Large Area Photo Diodes - V2 1586 Words

Large Area Photodiode; a report

BY THOMAS DAVID FOXEN (B00133844)

10th December 2008 Large Area Photodiodes; a report

Dated 1st December 2008

For Optics and Electronics (PHYS08002)

1 By Thomas David Foxen (B00133844)

For Optics and Electronics (PHYS02008)

10th December 2008 Large Area Photodiodes; a report

INDEX Page 1;

Index

Page 2;

Construction of a Large Area Photodiode

Page 3;

Operation of a Large Area Photodiode

Page 6;

Applications for a Large Area Photodiode

Page 7;

Solar Powered Garden Lights

Page 8;

The Circuit

Page 9;

The Functions of the Components

Page 10;

Conclusion

Page 11;

References

2 By Thomas David Foxen (B00133844)

For Optics and Electronics (PHYS02008)

10th December 2008 Large Area Photodiodes; a report

3 By Thomas David Foxen (B00133844)

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10th December 2008 Large Area Photodiodes; a report

Construction of a Large Area Photodiode Large area photodiodes (“LAPs”) used within Solar Panels are usually constructed in the following way(2);

A) Cover Glass; This is a layer of protective glass. B) Ant-reflective Coating; This can reduce energy loss by refection to less than 5%. It works by having a refractive index around half-way between the materials either side. C) Contact Grid; This is the negative contact and is most efficient when providing a balance of good contact, yet minimal shadowing of the layer below. D) N-type Silicon, and E) P-Type Silicon; These layers (which contain Phosphorus and Boron impurities respectively) are the key parts of the LAP. The N-type Silicon is the area of high electron concentration and where the photons fall. The P-type Silicon is the area high “electron-hole” density. F) Back Contact; This is the positive contact and is normally comprised of a conductive sheet coving the whole of the bottom of the P-type Silicon layer.

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For Optics and Electronics (PHYS02008)

10th December 2008 Large Area Photodiodes; a report

Operation of a Large Area Photodiode The process of generating a usable voltage with this device starts with the Silicon layers being electrically neutral. This is by way of the extra protons from the Phosphorus impurity balancing out the extra electrons in the Silicon and the missing electrons (‘holes’) being balanced out by the missing protons in the Boron. The ‘holes’ and electrons are then aloud to mix by joining the N-type and P-type Silicon layers, which disrupts the polarity balance (previously neutral) between them. The electrons filling these ‘holes’ at the PN-Junction form a barrier, which makes it increasingly hard for electrons to cross from the N to P-type Silicon. This continues until equilibrium, where we find a stable electric field separates the layers. At equilibrium the LAP acts as a diode, allowing (and even to some extent pushing) the electrons from the P to N-type Silicon, but hampering attempts for them to flow the other way. This is illustrated in the following diagram;

N-type Silicon

++++ + ++++ + ++++ + ++ - - - - - - - - - - - - - - - - -

Next we allow the N-type Silicon to be struck by a photon of a sufficient energy level to free an electron from the doped Silicon. The threshold energy level required is governed by the laws of the photoelectric effect, which show that an electron won’t be knocked out its orbit if the striking photon’s energy level is less than the work-function of the material. This is the minimum energy required to free the electron and leave it with zero kinetic-energy. However, should the photon have at least this sufficient energy, the electron will be knocked from its obit and then travel on as a free electron with any residual energy from the photon. Once the electron is released from its orbit, if it is close enough to the PN-Junction, it will feel the effects of the electric filed and flow in the direction governed by it. This of course all happens within the laws of the conservation of energy and momentum.

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For Optics and Electronics (PHYS02008)

10th December 2008 Large Area Photodiodes; a report In more detail, we can see the following process is going on at the atomic scale to keep the reaction sustainable as long as a continuous flow of photons is supplied to the N-type Silicon;

Nb. When looking at this diagram and reading the following cycle it is prudent to remember a few key points. Firstly, there is no scale represented in it (for obvious reasons). Secondly, there is an extra electron shown in the cycle, it represents the ‘hole’ and can be found by looking to the position any electron in the cycle is moving to. 1) A photon of sufficient energy enters the N-type Silicon Phosphorus and strikes an electron of the Phosphorus atom which is not tied up in a covalent bond with one of the Silicon atoms. The ensuing photoelectric effect causes it to gain enough energy to escape the valence shell orbit (represented as the space between the solid and dotted lines in the diagram).

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For Optics and Electronics (PHYS02008)

10th December 2008 Large Area Photodiodes; a report 2) At the same time, the hole which has formed from the electron leaving its orbit is taken up by an electron from closer to the PN-Junction. This process continues and the amount of holes forming close to the PN-junction causes the electric neutrality to become more and more unbalanced until the holes cross the PN-Junction by way of electrons flowing from the P-type to N-type Silicon. 3) This causes a potential difference between the sides which is settled by the easier route for the free electrons (from the photoelectric effect) to take. This route is down the wire through the load, leading to electro-motive force (voltage) cause by the electric field and also current from the flow of electrons away from the N-type Silicon. This is the origins of the voltage and current that is put to use in the load for whatever purpose the load is designed to perform. 4) These electrons, being pushed (through electro-negativity) by the process happening to the electrons behind them in the circuit then continue round and take up orbit in the valence shell of the Boron. They then continue to move towards the Phosphorus, taking up holes that come available from the process outlined in step 2 and upon reaching the P-type silicon, one lap of the cycle has been completed.

