Diode reverse-bias resistivity?

Check the datasheet specs for the terms Leakage Current or Peak Reverse Current. Typically on the order of a few microamps for small silicon diodes in the region before breakdown. This can be modeled as a high resistance of tens of megohms.

A sample data sheet:

Beware of another property, junction capacitance, which can be on the order of several picofarads. This can permit AC components of a signal to leak through even when reverse biased.

They pass a current called the saturation current that can be very small, depending on the materials and doping of the junction. But for some, especially high current or low voltage devices this reverse leakage can be in the milliamps or even hundreds of milliamps. Typical leakage for 1 amp silicon PN junctions is in microamps.

For each particular device, you find this information on the data sheet, which sometimes just lists a maximum value.

Here are a couple examples with graphs of reverse leakage versus voltage and temperature. Not that below breakdown the current is less than proportional to the reverse voltage (not resistive that follows Ohm's law, but closer to a constant current).

figure 5 on this one:

figure 2 in this one:

much higher leakage for Schottky low voltage type, figure 2:

Great Info. Thanks for the replies.

Should I assume that a transistor in its "off" state would exhibit similar behaviour?

Don

For sufficiently vague concepts of "similar", yes. There is definitely a reverse biased diode (collector to base junction) leakage current component that dominates the character of the leakage if the transistor has a reverse biased base emitter junction. If the transistor has a small positive base emitter bias it might still be considered to be off, but an additional leakage current component (gain times base current) is added.

The main similarity is that the current is less than proportional to the voltage over a considerable voltage range, till breakdown is approached. Then it takes off and becomes more than proportional to voltage.

Typically a nearly constant current, with a much less than resistive slope, as JP states. Schottkies are leakier and more "resistive" than silicon PN junctions. The reverse leakage current typically doubles with every 10 degree C temp increase.

It can vary wildly between "identical" devices, especially power diodes.

Some glass diodes are photosensitive, too!

John

When I was a kid that discovery made me jump for joy. No more plunking down more than a buck for an LDR. Just take a diode or transistor from the TV scrap heap, scrape the black paint off the glass body, and bingo!

Oddly, all (four, I think) answers (in other posts) are correct.

(1) If you don't have sub-picoamp current meters, a good quality instrument diode is an insulator.

(2) If you DO have sub-picoamp current meters, a good quality instrument diode leaks nearly constant 'satuation current'. The diode equation scales all diodes into a simple function, and the saturation current is the scale factor, I_s

(3) if your diode has significant surface area, and has aged and/or gotten 'dirty' there can be large leakage currents on the exterior crust of the semiconductor, not really covered by the diode equation at all, and the manufacturer has some data on how bad this can be for his design and with his chosen materials.

(4) It varies from device to device (i.e. a rectifier is 'good enough' if the leakage is a milliamp, but few are that bad, and NONE are that bad straight from the factory).