Chapter4 Detect v2

8/18/2019 Chapter4 Detect v2

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Chapter 4

Photodetectors


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Types of photodetectors:

Photoconductos

Photovoltaic

Photodiodes

Avalanche photodiodes (APDs)

Resonant-cavity photodiodes

MSM detectors

In telecom we mainly use PINs and APDs.


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Basic operation of a PN photodetector:

Absorption takes place in 3 places: (1) within depletion region, (2) away

from it, and (3) near it.

Those from category (1) generate photocurrent (as done in class)

Those from category (2) recombine and are not useful

Those from category (3) diffuse (if they don’t recombine) to the depletion

region and then generate photocurrent. Diffusion is a slow process,

however.

Bottom line: We want to confine the absorption to the depletion region

A PIN diode has this advantage


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p+

SiO2

Electrode

ρ net

– eN a

eN d x

x

E ( x)

R

E max

e – h+

I ph

hυ > E g

W

E

n

Depletion region

(a)

(b)

(c )

Antireflection

coating

V r

(a) A schematic diagram of a reverse biased pn junction photodiode. (b) Net space charge across the diode in thedepletion region. N d and N a are the donor and acceptor

concentrations in the p and n sides. (c). The field in thedepletion region.

Electrode

V out

© 1999 S.O. Kasap, Optoelectronics (Prentice Hall)


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e – h+

i ph(t )

Semiconductor

(a )

V

x

(b)

(a) An EHP is photogenerated at x = l. The electron and the hole drift in opposite

directions with drift velocities vh and ve. (b) The electron arrives at time t e = ( L − l)/ve an

the hole arrives at time t h = l/vh. (c) As the electron and hole drift, each generates an

external photocurrent shown as ie(t ) and ih(t ). (d) The total photocurrent is the sum of hole

and electron photocurrents each lasting a duration t h and t e respectively.

E

l L − l

t

v evhvh

0 Ll

t

e – h+

t h

t e

t

0

t h

t e

i ph(t )

i(t )

t

0

t h

t e

evh /L + eve /Levh /L

ie(t )

ih(t )

(c )

(d)

Charge = e

evh /L ev e /L



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0.2 0.4 0.6 0.8 1.2 1.4 1.6 1.8

Wavelength (µm)

In0.53Ga0.47As

Ge

Si

In0.7Ga0.3As0.64P0.36

InP

GaAs

a-Si:H

12

345 0.9 0.8 0.7

1×103

1×104

1×105

1×106

1×107

1×108

Photon energy (eV)

Absorption coefficient (α ) vs. wavelength (λ ) for various semiconductors(Data selectively collected and combined from various sources.)

α (m-1)

1.0

Figure 5.3


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E

CB

VB

k– k

Direct Bandgap E g Photon

E c

E v

(a) GaAs (Direct bandgap)

E

k– k

(b) Si (Indirect bandgap)

VB

CB

E c

E v

Indirect Bandgap, E g

Photon

Phonon

(a) Photon absorption in a direct bandgap semiconductor. (b) Photon absorption

in an indirect bandgap semiconductor (VB, valence band; CB, conduction band)© 1999 S.O. Kasap, Optoelectronics (Prentice Hall)


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Responsivity ( R) vs. wavelength (λ ) for an ideal photodiode with QE = 100% (η = 1) and for a typiccommercial Si photodiode.

0 200 400 600 800 1000 12000

0.1

0.2

0.3

0.4

0.5

0.6

0.70.8

0.9

1

Wavelength (nm)

Si Photodiode

λ g

Responsivity (A/W)

Ideal Photodiode

QE = 100% ( η = 1)



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Quantum efficiency η, definition.

Examples of QE for different detectors

Responsivity: definition: R=η q P/hν = ηλ (in microns) /1.24 units of

Amps per Watt

Speed: carrier transport time τtr ; RC-time constant τRC ∆f = 1/2π(τtr +τRC)

If the photodetector has gain, there is also a buildup time, τb, associated

with the gain that affects the bandwidth. τb

Tradeoff between responsivity and bandwidth


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p+

i-Si n+

SiO2

Electrode

ρ n et

– eN a

eN d

x

x

E ( x)

R

E o

E

e – h+

I p h

hυ > E g

W

(a)

(b)

(c )

(d)

V r

The schematic structure of an idealized pin photodiode (b) The netspace charge density across the photodiode. (c) The built-in field across the diode. (d) The pin photodiode in photodetection isreverse biased.

