Avalanche magnification ; F(M): Noise figure; B: receiver bandwidth

6.2 Photodetector Noise6.2 Photodetector Noise

6.2.1 Noise Source6.2.1 Noise Source

Example 6-6A given InGaAs pin photodiode has the following parameters at a wavelength of 1300 nm: ID=4 nA, η = 0.9, RL=1000Ω, and the surface leakage current is negligible. The incident optical power is 300 nW(-35 dBm), and the receiver bandwidth is 20 MHz. Let us find the various noise terms of the receiver.

6.2 Photodetector Noise6.2 Photodetector Noise

6.2.2 Signal6.2.2 Signal--toto--noise Rationoise Ratio

2Signal powerPIN : , (6.18)Noise power 2 ( ) 2 4 /

p

p D L p L

ISN q I I B qI B k TB R

= =+ + +

2 2

2

Signal powerAPD : , (6.18)Noise power 2 ( ) ( ) 2 4 /

p

p D L p L

I MSN q I I BM F M qI B k TB R

= =+ + +

Thermal noise (due to load resistor) dominates in PIN photodiodes, and negligible in APD photodiodes.

6.3 Photodetector Response Time6.3 Photodetector Response TimeThe response time of a photodiode together with its output circuit depends mainly on the following three factors:

1. The transit time of the photocarriersgenerated outside the depletion region.

2. The diffusion time of the photocarriersgenerated outside the depletion region

3. RC time constant of the photodiode and its associated circuit.

6.3 Photodetector Response Time6.3 Photodetector Response Time

The response time of a photodiode together with its output circuit depends mainly on the following three factors: 1. The transit time

2. The diffusion time3. RC time constant

Transit time (also called drift time) :is the time that photogenerated carriers across the width W of the intrinsic layer

d

, (6 27)vdWt = − W: depletion layer width

vd: carrier drift velocity

- Depletion layer width W: ↑ W allows more photons to be absorbed, but it slow down the drift time

- Carrier drift velocity vd : to reduce drift time, we may increase vd by increasing the applied field. However, at high fields, the drift velocity will saturate.

6.3 Photodetector Response Time6.3 Photodetector Response TimeDiffusion time tdiff :

is the time that it takes for an electron to diffuse across the p+ side (of length l ) to reach the depletion layer.

2

2diffe

ltD

= l: length of p+ sideDe: Diffusion coefficient

(see ref. “Optoelectronics and photonics”, p229)

ExampleA reverse biased pin photodiode is illuminated with a short wavelength photon that is absorbed very near the surface as shown in above figure. The photogeneratedelectron has to diffuse to the depletion region where it is swept into the i-layer and drifted across. What is the response time of this photodiode if the i-Si layer is 20 μmand the p+ layer is 1 μm and the applied voltage is 120 V ? The diffusion coefficient (De) of electrons in the heavily doped p+ region is approximately 3x10 -4 m2/s.

RC time constant :CT=Ca//CdRT=RL//Ra

TT CR=τ(with neglecting the small series resistor Rs)