4 ms·
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 f
by jeffwass 10y 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.
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- jerf 10y agoI have kept up my end of the bargain as of this post. As wise people have known for millenia, it's very important to keep one's word, but I submit it's even more important that the useless internet points be correctly awarded. I'm sure the millenia will bear me out on that.
- nathan_f77 10y agoI salute you for your generosity, and I thought you might like to know that I have granted one additional upvote.
- jjoonathan 10y agoAs usual, w2aew has an excellent video on the subject :) https://www.youtube.com/watch?v=PuG8CCUbg58 https://www.youtube.com/watch?v=PuG8CCUbg58
- jeffwass 10y agoFollowing 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/README.md 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 10y agoWhat's the difference between an RTD and a Gunn Diode (used in cavity oscillators)?