Negative Resistance with a Single Atom

http://sciencebulletin.org/archives/8902.html

by manojr • 10 years ago

136 33 10 years ago

jerf2• 10 years ago

I offer free useless internet points to anyone who can explain to the next level of detail what this might improve about current electronics, and a bit more about the "how".

51Cards• 10 years ago

Agreed. I felt the article gave a whole lot of nothing concrete? I don't think I learned anything specific in reading it about what this would be useful for.

codezero• 10 years ago

They went on about how this type of device was difficult to manufacture but didn't at all explain how the recent findings help make it more manufacturable :/

corkscrew• 10 years ago

I don't know much about this, but I'm assuming it is a replacement or supplement to a transistor.

jeffwass2• 10 years ago

In a much older role (nearly 20 yrs ago) my team tried using (quite successfully) a Resonant Tunneling Diode (RTD) as an electronic pulse generator to drive a fiber mode-locked laser.

Fun note - the pulses it produced were so narrow that one of the visiting oscilloscope dealers (I forget if it was LeCroy or Tektronix) was using it to test out timing resolution of their top model oscilloscopes! (Normally we had to use a sampling scope to see the pulses).

It was a "quantum well", and IIRC grown as using layers of doped GaAs. (I might be way off here, I didn't work on the fabrication).

If you plot current vs voltage across a device : an ideal textbook resistor has a straight line. Normal resistors will have some curvature but generally be monotonically increasing.

Out RTD showed a current that went up, then down a bit, then back up again. We biased it with a chosen current and load that put the current smack in the negative differential region. If you draw a horizontal line through this current, it intersects the graph at three voltage points. The middle voltage point has a negative slope, and is an unstable state. At this bias level, the device would flip back and forth between the two same-current points on either side where the slope was positive.

The cool part of this thing was that each 'bounce' sent an EM wave through a waveguide we attached, like an antenna, and the returning wave would trigger the next flip. So the length of the waveguide set the period of the oscillation. It had horrible timing jitter on its own, but we could make it very stable by driving it with a sinusoid oscillator.

semi-extrinsic2• 10 years ago

Give the man his internet points already, they're well earned.

jjoonathan• 10 years ago

As usual, w2aew has an excellent video on the subject :)

https://www.youtube.com/watch?v=PuG8CCUbg58

jeffwass2• 10 years ago

Following on my comment, back around 2004 or so when I was doing my PhD in physics, I made a Java applet showing how resonant tunnelling works (ie, the phenomenon that drives the negative differential resistance in the RTD above), and also how band structure arises in an ordered crystal.

I put these on GitHub, if anyone wants to play with it, this applet is from the top screenshot : https://github.com/jeffwass/Physics_Simulations/blob/master/...

Basically, in one dimension, we represented an atom as an infinitely-thin 'spike' in potential, a Dirac Delta function.

For a single 'atom', regardless of how much energy an incoming electron wave function has, some energy is always reflected back and a smaller amount goes through. (Here, energy and frequency are basically synonymous, where E = h * nu, h is Planck constant and nu is the frequency.)

For two atoms (a double barrier), certain input frequencies cause destructive interference of the reflected quantum wave function at the first atom (ie - the reflections bouncing off the first and second atom cancel out), and exactly 100% of the incoming energy transfers through! At certain other frequencies, the output waves cancel and the wave is entirely reflected. The plot of transmission vs input frequency looks sinusoidal, between 0% and 100%. The applet shows the impact of adjusting the spacing and relative 'height' of the barrier atoms.

This interference is the quantum mechanical basis of resonant tunnelling.

In the applet, you can control how many atoms are in the crystal. As you add more atoms to the system, the output transfer function starts forming notable regions of high transmission and regions of low transmission.

This is for a perfectly ordered crystal (same potential and spacing). The applet also shows what happens if you randomise the spacing or the potential heights.

Question for the HN community : as Java web applets are seemingly obsolete these days, any recommendations on current front-end tech for making these types of simulations?

madengr• 10 years ago

What's the difference between an RTD and a Gunn Diode (used in cavity oscillators)?

mafuyu• 10 years ago

This link seems to explain it in some detail: https://en.wikibooks.org/wiki/Circuit_Idea/Negative_Differen...

A typical transistor doesn't have the red part of the curve where resistance goes up with increased current. This effect might be useful for multi-level logic or for self current limiting (similar to a lamp)?

thirdhaf• 10 years ago

You can use the phenomenon of negative differential resistance to improve the design of some very common circuit elements. For example you can move from a classic six transistor SRAM cell to a two transistor design. It's a bit dense reading but here is a research synopsis from one group working on this in silicon/SiGe. http://www2.ece.ohio-state.edu/~berger/summ_ritd05.html

the8472• 10 years ago

Considering that sram takes up a lot of die space in processors, wouldn't that be a Huge Deal™ for chip manufacturers?

TFortunato• 10 years ago

From my background in EE, I can tell you a bit about the why at least. While I have some background in device physics, it's as an EE not as a physicist, so I probably won't be as helpful on the how.

