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Source: Photodetection and Measurement

Photodetection Basics

1.1 Introduction

The junction photodiode that is the focus of this book has been described in

detail in many other books and publications. Here only a few basics are

given, so that you can use the photodiode effectively in real circuits. A simple

model is presented that allows the main characteristics and limitations of

real components to be understood. The ability to correctly derive the polarity

of a photodiode’s output, guess at what level of output current to expect, and

have a feel for how detection speed depends on the attached load is necessary.

The model we begin with has little to do with the typical real component fab-

ricated using modern processing techniques; it is a schematic silicon pn-

junction diode.

1.2 Junction Diodes/Photodiodes

and Photodetection

Figure 1.1 a shows two separate blocks of silicon. Silicon has a chemical valency

of four, indicating simplistically that each silicon atom has four electron bonds,

which usually link it to neighboring atoms. However, the lower block has been

doped with a low concentration (typically 10 13 to 10 18 foreign atoms per cm 3 ) of

a ﬁve-valent element, such as arsenic or phosphorus. Because these dopants

have one more valency than is needed to satisfy neighboring silicon atoms, they

have a free electron to donate to the lattice and are therefore called donor

atoms. The donor atoms are bound in the silicon crystal lattice, but their extra

electron can be easily ionized by thermal energy at room temperature to con-

tribute to electrical conductivity. The extra electrons then in the conduction

band are effectively free to travel throughout the bulk material. Because of the

dominant presence of negatively charged conducting species, this doped mate-

rial is called n-type .

By contrast, the upper block has been doped with an element such as boron,

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Chapter

Photodetection Basics

Chapter One

(a) Before Contacting

(b) After Contacting

Space-

charge

density

Electric

field

Acceptor doping (e.g., B)

Depletion Region

Donor doping

(e.g., As,P)

Bound

Free

Figure 1.1 The pn-junction. The presence of predominately different polarities

of free carriers in the two contacted materials leads to asymmetrical conduc-

tivity, a rectifying action. Bound charges are indicated by a double circle and

free charges by a single circle.

0.02 to 0.05 eV) at room temperature the majority of

the dopant atoms are ionized.

If the two doped silicon blocks are forced into intimate contact (Fig. 1.1 b ),

the free carriers try to travel across the junction, driven by the concentration

gradient. Hence free electrons from the lower n-type material migrate into the

p-type material, and free holes migrate from the upper p-type into the n-type

material. This charge ﬂow constitutes the diffusion current, which tends to

reduce the nonequilibrium charge density. In the immediate vicinity of the

physical junction, the free charge carriers intermingle and recombine. This

leads to a thin region that is relatively depleted of free carriers and renders it

more highly resistive. This is called the depletion region . Although the free car-

riers have combined, the charged bound donor and acceptor atoms remain,

giving rise to a space charge, negative in p-type and positive in n-type material

and a real electric ﬁeld then exists between the n- and p-type materials.

If a voltage source were applied positively to the p-type material, free holes

would tend to be driven by the total electric ﬁeld into the depletion region and

on to the n-type side. A current would ﬂow. The junction is then termed

forward-biased . If, however, a negative voltage were applied to the p-type

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which exhibits a valency of three. Because boron lacks sufﬁcient electrons to

satisfy the four surrounding silicon atoms and tries to accept one from the sur-

rounding material it is termed an acceptor atom. As with donors, the bound

boron atoms can easily be ionized, effectively transferring the missing electron

to its conduction band, giving conduction by positive charge carriers or holes .

The doped material is then termed p-type . The electrical conductivity of the two

materials depends on the concentration of ionized dopant atoms and hence on

the temperature. Because the separation of the donor energy level from the

conduction band and the acceptor level from the valence band in silicon is very

small (energy difference

Photodetection Basics

Photon

Space-

charge

density

Electric

field

Depletion Region

Figure 1.2 When a photon with energy greater than the material

bandgap forms a hole-electron pair, a terminal voltage will be gen-

erated, positive at the p-type anode.

material, carriers would remain away from the depletion region and not con-

tribute to conduction. The pn-junction is then called reverse-biased , and has

very little current ﬂow. Note that the diffusion currents driven by concentra-

tion gradients and the ﬁeld currents driven by the electric ﬁeld can have dif-

ferent directions. The conventional designation of the p-type contact is the

anode (A); the n-type contact is the cathode (K).

This basic pn-junction diode model can also explain how a photodiode detec-

tor functions. Figure 1.2 shows the same diode depicted in Fig. 1.1 in schematic

form, with its bound dopant atoms (double circled) and free charge carriers

(single circled). A photon is incident on the junction; we assume that it has an

energy greater than the material bandgap, which is sufﬁcient to generate a hole-

electron pair. If this happens in the depletion region, the two charges will be

separated and accelerated by the electric ﬁeld as shown. Electrons accelerate

toward the positive space charge on the n-side, while holes move toward the p-

type negative space charge. If the photodiode is not connected to an external

circuit, the anode will become positively charged. If an external circuit is pro-

vided, current will ﬂow from the anode to the cathode.

1.3 TRY IT! Junction Diode Sensitivity and

Detection Polarity

The validity of this model can easily be tested. All diode rectiﬁers are to some extent

photosensitive, including those not normally used for photodetection. If a glass encap-

sulated small signal diode such as the common 1N4148 is connected to a voltmeter as

shown in Fig. 1.3 and illuminated strongly with light from a table lamp the anode will

become positive with respect to the cathode. The efﬁciency of this photodiode is

not high, as light access to the junction is almost occluded by the chip metallization.

