Photodetectors Read: Kasip, Chapter 5

Yariv, Chapter 11 Class Handout

ECE 162C Lecture #13

Prof. John Bowers

Definitions

•  Quantum efficiency η: Ratio of the number of electrons collected to the number of photons incident.

•  Responsivity: current out divided by optical power incident

AWhc

eheRd /24.1

ηληλ

νη ===

External Quantum Efficiency and Responsivity

Different contributions to the photocurrent Iph. Photogeneration profiles corresponding to short, medium and long wavelengths are also shown.

Schematic photogeneration profiles

photonsincident ofNumber collected and generated EHPfree ofNumber

=eη

External quantum efficiency (QE) ηe of the detector

υη

hPeI

o

phe /

/=

Responsivity R

o

ph

PI

==(W) Power OpticalIncident

(A)nt PhotocurreR

hce

he

eeλ

ηυ

η ==R

Responsivity R

Responsivity (R) vs. wavelength (λ) for an ideal photodiode with QE = 100% (ηe = 1) and for a typical inexpensive commercial Si photodiode. The exact shape of the responsivity curve depends on the device structure.

The line through the origin that is a tangent to the responsivity curve at X, identifies operation at λ1 with

maximum QE

pin Photodiode

The responsivity of Si, InGaAs and Ge pin type photodiodes. The pn junction GaP detector is used for UV detection. GaP (Thorlabs, FGAP71), Si(E), IR enhanced Si (Hamamatsu S11499), Si(C),

conventional Si with UV enhancement, InGaAs (Hamamatsu, G8376), and Ge (Thorlabs, FDG03). The dashed lines represent the responsivity due to QE = 100 %, 75% and 50 %.

pin Photodiode

WV

WV rr

o ≈+= EE

WAC roεε=dep

d

Wtv

=drift

pin Photodiode

Drift velocity vs. electric field for holes and electrons in Si.

d

Wtv

=drift

Width of i-region (Depletion region)

Drift velocity Transit time (Drift time)

pin Photodiode Speed

A reverse biased pin photodiode is illuminated with a short wavelength light pulse that is absorbed very near the surface. The photogenerated electron has to diffuse to the depletion region where it is swept into the i-layer and drifted across.

In time t, an electron, on average, diffuses a distance given by

= (2Det)1/2

Electron diffusion coefficient

PIN Impulse Response?

PIN Impulse Response?

Levevj hhee σσ +=

Displacement current flows. That is what is measured in an external circuit, not conduction current.

Avalanche Photodiodes (APDs) •  α Rate at which electrons multiply •  β Rate at which holes multiply

•  A large ratio of α/β or β/α results in a large gain bandwidth product and low noise amplification. True for Si

•  Most III-Vs have a small ratio, and limited gain bandwidth product. The noise is larger, but still lower than a PIN receiver.

E l e c t r i c F i e l d [ k V / c m ]

I n 0 . 5 3 G a 0 . 4 7 A s e l e c t r o n s h o l e s

G a A s e l e c t r o n s h o l e s

I n P h o l e s

e l e c t r o n s

F r o m : S i : P . P . W e b b , " M e a s u r e m e n t s o f I o n i z a t i o n C o e f f i c i e n t s i n S i l i c o n a t L o w E l e c t r i c F i e l d s " , G E C a n a d a I n c .

I n P : L . W . C o o k , e t . a l . , A p p l . P h y s . L e t t . 4 0 ( 7 ) , 1 A p r i l 1 9 8 2

I n G a A s : T . P . P e a r s a l l , A p p l . P h y s . L e t t . 3 6 ( 3 ) , 1 F e b r u a r y 1 9 8 0

G a A s : H . D . L a w a n d C . A . L e e , S o l i d - S t a t e E l e c t r o n i c s , 2 1 , 1 9 7 8

2 0 0 3 0 0 4 0 0 5 0 0 0 . 1

1

1 0

1 0 0

1 0 0 0

1 0 e 4

1 0 e 5

1 0 e 6

Si (electron

s)

Ioni

zatio

n C

oeffi

cien

ts[c

m -1

]

Ionization Coefficients for Semiconductors

- - - -

+ - - -

- -

- - +

+

- -

- -

+ + + +

P+ N+

Si Multiplication Layer

- - - + +

- +

- - + +

- +

- - + +

- +

- - +

InP Multiplication Layer

P+ N+

The Avalanche Multiplication Process

Avalanche Photodiode Gain or Multiplication M

Ionization coefficient ratio

αe = Aexp(-B/E) Chyoweth's law

Avalanche Photodiode Gain or Multiplication M

M = exp(αew) Ionization coefficient

kwkkM

e −−−−

=])1(exp[

1α

Electrons only

Electrons and holes

k = αh / αe

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.

Heterojunction Photodiodes: SAM

Heterojunction Photodiodes: SAM

(a)  Energy band diagrams for a SAM detector with a step junction between InP and InGaAs. There is a valence band step ΔEv from InGaAs to InP that slows hole entry into the InP layer.

(b) An interposing grading layer (InGaAsP) with an intermediate bandgap breaks ΔEv and makes it easier for the hole to pass to the InP layer for a detector with a graded junction between InP and InGaAs. This is the SAGM structure.

Heterojunction Photodiodes: SAM

Simplified schematic diagram of a more practical mesa-etched SAGM layered APD

SAM APDS: Need for Separate Absorption and Multiplication Regions

Small bandgap avalanche regions tend to have large dark current.

Absorption Region

Multiplication Region

e- e- e-

Electric Field

position

Electric Field Simulation for SHIP

-600

-500

-400

-300

-200

-100

0

100

0.4 0.8 1.2 1.6 2

Elec

tric

Fiel

d [k

V/c

m]

Distance into Detector [µm]

Si InGaAs

5 V 10 V 23 V

Gain and Dark Current vs. Bias. 23 µm diameter SHIP

0.01

0.1

1

10

100

0.1

1

10

100

1000

10000

0 5 10 15 20 25

Gai

n

Dar

k C

urre

nt [µ

A]

Reverse Bias [V]

Theoretical Gain Measured Photocurrent Gain

0.1

1

10

1 10 100 1000

3 dB

Ban

dwid

th [G

Hz]

Gain

SHIP Detector 3-dB Bandwidth versus gain

315 GHz GB

Comparison of Acheivable GB Product for SHIP, SL, and InP APDs

0

100

200

300

400

500

600

700

800

0.1 1

Gai

n-B

andw

idth

-Pro

duct

(GH

z)

Multiplication Layer Thickness [µm] 5

SHIP APD SL APD

InP APD

Staircase APD: Use of bandgap engineering to increase the ratio of ionization coefficients.