PHOTODIODESPHOTODIODESIntroduction to Semiconductor EngineeringIntroduction to Semiconductor Engineering

MATERIALS AND CONSTRUCTION

Materials• The material used to make a photodiode is

critical to defining its properties, because only photons with sufficient energy to excite electrons across the material's bandgap will produce significant photocurrents.

Materials commonly used to produce photodiodes include:

• Material Wavelength range (nm)

Silicon190–1100

Germanium400–1700

Indium gallium arsenide800–2600

Lead sulfide<1000-3500

• Because of their greater bandgap, silicon-based photodiodes generate less noise than germanium-based photodiodes, but germanium photodiodes must be used for wavelengths longer than approximately 1 µm

• Since transistors and ICs are made of semiconductors, and contain P-N junctions:

almost every active component is potentially a photodiode.

• Many components, especially those sensitive to small currents, will not work correctly if illuminated, due to the induced photocurrents.

• In most components this is not desired, so they are placed in an opaque housing. Since housings are not completely opaque to X-rays or other high energy radiation, these can still cause many ICs to malfunction due to induced photo-currents.

Construction • Hamamatsu photodiodes can be classified by

manufacturing method and construction into: five types of silicon photodiodes and two types each of GaAsP and GaP photodiodes.

Photodiode Construction • Silicon photodiodes are

constructed from single crystal silicon wafers similar to those used in the manufacture of integrated circuits

• The major difference is that photodiodes require higher purity silicon

• The purity of silicon is directly related to its resistivity, with higher resistivity indicating higher purity silicon.

• Centro Vision products utilize silicon whose resistivities range from 10 Ohm-cm to 10,000 Ohm-cm.

A cross section of a typical silicon photodiode is shown in the figure:

• N type silicon is the starting material.

• A thin "p" layer is formed on the front surface of the device by thermal diffusion or ion implantation of the appropriate doping material (usually boron).

• The interface between the "p" layer and the "n" silicon is known as a pn junction.

• The back contact is the cathode, the front contact is the anode.

• The active area is coated with either silicon nitride, silicon monoxide or silicon dioxide for protection and to serve as an anti-reflection coating.

• The thickness of this coating is optimized for particular irradiation wavelengths.

Planar Diffusion Type• An SiO2 coating is

applied to the P-N junction surface, yielding a photodiode with a low level dark current.

Low-Capacitance Planar Diffusion Type• A high-speed version of the planar

diffusion type photodiode. This type makes use of a highly pure, high-resistance N-type material to enlarge the depletion layer and thereby decrease the junction capacitance, thus lowering the response time to 1/ 10 the normal value. The P layer is made extra thin for high ultraviolet response.

PNN+ Type• A low-resistance N+ material

layer is made thick to bring the NN+ boundary close to the depletion layer. This somewhat lowers the sensitivity to infrared radiation, making this type of device useful for measurements of short wavelengths.

PIN Type• An improved version of the low-

capacitance planar diffusion device.

• It Uses: extra high-resistance I layer between the P- and N-layers to improve response time.

• exhibits even further improved response time when used with reversed bias.

• designed with high resistance to breakdown and low leakage for such applications

Schottky Type• A thin gold coating is

sputtered onto the N material layer to form a Schottky Effect P-N junction. Since the distance from the outer surface to the junction is small, ultraviolet sensitivity is high

Avalanche Type• If a reverse bias is applied to a P-N

junction and a high-field formed within the depletion layer, photon carriers will be accelerated by this field.

• They will collide with atoms in the field and secondary carriers are produced, this process occurring repeatedly.

• known as the avalanche effect and, since it results in the signal being amplified .

• this type of device is ideal for detecting extremely low level light

ELECTRICAL CHARACTERISTICS

Photodiode Equivalent Circuit

IL = current generated by

the incident light

ID = diode current

CJ = junction capacitance

RSH = shunt resistance

RS = series resistance

I’ = shunt resistance current

VD = voltage across the diode

Io = output current

Vo = output voltage

Shunt Resistance

• Shunt resistance is used to determine the noise current in the photodiode in photovoltaic mode.

• It is the slope of the current-voltage curve of the photodiode at the origin.

• Ideal photodiode should have an infinite shunt resistance.

• Non-ideal photodiodes have typical values ranging from 10’s to 1000’s of Mega ohms.

Series Resistance• Series resistance is used to determine the linearity of the photodiode in

photovoltaic mode.

