For example, one might start with a p-type block, change a thin layer near the surface to n-type by diffusion, and then make a thin p-layer next to the surface by diffusion of an acceptor:
Photoconductivity of Selenium Smith was an electrical engineer who worked on several major under-sea cable laying projects. While developing a system of testing the cables while they were being laid Smith needed a means by which the high resistance of a long cable could be simulated reliably.
He obtained some resistors which were made from selenium wire hermetically sealed in glass tubes connected to platinum wires at each end of the tube.
Selenium has a high resistance and Smith notes that wires of up to mm long and 1. While investigating the cause of such great differences in the resistance of the bars, it was found that the resistance altered materially according to the intensity of light to which they were subjected.
When the bars were fixed in a box with a sliding cover, so as to exclude all light, their resistance was at its highest, and remained very constant, fulfilling ail the conditions necessary to my requirements; but immediately the cover of the box was removed the conductivity increased from 15 to per cent, according to the intensity of the light falling on the bar.
Merely intercepting the light by passing the hand before an ordinary gas-burner, placed several feet from the bar, increased the resistance from 15 to 20 per cent.
If the light be intercepted by glass of various colours, the resistance varies according to the amount of light passing through the glass. To ensure that the temperature was in no way affecting the experiments, one of the bars was placed in a trough of water so that there was about an inch of water for the light to pass through, but the results were the same; and when a strong light from the ignition of a narrow band of magnesium was held about 9 in above the water the resistance immediately fell more than two-thirds, returning to its normal condition immediately the light was extinguished.
Thus Smith discovered a potent photoconductivity effect and inspired extensive research into selenium and commercial applications. Had he done so he would have found a significant temperature effect over a few degrees: Selenium presents in several allotropic forms of which only one, the most stable and common trigonal form is a semiconductor.
Accordingly it is highly temperature sensitive. Photconductivity of Silver Sulfide Later, in Shelford Bidwell reported doubts about the proposition that these observations were due to the intrinsic properties of selenium speculating that the results could relate to impurities introduced by the electrodes and the annealing process.
Noting that selenium and sulfur are closely related he constructed cells of sulfur with silver added to improve its conductivity. In doing so he prepared cells of silver sulfide suspended in excess sulfur and obtained readily measurable photoconductivity effects.
London 7 Exhibit Three: Photovoltaic Effects Most reviews of the history of semiconductor research credit Becquerel with noting the first photovoltaic effect. He observed potential differences between electrodes where one is illuminated and one is not.
But it is difficult to deduce whether he was observing photoelectric or photochemical reactions and therefore whether his work should be given credit in the context of the history of semiconductors. William Adams and Richard Day Adams and Day prepared their selenium cells by taking a short piece of vitreous selenium 5 mm to 25 mm in length and pressed red hot platinum electrode wires formed into a circle into each end of the rod thereby forming a good connection to each end.
The assembly was enclosed in a glass tube and sealed with corks prior to annealing to convert the selenium to the conducting trigonal form.
In they first report three rectifying junction properties: The question at once presented itself as to whether it would be possible to start a current in the selenium merely by the action of light.
Accordingly the same piece of selenium was connected directly with the galvanometer. While unexposed there was no action whatever. On exposing the tube to the light of a candle, there was at once a strong deflection of the galvanometer needle.
On screening off the light the deflection came back at once to zero. With these they eliminated possible thermoelectric effects and thermal heating from their light source.Jan 24, · Making Men of Myths – Part 1 Author’s Note: this post contains extracts from Rick Riordan’s Percy Jackson & The Olympians series and some big spoilers, especially for the first book; Percy Jackson and the Lightning Thief, so read with caution if you are planning on trying these books out and don’t want to be spoiled.
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