PDS Fire Detection, Physics and materials

The Photovoltaic Effect

When light is absorbed in the active area an electron-hole pair is formed. The electrons and holes are separated electrons passing to the "n" region and holes to the "p" region. This results in a current generated by light . The migration of electrons and holes to their respective region is called "The Photovoltaic Effect".

Why are We Detecting Fires?

How do you decide what to make a photo diode out of ?

• Fire detectors are designed to save lives.
• They are used to detect the smoke caused by a fires
• There are many different variations of this device

The materials chosen to make a photo diode depends on the wavelength of light that the diode to needs to see.

The following shows which material is best for which wave lengths.

So how do they work?

Material Wavelength range (nm)

Silicon 190–1100

Germanium 400–1700

Indium gallium arsenide 800–2600

Lead sulfide <1000-3500

Constructing a Diode

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

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 layers is known as a pn junction. Small metal contacts are applied to the front surface of the device and the entire back is coated with a contact metal. 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.

Quoted from http://home.sandiego.edu/~ekim/photodiode/pdtech.html

Review

The most basic kind is a photoelectric smoke detector

Structure:

• The choice of materials is bases on what wavelength of light you want to detect. Infrared light has a wave length of 700 nm - 1 mm.
• this Indicates silicon is the best option.

Processing: A P-type film is attached to single crystal silicon (n-type) using thermal diffusion or ion implantation.

Properties:The photodiode is a PN Junction between to semi conductors. Run in reverse bias this allow for it to detect light.

Performance:

• When the infrared light is detected current is created because of the photovoltaic effect.
• Current act as a trigger in the smoke detector and sounds the alarm.

Reverse Bias: Our Photodiode

Photoelectric Smoke Detector

• In reverse bias the battery is flipped causing the electron and holes to be attacked to the battery. This creates a large depletion zone at the junction and prevents current from flowing.
• In the case of our photodiode, this system can be broken.When the light from the LED hits the diode, it can impart enough energy to temporarily create current flow. This is useally used as a trigger.

This model has two main parts.

The light emitting source and a light receiving senor.

• The light emitting source is most commonly an infrared LED source.
• The light receiving sensor is a photodiode.

References

Forward Bias: Our LED

How do these to components work together?

1."Photodiode Design Note." ElectroSchematicscom RSS. N.p., n.d. Web. 05 Dec. 2013.

2."Photodiode Light Detector." Photodetectors. N.p., n.d. Web. 05 Dec. 2013.

3."Photodiode." Wikipedia. Wikimedia Foundation, 12 Jan. 2013. Web. 05 Dec. 2013.

4.SLR-24VN PHOTOELECTRIC SMOKE DETECTOR. Buena Park, CA: Hochiki America Corporation, n.d. PDF.

5. "Photoelectric Sensors." Omron Industrial Automation Singapore. N.p., n.d. Web. 05 Dec. 2013.

6."Smoke Detector." Wikipedia. Wikimedia Foundation, 17 Nov. 2013. Web. 05 Dec. 2013.

7."How Smoke Detectors Work." HowStuffWorks. N.p., n.d. Web. 05 Dec. 2013.

8."Photoelectric Sensor." Wikipedia. Wikimedia Foundation, 11 June 2013. Web. 05 Dec. 2013.

In the Forward Bias, the positive side of the battery is connected to the P-type side and the negative side of the battery is connected to the N-type. This cause the electron to repel away from the battery and toward the junction. The electrons in the N-type fall into holes in the P-type emitting enegry in the form of light.

However when a fire happens and smoke appears, the smoke particles that float into the chamber disrupt the light beam. This causes the light to hit the sensing unit . This triggers the alarm.

When there is no fire or smoke present the Infrared light beam pass by the detector and nothing happens.

How Does a Semiconductor Work?

Semiconductors are made by doping.

Doping is the process of adding a small amount of atoms that have either more electrons to create N-type or less electrons to create P-type.

When these two are put together the create a PN junction.

This junction can work in two ways, either forward or reverse bias.

An LED runs in forward bias and the photodiode runs in Reverse bias.

More about Infrared LEDS

What is a LED?

• A semiconductor diode that converts applied voltage to light.

What is a Infrared LED made of?

• An Infrared LED is made of gallium arsenide or aluminum gallium arsenide.

How can you see an infrared LED?

• Infrared light is not visible to the naked eye so when installed it will be impossible to tell if it is working with out the help of a camera.

What other thing can an infrared LED be used for?

• Infrared LED's are security systems and electronics like a TV remote

How are the two components similar?

Both of these components are semiconductors.

Detecting A Fire

More on Photodiodes

What is a semi conductor?

• a solid substance that has a conductivity between that of an insulator and that of most metals, either due to the addition of an impurity or because of temperature effects.
• Devices made of semiconductors, notably silicon, are essential components of most electronic circuits.

What is a Photodiode?

• a semiconductor diode that, when exposed to light, generates a potential difference or changes its electrical resistance.

What are they made of?

• They can be made of Silicon , Germanium, Indium gallium arsenide, or Lead(II) sulfide
• Each option react with different wavelengths of light for a smoke detector the Silicon is the best option.

What are other uses for a photodiode?

• They are also used in spectroscopy and night vision devices.