Lecture 21 Semiconductor Detectors - Photodetectors §  Principle of the pn junction photodiode §  Absorption coefficient and photodiode materials §  Properties of semiconductor detectors §  The pin photodiodes §  Avalanche photodiodes

milimeter size components

Array waveguide grating

DFB/DBR Lasers Ø  High threshold Ø  Non-planar Ø  Integration Ø  Slow Ø  Expensive  

Optical-electronic Conversion: Ø  For switching & routing Ø  Very expensive Ø  Time consuming

Source 1

Source 2

Source N

Source M or Receiver M Source 1

Source 2

Source N Add/drop node

electronics fiber

Source 1

Source 2

Source N

Multiplexer D

e-M

ultip

lexe

r Multiplexer

Optical à Electrical Electrical à Optical

wire

modulators

Fiber Optic Communications

pn Junction Photodiode

§  Photons generates electron hole pairs -EHP (photogeneration)

§  Separates charged carriers (electrons and holes)

§  Annular electrode for light to enter

§  Antireflection (AR) coatings at the ends

§  Highly doped p+ region (1 micron thick)

§  Depletion zone extends to n-region (few microns)

§  Reverse biased (5-20 V) much larger than V0=1V

§  Drifting carriers cause photocurrent

Photodetector: Converts incident light to an electrical signal (voltage or current)

ndpa WNWN =

§  Photogeneration inside the SCL generates an electron and a hole.

§  Both fall their respective energy hills i.e. they drift, and cause a photocurrent Iph in the external circuit.

pn Junction Photodiode

§  Photogeneration occurs in the neutral region.

§  The electron has to diffuse to the depletion layer and then roll down the energy hill i.e. drift across the SCL.

Photosynthesis & Solar Power Harvesting

Electrons and holes must be separated without energy input !!

hυ → e− + h+

A shorted pn junction. The photogenerated electron and hole in the SCL roll down their energy hills, i.e. drift across the SCL, and cause a current Iph in the external circuit.

pn Junction Photodiode (Photovoltaic Mode) Short Circuit

The pn junction in open circuit. The photogenerated electron and hole roll down their energy hills (drift) but there is a voltage Voc across the diode that causes them the diffuse back so that the net current is zero.

Open Circuit

Photodetection Modes

Photodetection Modes (Forward)

pn junction diode

Photodetection Modes (Reverse)

pn junction diode

Photodetection Modes

Photodetector Photodiode Mode

Photodetection Modes

Photodetector Photovoltaic Mode (short circuit)

Photodetection Modes

Photodetector Photovoltaic Mode (open circuit)

An electron and hole pair (EHP) is photogenerated at x = l. The electron and the hole drift in opposite directions with drift velocities vh and ve.

Question

What is the induced current iph(t)?

Shockley-Ramo Theorem

Work done eEdx is provided by the battery in time dt

Electrical energy provided by the battery in time dt = Vie(t)dt

Thus, eEdx = Vie(t)dt. In time dt, the electron drift a distance dx = vedt

eEdx = Vie(t)dt e(V/L)(vedt) = Vie(t)dt Leti e

ev=)(

Shockley-Ramo Theorem

ee

lLtv)( −=

th =

lvh

Electron transit times

Hole transit time

Shockley-Ramo Theorem

Qcollected = ie(t)0

te

∫ dt + ih (t)0

th

∫ dt = e

Leti e

ev=)(

Leti h

hv=)(

t < te

t < th

Total collected charge = e

Wavelength in microns

(micrometers)

)eV(24.1)µm(

gg E

≈λ

Absorption cutoff wavelength

Bandgap in eV

Absorption and Bandgap

d = 1/α = penetration or absorption depth

I x( ) = I0 exp −α x( )

Absorption and Direct/Indirect Transitions

d = 1/α = penetration or absorption depth

Absorption and Direct/Indirect Transitions

I x( ) = I0 exp −α x( )

Semiconductor Eg (eV) λg (µm) Type

InP 1.35 0.91 Direct

GaAs0.88Sb0.12 1.15 1.08 Direct

Si 1.12 1.11 Indirect

In0.7Ga0.3As0.64P0.36 0.89 1.4 Direct

In0.53Ga0.47As 0.75 1.65 Direct

Ge 0.66 1.87 Indirect

InAs 0.35 3.5 Direct

InSb 0.18 7 Direct

Band gap energy Eg at 300 K, cut-off wavelength λg and type of bandgap (Direct and Indirect) for some photodetector materials

Absorption and Direct/Indirect Transitions

photonsincident ofNumber collected and generated EHPfree ofNumber =eη

υη

hPeI

o

phe /

/=

External Quantum Efficiency

Number of Electrons

Number of Photons

Responsivity R

o

ph

PI

==(W) Power OpticalIncident

(A)nt PhotocurreR

hce

he

eeλη

υη ==R

Photogeneration profiles corresponding to short, medium and long wavelengths are also shown.

Schematic Photogeneration profiles

External Quantum Efficiency

photons absorbed ofNumber atedphotogener EHPofNumber y EfficiencQuantumInternal ==iη

Assuming lp is very thin, and assuming W >> Lh

)]exp(1[)0( WhPeI oi

ph αυ

η −−≈ T

T = Transmission coefficient of AR coating α = Absorption coefficient

Internal Quantum Efficiency

pn Photodiode Limitations

1)  Narrow depletion zone lead to less absorption 2)  Low RC times due to low capacitance

pin Photodiode

§  Larger absorption region (intrinsic region) §  Smaller capacitance lead to faster RC time (50-100 ps) §  Response time is limited by electron travel time in intrinsic region