V out

Electrode



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hυ > E g

p+ i-Si

e – E

h+

W l

Drift

Diffusion

A reverse biased pin photodiode is illuminated with a shortwavelength photon that is absorbed very near the surface.

The photogenerated electron has to diffuse to the depletionregion where it is swept into the i-layer and drifted across.

V r



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Operation of PIN diodes

Properties;

1. Controllable increase in width of the depletion region (good)

2. Reduced junction capacitance (reduced RC constant, good for speed)

3. Diffusion current is reduced due the increase of the depletion region

width

4. Higher carrier transport time (bad for speed).

5. Next step: reduce absorption in doped layers in a PIN by using higher

bandgap material such as InP (in the case of InGaAs absorber)

6. This is what PIN heterostructures do

7. Next thing is to have gain: impact ionization


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š p+

SiO2Electrode

ρ net

x

x

E ( x)

R

E

hυ > E g

p

I ph

e – h+

Absorptionregion

Avalanche

region

(a)

(b)

(c )

(a) A schematic illustration of the structure of an avalanche photodiode (APD) biased for avalanche gain. (b) The net space charge density across the photodiode. (c) Thefield across the diode and the identification of absorption and multiplication regions.

Electrode


n+


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h+

E

š n+ p

e –

Avalanche region

e –

h

+

E c

E v

(a ) (b)

E

(a) A pictorial view of impact ionization processes releasing EHPs and the resulting avalanche multiplication. (b) Impact of an energeticconduction electron with crystal vibrations transfers the electron's

kinetic energy to a valence electron and thereby excites it to theconduction band.


V


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E

N n

Electrode

x

E ( x)

R

hυ

I p h

Absorption

region

Avalanche

region

InP InGaAs

h+

e – E

InP

P+ n+

Simplified schematic diagram of a separate absorption and multiplication(SAM) APD using a heterostructure based on InGaAs-InP. P and N refer to p and n -type wider-bandgap semiconductor.

V r

V out



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Modes of Operation of an APD

• Recall that APDs are photodetectors that haveoptoelectronic gain

• Linear mode (sub-breakdown):

photocurrent ∝ optical power e.g., optical communication

• Geiger mode (post-breakdown): Ideally, eachdetected photon results in breakdown

e.g., photon counting, coincidencecounting


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Current Photodetectors Needs for Long-haul

Optical Communication

• High quantum efficiency at 1.55 µm (beyond

80%).

• High speed: 10 Gbps (OC 192) and beyond.

• Internal gain: ~10-50 (preferred over externalEDFA amplification).

• Wavelength selectivity (WDM).

• Low noise (excess noise, dark current).

• Compactness, reduced cost, OEIC (solid-state).

Thi M l i li i & Ab i R i


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Thin Multiplication- & Absorption-Region

APDs• Benefits:

– High speed (up to 40 GHz +)

– Low multiplication noise (factor of ~2)

• similar mechanism as noise suppression in superlattice MQW

APDs).

– Higher optimal gain values: better SNR and BER

– Breakdown characteristics (Geiger mode)— “not so good”

• Challenges:

– Quantum efficiency must be enhanced by employing new structures:

• Waveguide structures (lateral absorption)• Resonant-cavity structures (vertical absorption)

• Detection efficiency is low 1.55µm and beyond

• Dark current is always a problem

Ed l d id APD’


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• Edge-coupled waveguide APD’s:

– Idea: Reduce absorption-region (~0.8 µm or less) width

without killing quantum efficiency.

– High gain-bandwidth (> 12 GHz at gain of 10)

– Reduce charge-space effects

– Challenge remains: coupling efficiency (QE ~25%)

InGaAsabsorption

InAlAs

multiplicationInP buffer n: InAlAs

InGaAs cap

p: InAlAs

800nm

p: charge

InP Substrate

[Kinsey et al, 00]

l i g h t

400 nm

1 0 µ m

800 nm


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• Resonant cavity photodiodes

n- contact

}

InP/InGaAsP

Bragg reflector

InGaAs

absorption layer n

p p: InP

n: InP

Input light

.

.

.

i

•Increase: Quantum efficiency

•Increase: Bandwidth

•Wavelength selectivity

•Drawbacks:•Increased fab. complexity

•Selectivity may not

be desirable in someapplications

[Unlu et al, 1995]

Chapter4 Detect v2

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