Threshold voltage is a term not normally used with diodes.
What is you definition of that?

"Semiconductor diodes begin conducting electricity only if a certain threshold voltage or cut-in voltage is present in the forward direction (a state in which the diode is said to be forward-biased)".

Main Functions section

https://en.wikipedia.org/wiki/Diode

 

Forward drop is the voltage across the diode after the curve turns to the part where the voltage increases very little as the current increases quite a bit. And the "threshold' is also referred to as the diode knee. Diodes have a little current flowing below that point, thus "threshold" is not the best term,

Thanks for the clarification.

 

Neither of the ones you mention is germanium. 1N4002 is a 100V, 1A silicon rectifer diode. The 1N5819 is a schottky diode. The voltages you quote are normal for those diodes.
The only diodes around with those threshold voltages are zener diodes.

Can I ask you how you know these are silicon? Is it the style of the package? Colour? Coding? I have just ordered some Zener diodes and they are red. Is there a standard here?

 

Can I ask you how you know these are silicon? Is it the style of the package? Colour? Coding? I have just ordered some Zener diodes and they are red. Is there a standard here?

You can tell by looking at the datasheet for them not by anything about their appearance. The ones you have are very familiar to me and many others here. They are commonly used diodes.

 

You can tell by looking at the datasheet for them not by anything about their appearance. The ones you have are very familiar to me and many others here. They are commonly used diodes.

Thanks Albert. I did check out the datasheets for the ones I referenced. There's not a great deal of info on them compared to the datasheets of transistors for example. here's one of teh ones I checked out. Can't see any reference to material. That's why I asked.

https://pdf1.alldatasheet.com/datasheet-pdf/view/725218/ONSEMI/1N4002.html

 

That isn't what I would call a datasheet, however a proper datasheet for this diode also doesn't mention silicon. I think that is because nowadays the vast majority of diodes are silicon. If you look at a germanium diode (e.g. OA91) the fact that it is germanium is prominent.

 

Thanks Albert. I did check out the datasheets for the ones I referenced. There's not a great deal of info on them compared to the datasheets of transistors for example. here's one of teh ones I checked out. Can't see any reference to material. That's why I asked.

https://pdf1.alldatasheet.com/datasheet-pdf/view/725218/ONSEMI/1N4002.html

You have made an interesting discovery. I would not be surprised if a lot of us on this forum assumed the material of the 1N400x diodes was on the data sheet somewhere. The weak answer is "It's something we just know". The 1N400x are quite generic. Maybe other manufacturers than On Semiconductor include the word "silicon"?

 

Neither Vishay nor Diodes Incorporated include the fact that they are silicon. I got bored after that.

 

Well, according to Vishay there is other info much more relevant; from their datasheet:

General Purpose Plastic Rectifier

Go figure

 

That's interesting. But you say "forward drop". I am talking about threshold voltage. Are we on the same page? Are we talking about the same thing.

Like @Reloadron said, you'll get much more help if you describe what you expect to do with this "threshold" voltage. For me at least, the more I learn about diodes, the more complex they are. Any attempt to simplify them into a simple Wikipedia description skips important details.

Depending on your application, you might be able to use a Zener of appropriate voltage, an LED, or a string of more common silicone diodes. However, all of those have somewhat "soft" transitions, especially the Zeners. Furthermore, the threshold varies as a function of both current through the device and temperature. So, depending on your application, diodes may not be right for you at all. Another possibility might be a comparator or a "programmable Zener" (I'm never sure what their real name is, maybe precision Zener shunt?)

 

"programmable Zener"

e.g TL431.

 

Diodes and transistors are not made from silicone which is a soft synthetic rubber. Instead thay are made from silicon which is a hard crystalline mineral similar to ceramic or glass.

 

Like @Reloadron said, you'll get much more help if you describe what you expect to do with this "threshold" voltage. For me at least, the more I learn about diodes, the more complex they are. Any attempt to simplify them into a simple Wikipedia description skips important details.

Depending on your application, you might be able to use a Zener of appropriate voltage, an LED, or a string of more common silicone diodes. However, all of those have somewhat "soft" transitions, especially the Zeners. Furthermore, the threshold varies as a function of both current through the device and temperature. So, depending on your application, diodes may not be right for you at all. Another possibility might be a comparator or a "programmable Zener" (I'm never sure what their real name is, maybe precision Zener shunt?)

For almost all diodes the bend in the voltage/current plot is a curve, not a sharp break. The reason is that there are at least two types of current flow by at least two different mechanisms. starting around zero there is leakage current based on the resistive properties of the material. That contribution to the current is close to a straight line. As the voltage increases the current reaches the start of the Band Gap Energy level, which is not a single value, but rather a distribution around some point. That is where the curve part begins. At higher voltage all of the atomic band gap levels are exceeded and the curve is much flatter. At this point the remaining influence is mostly the effective series resistance effect, which is complicated. There are some diodes that include some sort of arrangement so that the curve is much sharper, and also "Tunnel" diodes that do not quite follow this explanation. But not a lot of them.
You can do an interesting experiment with different LEDs and a higher resolution voltmeter, plus a variable power supply. In a dark room you will see that the LED lights just a bit at a much lower voltage than the rated forward voltage. As the voltage is increased the light will increase more and more for very small increases in voltage. At the same time, the current is also increasing, but it takes two meters to watch both at the same time. The "threshold voltage" is where the light output achieves some specified level. Non-LED diodes mostly have a similar curve, but no light output.