IV.32 - Resonant Tunneling Diodes?
People thought that regular tunnel diodes, or Esaki diodes, were pretty neat and bound to take over the world. Well, they were pretty neat and did win a Nobel Prize for Esaki, but their relatively high capacitance kept them from becoming widely used high-speed switches.
A newer technology, the resonant tunneling diode, may change that. These diodes have one or more "quantum wells" - thins layers of one type of semiconductor sandwiched in between thicker layers of another type of semiconductor. The semiconductor energy bands warp at these discontinuities of course, and can form "traps", or "wells" for electrons and holes. If these layers are spaced properly, conditions can exist where electrons or holes will "tunnel" between the well with very little resistance and with very high speed. It's sort of analogous to a stone skipping across water, only touching the water surface at a few equally spaced points.
Since these wells are separated by insulating barrier regions between them, the overall device capacitance can be lower than for a regular tunnel diode.
As far as I know, no one makes these diodes available commercially (although I believe that HP uses them in high-end high-speed oscilloscope triggering circuits). Here are some educational links:
- The Resonant Tunnelling Transistor. Has an introduction section on resonant tunneling diodes.
- Resonant Tunneling Diodes and High Frequency Oscillators. Research at the Microelectromagnetic Device Group of The University of Texas at Austin.
- Heterostructure and Quantum Well Physics. Quantum physics is fun!
- Silicon RTD announced by Texas Instruments
- A Review of the State of the Art in Resonant Tunneling Diodes by me. Something I wrote while I was at Carleton University. I know, most of the figures are missing.
- Altavista search for "resonant tunneling diode".