EE529 Semiconductor Optoelectronics

Lih Y. Lin

EE 529 Semiconductor Optoelectronics – Photodetectors and Solar Cells

EE529 Semiconductor Optoelectronics

Photodetectors and Solar Cells 1. Photodetector noise 2. Performance parameters 3. Photoconductors 4. Junction photodiodes 5. Solar cells

Reading: Liu, Chapter 14: Photodetectors; Bhattacharya, Chapter 10: Solar Cells Ref: Bhattacharya, Sec. 8.2-8.3

Lih Y. Lin


Photodetector Noise

2

Shot noise: 2, 2 ( )n sh s b di eB i i i= + +

2, 2 ( )n sh s b di eBGF i i i= + +: signal currentsi: background radiation currentbi: dark currentdi

Thermal noise: 2 2, , ,4 /n th B n th n thP k TB i R v R= = =

Exercise: A photodetector without internal gain has a load resistance of 50 R = Ω and a bandwidth of B = 100 MHz. Input optical power is adjusted to generate photocurrent ranging from 1 µA to 10 mA. Discuss the behavior of its SNR vs photocurrent. At what photocurrent is the shot noise equal to thermal noise?

for photodetectors with internal gain G

2 2/ :F G G= Excess noise factor

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Noise Characteristics of Photodetectors

3

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EE 529 Semiconductor Optoelectronics – Photodetectors and Solar Cells Discussion: Noise Characterization

for a QD Photoconductor

4

This figure is from the paper “Ultrasensitive solution-cast quantum dot photodetectors” published in Nature in 2006. The device structure is shown in Slide 7. The noise characterization was done using a lock-in amplifier, which reported a noise current in A/Hz1/2. From the experimental results presented in Figure (b), determine the NEP and root-mean-square noise current at various modulation frequencies.

1/21/2

rms( )

(2 2 4 / ) = (W)

n

b d B

iNEP

ei ei k T R B

=

+ +R

R1/2

1/2 1( )* (cm Hz )( )

BD WNEP

−= ⋅ ⋅A

Normalized detectivity

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Linearity and Dynamic Range

5

Dynamic Range (DR) 10logsat

sPNEP

=

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Speed and Frequency Response

6

3

0.35dB

r

ft

=

Considering the rectangular time interval used to define the electrical bandwidth B when discussing noise,

3

0.443 0.886dBf BT

= =

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Photoconductor Structure and Principle

“Ultrasensitive solution-cast quantum dot photodetectors,” Nature (2006)

Photogenerated carriers drift across the photoconductor multiple times during their lifetime. Gain

7

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Exercise: Photoconductor Gain

8

An n-type GaAs intrinsic photoconductor for 850λ = nm has the following parameters: 100 ml w= = µ , 1 md = µ , 4 11 10 cm−α = × at 850 nm, 1collη = , and 1tη = with

antireflection coating on the incident surface. It’s lightly doped with 12 30 1 10 cmn −= × .

GaAs has the following characteristic parameters at 300 K: 013.2ε = ε at DC or low frequencies, 2 1 18500 cm V se

− −µ = , 2 1 1400 cm V sh− −µ = , 6 32.33 10 cmin −= × . The

bimolecular recombination coefficient 11 3 18 10 cm sB − −= × . (a) Find the external quantum efficiency for this device. (b) Under an incident optical power of 1sP = µW on the detection area, what is the carrier lifetime assuming bimolecular recombination dominates? (c) Find the dark conductivity. The device is biased at V = 2 V. Is the device limted by a

space-charge effect at any level of input optical signal? (d) What are the gain and the responsivity of this device? (e) What is the space charge-limited gain?

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Exercise: Photoconductor Noise

9

The photoconductor considered in the previous exercise is loaded with a sufficiently large resistance such that the resistive thermal noise is negligible compared to the shot noise from its dark current at the operating temperature of 300 K. The background radiation noise is also negligible. The incident wavelength 850λ = nm. (a) Find the dark resistance of the device. Then, find its dark current at 2V bias. (b) Find the NEP of the device for a bandwidth of 1 Hz, NEP/ 1/2B (W Hz-1/2). (c) Find the specific detectivity D* for the device. (d) Discuss how gain affects the NEP for a photoconductor.

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p-n Junction Photodiode

10

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p-i-n Photodiode

11

p+

i-S i n+

SiO2E lec trode

ρnet

–eNa

eNd

x

x

E(x)

R

Eo

E

e–h+

Iph

hυ > Eg

W

(a)

(b)

(c)

(d)

Vr

Vo ut

E lec trode

Drawbacks of p-n junction photodiode: (1) High junction capacitance → long RC time. (2) Thin depletion layer → low quantum efficiency. (3) Depletion width changes as bias changes. (4) Non-uniform e-field in the depletion region. Transit time across the depletion layer /tr dW vτ =→ Desirable to operate at saturation velocity (vd = vsat)

[ ](1 )1 exp( )s

pheP R

i Wh

−= − −α

ν

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Photodetection Modes

12

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Exercise: Si p-i-n Photodiode

13

Discuss the responsivity of a Si p-i-n photodiode at λ = 900 nm, given Ps = 100 nW and the reflection coefficient of the top surface = 32%. What would be the photocurrent and responsivity if the depletion layer thickness is 20 µm? What would be the maximum responsivity given an ideal device structure? Discuss the possible drawbacks of such a structure. For 20 µm-thick depletion layer, what’s the 3-dB cutoff frequency assuming saturation velocities are achieved for both electrons and holes, and the photodiode bandwidth is limited by its transit time?

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Solar Radiation Spectrum

14

AM0: Solar spectrum in outer space

AM1: Solar spectrum at sea level under normal light incidence

AM2: Solar spectrum at an incident angle resulting in twice the path length through the atmosphere

Solar cell aircraft Helios (Source: NASA Dryden Research Center)

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Example: Solar Cell Driving a Load

15

Solar cell area: 1cm x 1cm Illumination light intensity: 900 W m-2

Load resistance: 16 Ohm

What are the current and voltage in the circuit? What is the power delivered to the load? What is the efficiency of the solar cell? Assume it is operating close to the maximum efficiency point, what is the fill factor?

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Absorption isn’t the Whole Story

16 (Atwater and Polman, “Plasmonics for improved PV devices,” Nature Materials 2010)

Light-trapping structures are desirable Anti-reflection surface is necessary

Si nanoshells

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Utilizing the Full Solar Spectrum

17

Multi-junction or tandem structure

Tandem colloidal QD solar cell

Sargent group, Nature Photonics (2011)

(b)

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EE529 Semiconductor Optoelectronics