Nevertheless you should see a few tens of millivolts close to a bright desk lamp.

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Photodetection Basics

Chapter One

Anode becomes

positive

Light

Any glass-encapsulated

silicon diode (e.g., 1N4148)

or LED

Similar LEDs detect

their own light

Figure 1.3 Any diode, even a silicon rectiﬁer, can show photosensitivity if the

light can get to the junction. LEDs generate higher open circuit voltages than

the silicon diode when illuminated with light from a similar or shorter wave-

length LED.

Rather more efﬁcient are light emitting diodes (LEDs), having been designed to let

light out of (and therefore into) the junction. Any common LED tested in this way

will show a similar positive voltage on the anode. LEDs have the advantage of a higher

open-circuit voltage over silicon diodes and photodiodes. This voltage gives an indi-

cation of the material’s bandgap energy ( E g , see Table 1.1). Although with a silicon

diode ( E g ª 1.1 V) you might expect 0.5 V under an ordinary desk lamp, a red LED ( E g

ª 2.1 V) might manage more than 1 V, and a green LED ( E g ª 3.0 V) almost 2 V. This

is sufﬁcient to directly drive the input stages of low voltage logic families such as 74

LVC, 74 AC, and 74 HC in simple detection circuits.

This works because the desk lamp emits a wide range of energies, sufﬁcient to gen-

erate photoelectrons in all the diode materials mentioned. However, if the photon

energy is insufﬁcient, or the wavelength is too long, then a photocurrent will not be

detected. My 470-nm blue LEDs generate negligible junction voltage under the desk

lamp. Try illuminating different LEDs with light from a red source, such as a red-

ﬁltered desk lamp or a helium neon laser. You should detect a large photovoltage with

the silicon diode, and perhaps the red LED, but not with the green LED. The bandgap

in the green emitter is simply too large for red photons to excite photoelectrons. You

can take this game even further if you have a good selection of LEDs. My 470-nm LED

gets 1.4 V from a 660-nm red LED as detector but nothing reversing the illumination

direction. Similarly the 470 nm generates 1.6 V from a 525-nm emitting green LED

but nothing in return. These results were obtained by simply butting together the

molded LED lenses, so the coupling efﬁciency is far from optimized. The above

bandgap model suggests that LED detection is zero above the threshold wavelength

and perfect below. In reality the response at shorter wavelengths is also limited by

excessive material absorption. So they generally show a strongly peaked response only

a few tens of nanometers wide, which can be very useful to reduce sensitivity to inter-

fering optical sources. See Mims (2000) for a solar radiometer design using LEDs as

selective photodetectors. Most LEDs are reasonable detectors of their own radiation,

although the overlap of emission and detection spectra is not perfect. It can occa-

sionally be useful to make bidirectional LED–LED optocouplers, even coupled with

fat multimode ﬁber. Chapter 4 shows an application of an LED used simultaneously

as emitter and detector of its own radiation.

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Photodetection Basics

probe against the top surface.

Illuminate the contact point with a bright red LED modulated at 1 kHz. You should

see a strong response on the scope display. This “cat’s whisker” photodetector is about

as simple a demonstration of photodetection as I can come up with! This isn’t a semi-

conductor pn-junction diode, but a metal-semiconductor diode like a Schottky diode.

It seems that almost any junction between dissimilar conducting materials will

operate as a photodetector, including semiconductors, metals, electrolytes, and more

fashionably organic semiconductors.

1.4 Real Fabrications

Although all pn-junction diodes are photosensitive, and a diode can be formed

by pressing together two different semiconductor (or metal and semiconductor)

materials in the manner of the ﬁrst cat’s whisker radio detectors or the

previous TRY IT! demonstrations, for optimum and repeatable performance we

usually turn to specially designed structures, those commercially produced.

These are solid structures, formed, for example, by diffusing boron into an n-

type silicon substrate as in Fig. 1.4 (similar to the Siemens BPW34). The dif-

fusion is very shallow, typically only a few microns in total depth, and the

pn-junction itself is thinner still. This structure is therefore modiﬁed with

respect to the simple pn-junction, in that the diffusions are made in a high resis-

tivity (intrinsic conduction only) material or additionally formed layer with a

doping level as low as 10 12 cm -3 , instead of the 10 15 cm -3 of a normal pn-junction.

This is the pin -junction photodiode, where “i” represents the thick, high-

resistivity intrinsic region. Most photodetectors are fabricated in this way. The

design gives a two order of magnitude increase in the width of the space-charge

region. As photodetection occurs only if charge pairs are generated close to the

high-ﬁeld depleted region of the structure, this helps to increase efﬁciency and

Photon

AR-coating

Contact metal (Al)

Isolation (SiO 2 )

Anode

p-diffusion

(e.g., Boron)

Space-

charge

Epitaxial

intrinsic layer

(1–10kW cm)

n-type substrate

(5mW cm)

Contact metal

(AuSb)

Cath ode

Figure 1.4 Most photodiodes are formed by diffusing dopants into epi-

taxially formed layers. The use of a low conductivity intrinsic layer

leads to thickening of the space-charge region, lower capacitance, and

improved sensitivity.

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For another detection demonstration, ﬁnd a piece of silicon, connect it to the ground

terminal of a laboratory oscilloscope and press a 10-M

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