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For Optics and Electronics (PHYS02008)

10th December 2008 Large Area Photodiodes; a report

Applications for a Large Area Photodiode Some of the most common places you will find LAPs are as follows; • • • •

Solar Powered Garden Lights Desktop Solar Calculators Solar Trickle Chargers for use with boat and car batteries Satellites

This list is not finite as any electrical device can be powered off the charge stored from recharging a battery with an LAP array. The reasons they are not more common is due to their low efficiency and the high power consumption required by most electronics. A good example of this is the following Solar Panels for Laptops(3);

As you can see, it is over twice the size of the Laptop, requires a bulky external 12V battery and for that you get an extra 3 to 6 hours of runtime from when the 12V battery is fully charged. In other words, even this bulky LAP array (costing over £200) running at its peak does not output enough power to run your laptop without charge in the 12V battery pack.

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For Optics and Electronics (PHYS02008)

10th December 2008 Large Area Photodiodes; a report

Solar Powered Garden Lights For the purposes of this report, I’m looking at running a solar powered garden light. These are very common, can be found in many shops and often look like these(4);

The devices comprise of a LAP array which charge cells in the day time and then discharge them through LEDs at night.

9 By Thomas David Foxen (B00133844)

For Optics and Electronics (PHYS02008)

10th December 2008 Large Area Photodiodes; a report

The Circuit There are lots of different solar powered LED circuits on the market and I have chosen the following because it has a few key point which the designer has pulled together to increase efficiency and keep cost low(5);

The first of these key points is that the circuit runs the LED off a high frequency plus, rather than a DC supply, meaning that the same brightness is achieve from the LED while using less than 50% of the energy that a standard DC circuit would. This allows a single rechargeable power cell to be used, lowering the production costs. The second is that the designer has omitted including a dropper resistor for the LED, which also increase its efficiency. Thirdly, another product of using one cells, and also having four Solar Panels is that the low minimum charging voltage of 1.2v is easily achievable from the LAPs on a dull day. Finally, the circuit turns the LED on by detecting when the voltage from the solar cells drop lower than 0.7v, rather than using an extra component to switch between charging and lighting.

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For Optics and Electronics (PHYS02008)

10th December 2008 Large Area Photodiodes; a report

The Functions of the Components The previous circuit diagram can be represented as follows to help describe its operation(5);

When the output from the four LAP arrays (the Solar Panel) is over 0.7v the cut-off transistor (BC547) keeps the LED part of the circuit off. This allows the energy from the LAP arrays to charge the cell through a simple series circuit. When the output from the LAP arrays is less than 0.7v the oscillator transistor (BC547) along with the resistor (6K8) and the timing component (1n2) create oscillations when connected in this configuration with the high-voltage generator (4R9) and the 10R component. This is the reason the circuit uses less than half the power of a normal DC circuit. However, how this works is outside the scope of this report as it is not related to the function and operation of LAPs.

11 By Thomas David Foxen (B00133844)

For Optics and Electronics (PHYS02008)

10th December 2008 Large Area Photodiodes; a report

Conclusion We have found how the arrangement of two ‘doped’ Silicon semiconductors can take the energy from photons in order to release electrons from their valence shell orbits and cause them to flow through a circuit in order to get from one semiconductor to the other and then back into the ‘electron-holes’ in the semiconductor they were released from. We have shown this to be done using the principles of the photoelectric effect, like charges repelling and electrons flowing from areas of high to areas of low potential, by way of the easiest path, in order to balance charge. All this put together then allows us to create LAPs in order to harness the energy in photons and use it to free electrons for them to flow through a load, in order to do useful work for us; either; right then (such as in solar powered calculators) or later on by storing chemical potential-energy in a rechargeable battery (for devices such as garden lights).

12 By Thomas David Foxen (B00133844)

For Optics and Electronics (PHYS02008)

10th December 2008 Large Area Photodiodes; a report

References

(1) (2) (3) (4) (5)

http;//en.wikipedia.org/wiki/Photovoltaic_cells http;//science.howstuffworks.com/solar-cell3.htm http;//www.sunshinesolar.co.uk/khxc/gbu0-prodshow/LAPTOP1.html http;//www.greenrewards.co.uk/images/products/FINAL%20PRODUCT%20IMAGES/Prod uct%20images/Eco%20Gadgets/Venetian-Light-Garden-big.jpg http;//www.talkingelectronics.com/projects/SolarLight/SolarLight.html

13 By Thomas David Foxen (B00133844)

For Optics and Electronics (PHYS02008)