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For example, Can a silicon diode be designed to have a forward bias voltage drop of 0.4V or may be 0.9V?

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Without going into solid state physics more than I am competent to : no. You might be able to shift the bias by a few millivolts this way by changing doping, but you can do that more easily by changing the temperature or the bias current. Read up about the Shockley equation - you can use a diode to generate logarithms that way...

To get a bigger change you need to move to a different semiconductor material (germanium for about 0.2-0.3V, silicon carbide for higher voltages (dim memory says about a volt...) anyone know the numbers for 3-5 compounds like Ga-As, Ga-InP etc?

Finally, Schottky diodes involve a metal-semiconductor boundary and have quite a low bias voltage (about 0.3V)

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Thank you, that was very helpful. – Nomee Ash Nov 24 '12 at 12:22
Found a Silicon Carbide schottky diode here, st.com/internet/com/TECHNICAL_RESOURCES/TECHNICAL_LITERATURE/… with a forward voltage of 1.4-1.6 volts at 10 amps, 25-150 Celsius, and about 1V at 1 amp. – Brian Drummond Nov 25 '12 at 0:15

If a gold dope injected in the PN junction , the minority carriers will gain speed and at the expense of the higher potential (the majority of investors) . ( pun intended) eg. 1N914 is gold doped.

PN materials, doping and geometry each affect junction voltages.

E.g. undoped silicon is lower as in PIN diodes but doping more does not increase beyond standard silicon voltages. More examples

Composition appears to be a trade secret and for those who need to know.

"It consists of an active part of each device and is made of single crystal silicon. It contains deminimus amounts, usually in parts per billion (ppb) levels, of doped elements such as arsenic, phosphorus and boron. "

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Where would I have to look to find more info about what dopant in used in what device? A datasheet will tell me that a 1N4148 is fast and a 1N4001 has a somewhat lower forward voltage - but how exactly are they made? How do they influence the ideality factor in the Shockley equation? – zebonaut Nov 24 '12 at 22:40

This reference on diodes may be of help to you. And if that isn't enough take a look at this pdf from Berkley.

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