- For analog type devices, negative resistance gives you a way to easily make oscillators, and negative feedback loops. Practical applications could be things like smaller or lower power wireless chips, or more accurate filters / oscillators in those chips. Useful for everything from wireless chipsets to things like google's project soli.

- For digital type devices, the big draw is that you can make a memory cell (aka flip-flop) from one active component instead of 2. This could potentially halve the size of some of the digital logic in your microprocessor (at least the registers and all the temporary storage used in your processing pipelines!)

- Talking more pie-in-the-sky, some models of neurons include negative resistance areas...analog neural nets on a chip using these will probably support a grad student or two as a research project!

Start reading here: https://en.m.wikipedia.org/wiki/Negative_resistance

ChuckMcM3• 10 years ago

I agree that the article was amazingly content free. "negative differential resistance" in this context means simply that the relationship of current and voltage across the device is not a linear function.

Looking around the web the most common way that is taken advantage of is in small oscillators; transistor turns on as the input voltage rises above a gate threshold but then turns off again once you get enough gate current to be in a different part of the resistance curve. There were some EE times articles suggesting multi-level memory (non-binary) as another application.

The weird thing is "negative resistance" isn't something that makes sense electrically, and what they are really talking about is plotting the value of current with respect to voltage, a resistor is a simple line, diodes are a discontinuous at their forward avalanche voltage, and these things have lower current passing through them at some higher voltage. So the slope of the line is 'negative' at that point. (and the differential part comes in because it is between two specific voltages).

semi-extrinsic2• 10 years ago

means simply that the relationship of current and voltage across the device is not a linear function.

That's not correct. An ordinary resistor will have a (weakly) non-linear V-I relationship as well. The key here is that in some small region, the V-I graph is "going down", whereas for normal components it's always "going up".

pizza3• 10 years ago

*non-monotone

lightedman• 10 years ago

Massive improvement in LED efficiency, for starters, now that we've also got a minor handle on the Auger effect. Essentially we could get a lot closer to the theoretical maximum of ~683 lumens per watt with this.

conistonwater• 10 years ago

I liked this explanation better, it's rather clearer: https://physics.aps.org/articles/v9/155

Here is the original report of the effect: https://journals.aps.org/pr/abstract/10.1103/PhysRev.109.603

pizza3• 10 years ago

That explanation really reminded me a lot of the Petkau effect [0]:

The Petkau effect is an early counterexample to linear-effect assumptions usually made about radiation exposure.
Petkau had been measuring, in the usual way, the radiation dose that would rupture a simulated artificial cell membrane. He found that 3500 rads delivered in 2 1⁄4 hours (26 rad/min = 15.5 Sv/h) would do it. Then, almost by chance, Petkau repeated the experiment with much weaker radiation and found that 0.7 rad delivered in 11 1⁄2 hours (1 millirad/min = 0.61 mSv/h) also ruptured the membrane. This was counter to the prevailing assumption of a linear relationship between total dose or dose rate and the consequences.

[0] https://en.wikipedia.org/wiki/Petkau_effect

jpmattia• 10 years ago

I know the HN protocol is to use the headline, but the sciencebulletin article headline is pretty broken. Negative differential resistance (NDR) in tunneling diodes has been understood for several decades, and is pretty far from a mystery.

The original article is about an I-V curve from a structure involving a single-atom + a scanning-tunneling microscope (STM). That such a system would also exhibit NDR is not particularly surprising. Tunneling to a structure with discrete energy levels will have current flow when energies line up, and less current flow when energies don't line up. So even the "mystery" isn't much of a mystery.

The interesting results imho are:

1. The team got reproducible single-atom tunneling with an STM tip, and

2. They measured the time-response of about 10 microseconds.

fwiw.

Edit: Resonant tunneling diodes used to be a hobby of mine: https://dspace.mit.edu/handle/1721.1/38419

steamer25• 10 years ago

For anyone else who wondered, in this context STM stands for Scanning Tunneling Microscope (as opposed to Software Transactional Memory).

justifier• 10 years ago

it reads like there is a bit more 'mystery', or 'the yet explained', explained(o):

measuring current vs voltage in a new way, by applying brief voltage pulses to the STM tip.

      When the pulses lasted 10 microseconds or longer, 
    they saw NDR. 
      But when they reduced the pulse length to 10
    nanoseconds, the effect disappeared. 
    ...
      Combining this information with other data, 
    the team determined the timescale for electrons
    refilling the empty lower level.

unreal

(o) https://physics.aps.org/articles/v9/155

mtdewcmu• 10 years ago

Do you agree that this is a major breakthrough, worth billions of dollars?

pmalynin• 10 years ago

I'm glad to see my University up there. As Canadians we are often left out of the global scene when it comes to news and publications.

tehwalrus• 10 years ago

Full text of the original article,

https://arxiv.org/abs/1608.06344

tdeck• 10 years ago

Those of you who're interested in obscure engineering history might like to read up on Oleg Losev[1] who developed and built solid-state negative resistance radios and amplifiers long before the first practical transistors.

[1] https://en.wikipedia.org/wiki/Oleg_Losev?wprov=sfla1

Negative Resistance with a Single Atom

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