Weird I-V

I'm trying to understand this I-V curve (see Ascii art below.)* the sample is a piece of n-type Si (I don't know doping density.. lightly doped.) There are Ti/Au contacts (made into a Hall bar, A stripe of current input on each end and four voltage probe points along the length (3mm x 6mm maybe)) This is just the I-V across the large current contacts. |(I) ~150 uA -| -------- | / | / | / Slope ~20k ohm | / |/ ----------+-------------- (V) /| ^ / | 3 V / | / | symmetrical at reverse bias.

So the contacts look ohmic to about 150 uA @ 3V where they roll off and become constant current (tested only to 10 V bias) I'm not sure the process steps in making the contacts, but I'm assuming a layer of silicon oxide, or some other surface crud. (I'll be sending off an email on Monday to gather more information.) I'm trying to understand what's going on. It's like a depletion fet with S-G shorted.

I cooled the device down and the curve collapses rapidly (data book's at work) at something like 200 K the resistance had increased the 'corner' voltage decreased to (~300 mV) and the 'saturation' current decreased proportionally. The 'resistance' increase may be partially due to the round-i-ness of the corners. (not shown in ascii art.)

Any thoughts?

Oh I should add that I've seen similar things (not as high a voltage) with 'crappy' rectifying Schottky diode contacts. So a string of crappy diodes at each end is a possible model... in which case I'm just trying to understand the 'crappy' part.

George H.

*I wanted to start this with a rant about co-worker, but instead put it here.
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George Herold
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Limited number of available carriers? ...Jim Thompson

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| James E.Thompson                                 |    mens     | 
| Analog Innovations                               |     et      | 
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Jim Thompson

And mobility. ...Jim Thompson

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| James E.Thompson                                 |    mens     | 
| Analog Innovations                               |     et      | 
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Jim Thompson

In the Silicon? I made some of these a few years ago (maybe higher doping.) and they looked fine to ~10-20V. I stuck a 60W incandescent bulb up next to it. That increased the slope (decreased resistance) a bit. But didn't change the current limit.

There's a more heavily doped one I looked at quickly so don't have the numbers. But it turned off at ~200mV and not 3V. (I'm not sure of current.) I think it has to be the contacts. limited carriers in the contacts? Maybe the oxide layer has 'air crude' in it, some shallow impurity that freezes out.

I assume there was a HF acid etch, but I have no idea.

George H.

George H. I

Reply to
George Herold

What happens if you crank the voltage way up? If you hit reach-through, i.e. the I(V) curve goes vertical at some point, then what you've got is two leaky Schottky diodes back-to-back and not real ohmic contacts.

Cheers

Phil Hobbs

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Dr Philip C D Hobbs 
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Phil Hobbs

Right, there's definitely diode behavior. If you look at the voltage probes along the sample, you can see that one side follows the current and the other follows the voltage.

I was thinking I could crank up the voltage and see if anything happens. George H.

Reply to
George Herold

I haven't turned up the voltage yet. Turns out there was no HF etching step. Nice patterns laid down and Ti/ Au sputtered on. (I'm assuming plasma, sputtering, but don't know.) I got an email from the guy there saying he didn't think HF etch would make much difference. I'm scratchin' my head wondering if we need a different fabricator? I've been saying that Ge may be easier than Si. (I don't know if that's true or not but you may not need HF to clean the oxide on Ge.) (So many experiments, so little time.)

Ge is more interesting from a physics teaching lab stand point. With a smaller band gap you see thermal excitation of carriers at a lower temperature.

George H.

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George Herold

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