Optical Detectors, Exercises of Optics

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Optical Detectors

Wei-Chih Wang

Department of Power Mechanical Engineering

National Tsing Hua University

• Photodiodes are designed to detect photons andcan be used in circuits to sense light.– Phototransistors are photodiodes with someinternal amplification. Note

: Reverse current flows through thephotodiode when it is sensing light.

If photons excite carriers in a reverse-biased pn junction, a very smallcurrent proportional to the light

intensity flows.

The sensitivity depends on the

wavelength of light.

Photodiodes and Phototransistors

Semicoductor types (interval photoemission)P-N junction (no bias, short circuit)1. Absorbed h w.wang

^ excited e from valence to conduction, resulting in the creation of e-h pair2. Under the influence of a bias voltage these carriers movethrough the material and induce a current in the external circuit.3. For each electron-hole pair created, the result is an electronflowing in the circuit.

A photodiode behaves as a photocontrolledcurrent source in parallel with asemiconductor diode and is governed by thestandard diode equationwhere^ I^ is the total device current,w.wang

I^ is the p^

photocurrent,

I^ is the dark current (leakage dk^ current),

V is the voltage across the diode^0 junction,

q^ is the charge of an electron,

k^ is

Boltzmann's constant, and

T^ is the

temperature in degrees Kelvin.

Two significant features to note from boththe curve and the equation are that thephotogenerated current (

I )^ is additive top

the diode current, and the dark current ismerely the diode's reverse leakagecurrent. Finally, the detector shuntresistance is the slope of the

I - V^ curve

( dV / dI^

) evaluated at

V = 0.

Photodiode Operation

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Quantum efficiency

A photodiode's capability to convert light energy to electricalenergy, expressed as a percentage, is its Quantum Efficiency,(Q.E.). Depends on

, through absorption coefficient, thickness of layers,

Doping, geometry, etc. Operating under ideal conditions ofreflectance, crystal structure and internal resistance, a high qualitysilicon photodiode of optimum design would be capable ofapproaching a Q.E. of 80%.

# of electrons (holes) collected as

I /sec p

r

/r= ---------------------------------------------------e p^

# of incident photons/sec

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Photodiode Responsivity

I^ P

R^

p 

Responsivity

R is defined as the ratio of radiant energy (in watts),^ ^

P ,

incident on the photodiode to the photocurrent output in amperes

I^. It is p

expressed as the absolute responsivity in amps per watt. Please note thatradiant energy is usually expressed as watts/cm^2 and that photodiodecurrent as amps/cm^2. The cm^2 term cancels and we are left withamps/watt (A/W).Since

h ^ = energy of photon,

P = r

h  p^

where

r^ = photon flux = p^

P/ h

 # photons/ sec

(A/W)

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Electron rate then

r^ =^ e^

r^ =p^

P/(h

)

Therefore, the output photo current is

I^ =p^ e P/(h

)

The responsivity may then be written

R=^

e /(h

) =

e 

/(hc) =



(A/W)

h= plank constant = 6.63x

-34^ joule-sec

A typical responsivity curve that shows A/W as a function of wavelengthw.wang

R ^

Silicone baseddetector

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Equivalent Operating Circuitsw.wang

A photodiode behaves as a photocontrolled current source in parallelwith a semiconductor diode

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Types of Optical Detectors

Photon detectors may be further subdivided according to the physical effect that produces thedetector response. Some important classes of photon detectors are listed below.• Photoconductive

. The incoming light produces free electrons which can carry electrical current so that the electrical conductivity of the detector material changes as a function of theintensity of the incident light. Photoconductive detectors are fabricated from semiconductormaterials such as silicon.• Photovoltaic . Such a detector contains a junction in a semiconductor material between a region where the conductivity is due to electrons and a region where the conductivity is dueto holes (a so-called pn junction). A voltage is generated when optical energy strikes thedevice.• Photoemissive . These detectors are based on the photoelectric effect, in which incident photons release electrons from the surface of the detector material. The free electrons are thencollected in an external circuit.

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Photovoltaic

(a) Photovoltaic Operation - R

>>R^ l d

, load line

The generated photocurrent flows through R

causing a voltage across the diode.d

This voltage opposes the band gap potential of the photodiode junction, forwardbiasing it. The value of R

drops exponentially as the illumination increases.d

Thus the photo-generated voltage is a logarithmic function of incident lightintensity. The major disadvantage of this circuit is that the signal depends on T

,d

which typically has a wide spread of values over different production batches.The basic circuit is shown below:

(c) Photoconductive Operation - load line In the photoconductive mode, the generatedphotocurrent produces a voltage across a loadresistor in parallel with the shunt resistance.Since, in the reverse biased mode Rw.wang

isd

substantially constant, large values of Rl maybe used still giving a linear response betweenoutput voltage and applied radiation intensity.This form of circuit is required for high speedof response. The main disadvantage of thismode of operation is the increased leakagecurrent due to the bias voltage, giving highernoise than the other circuit modes alreadydescribed. (Note that the photodiode is reverse-biased.)

Photoconductive

-^ Reverse bias current is mainly due to minority carriers- Photo current increases significantly in reverse bias –- diffusion current outside the depletion region diffusionis slow process (high potential barrier)w.wang