Do we operate diodes past their barrier or cut off voltage?

Question

I think I get the idea of barrier or cut of voltage (e.g., 0.7 V for Si). So diodes start conducting after cut voltage. I have few questions for the cut off voltage

1. What happens if I increase the voltage to say 1 Volt? My understanding is the current will exponentially increase and may damage the diode. Is that correct?

2. If 1 above is true then is it fair to say that diodes are operated at around their cut off voltage?

Answer 1

Your statements are correct.

Except for your first sentence, which assumes an ideal diode. Real diode conduct even if the voltage across them is lower than their forward voltage, it's just that the current will be low. The relationship between voltage and current for a diode is given by the Shockley diode equation and looks like this:

As the graph shows, a small increase in voltage results in a huge increase in current. For this reason, diodes are not voltage-driven but current-driven: your design circuits to have a desired current through a diode.
Also, notice that even below 0.5V, the current is not really equal to 0, it is just very small (probably in the order of tens microamps around 0.4V).

Answer 2

is it fair to say that diodes are operated at around their cut off voltage

Diodes are operated at significantly less than their: -

• maximum continuous current
• peak instantaneous current
• values in between (or less) depending on the amount of heatsinking provided.
• maximum peak reverse voltage

How significantly less that is depends on how reliable you want them to be.

What happens if I increase the voltage to say 1 Volt? My understanding is the current will exponentially increase and may damage the diode. Is that correct?

Some diodes will be fine (like the bulk diode inside most SiC MOSFETs; others will be destroyed. Anywhere between those two extremes lies the reality for any particular diode.

Take the 1N4148: -

• at around 0.8 volts there might be 15 mA of current flow
• at around 0.7 volts there might be 3 mA flowing
• at around 0.6 volts about 0.8 mA will flow
• at around 0.5 volts about 0.1 mA flows

Diodes conduct at voltages lower than 0.7 volts. Some more than others. A lot of diodes conduct mA even when reverse biased when the temperature is high (especially Schottky devices).

Answer 3

1. The magic smoke comes out, and you need to buy a new diode.

The current through a diode increases rapidly as you increase the voltage (see the graph in the post by Sacha). So once you go much above 0.7V for a silicon diode, the current will be so large as to blow the diode.

2. Yes. Because that's the voltage you get for any sensible current, with the diode forward biased.