• Series resistance of a photodiode arises from the resistance of the contacts and the resistance of the undepleted silicon is given by:

• Ideal photodiodes have no series resistance

• Non-ideal photodiodes have typical values ranging from 10 to 1000 Ω.

Junction Capacitance• Junction capacitance is used to determine the speed of the

response of the photodiode.

• Junction capacitance is the capacitance that exist at PN junction which is dependent on the thickness of depletion region.

• The junction capacitance is directly proportional to the diffused area and inversely proportional to the width of the depletion region.

• Furthermore, the capacitance is dependent on the reverse bias as follows:

Response Time• Response time is the certain amount of time to respond to a

sudden change in light levels.

• The response time is expressed in terms of the following:

1. tR = Rise time Rise time is the time required for the output to rise from 10% to 90% of its final value.

2. tF = Fall time Fall time is the time required for the output to fall from 90% to 10% of its final value.

• There are three factors defining the response time of a photodiode:

1. tDRIFT, the charge collection time of the carriers in the depleted region of the photodiode.

2. tDIFFUSED, the charge collection time of the carriers in the undepleted region of the photodiode.

3. tRC, the RC time constant of the diode-circuit combination.

tRC = 2.2RC

Rise Time and Fall Time

Rise Time and Fall Time• Total rise time is determined by:

• In photovoltaic mode, rise time is dominated by the diffusion time for diffused areas less than 5mm2 and by RC time constant.

• In fully depleted photoconductive mode, the dominant factor is drift time.

• In non-fully depleted photoconductive mode, all the drift time, diffused time and RC time constant contribute to the response time.

OPTICAL CHARACTERISTICS

ResponsivityResponsivity

• The responsivity of a silicon photodiode is a measure of the sensitivity to light, and it is defined as the ratio of the photocurrent IP to the incident light power P at a given wavelength: R= Ip / P

• it is a measure of the effectiveness of the conversion of the light power into electrical current.

A typical responsivity curve that shows A/W as a

function of wavelength is given below.

Quantum EfficiencyQuantum Efficiency

• A photodiode's capability to convert light energy to electrical energy, expressed as a percentage, is its Quantum Efficiency, (Q.E.). The sensitivity of a photodiode may also be expressed in practical units of amps of photodiode current per watt of incident illumination.

Relationship of Responsivity and Quantum Efficiency

• It is a measure of the effectiveness of the conversion of the light power into electrical current. It is related to responsivity by:

Q.E= R observed

R ideal (100%)

= R hc

q

= 1.24 10^3 R

Quantum Efficiency for a PhotodiodeQuantum Efficiency for a Photodiode

• η ≡Number of corresponding electrons in the external circuit Number of incident photons

• The quantum efficiency η is less than one because of:

a. Fresnel reflection at the photodiode surface

b. Absorption of photons in areas other than the depletion region

c. Recombination in the depletion region

Non- uniformityNon- uniformity• Non-Uniformity of response is defined as variations of

responsivity observed over the surface of the photodiode active area with a small spot of light. Non-uniformity is inversely proportional to spot size, larger non-uniformity for smaller spot size.

• Accurate determination of the responsivity of silicon photodiodes are highly desired in photometry. It affects power measurements especially in photodiodes with large active areas.

Non- linearityNon- linearity• Non-Linearity is the variation of the ratio of the change

in photocurrent to the same change in light power, i.e. I/ P.

• The linearity exhibits the consistency of responsivity over a range of light power.

• The lower limit of the photocurrent linearity is determined by the noise current and the upper limit by the series resistance and the load resistance.

PHOTODIODES APPLICATIONSPhotodiodes are used in many different types of circuits and

applications. Here are a few examples of where photodiodes have been used:

A handheld digital ambient light meter, showing an f-stop of 5.6 for 24 frame/s 500 ISO filming

Medical

A portable pulse oximeter registering a satisfactory saturation reading

Safety Equipment

A residential wall-mounted smoke detector. The "test" button is visible on the lower part of the image.

Automotive

Automatic Headlight Dimmers

Communications

1.) Optical Fiber - An optical fiber (or fibre) is a glass or plastic fiber that carries light along its length. Fiber optics is the overlap of applied science and engineering concerned with the design and application of optical fibers. Optical fibers are widely used in fiber-optic communications, which permits transmission over longer distances and at higher bandwidths (data rates) than other forms of communications. Fibers are used instead of metal wires because signals travel along them with less loss, and they are also immune to electromagnetic interference.

2.) Optical Communications

3.) Optical Remote Control A bundle of optical fibers

Industry

A typical handheld barcode scanner