kenneth patterson gwinnett school of math science and ...gunn diode fits into, that produce...
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Kenneth Patterson
Gwinnett School of Math Science and Technology
Georgia Tech STEP-UP Summer 2018
Faculty Advisor: Doug Yoder
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¡ Electronics oscillate at low frequencies, photonics at high frequencies
¡ Very few technologies reliably produce EM radiation in 300 GHz - 10THz range
¡ Between microwaves and infrared¡ Applications include: Radar, high-speed
data transmission, astronomy, medical imaging “Mr. President, we must not
allow a Tera-Hertz gap!”
Image source: https://media.nature.com/m685/nature-assets/nature/journal/v417/n6885/images/417132b-f1.2.jpg
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§ Group is working on a semiconductor devicecalled a Gunn Diode
§ Should oscillate at a frequencyabove 1 THz
§ This project’s goal is to simulate a circuit that the Gunn Diode fits into, that produce oscillations
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LC Circuit
Capacitor starts fully charged, energy stored in electric field
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Capacitor discharges, and current creates magnetic field in Inductor
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Inductor keeps current going, and re-charges capacitor in opposite direction
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Capacitor discharges again in opposite direction, and cycle repeats!
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Frequency of oscillation:
Of course, real physical elements have nonzero resistance, meaning loss of energy over time
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¡ An oscillator requirestwo things:§ Amplification§ Feedback
Desired frequency L C
100 Hz 1.0 H 3.0 uF
99.7 MHz 1.5 nH 1.75 nF
2.4 GHz 2.8 nH 1.5 pF
1.0 THz 0.01 nH 0.001 pF
LC circuit is called a“tank circuit”
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¡ Exhibits “negative resistance” over a certain voltage range
¡ Acts as its own source of amplification and feedback!
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¡ James Gunn in 1964 observedoscillations in current in aGaAs semiconductor device
¡ Later researchers discovered it to be explained by NDR ¡ GaAs Gunn Diodes are commercially available in waveguides
up to ~200GHz¡ Used in microwave devices like radar guns
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¡ GaN has been investigated and simulated as a potential NDR semiconductor material since year 2000
¡ Group has a design for fabricating such a device, but needs the resonant circuit to plug it into!
¡ Goal is to design andsimulate a resonantcircuit with Gunn-likeNDR device
Source: E. Alekseev, A. Eisenbach, D. Pavlidis, S. M. Hubbard, and W. Sutton, “GaN-based NDR Devices for THz Generation,” p. 10, 2000.
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¡ Circuit simulator: PSPICE¡ Schematic CAD software: Cadence OrCAD
¡ Two goals to accomplish:§ “Gunn Diode” is not a device in SPICE, so needed
an equivalent subcircuit that exhibits the NDR V-I curve
§ Place that subcircuit into a simple resonant circuit and demonstrate oscillations
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¡ Subcircuit uses an opamp to produce “-R”
¡ In practice, this subcircuit only lowered resistance of the total circuitrather than show NDR
Source: http://adamsiembida.com/negative-resistor-tutorial/
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¡ 50.75 GHz
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¡ Proof of concept, success!¡ Simplest possible resonant circuit, too simple
for real-world fabrication
¡ Handed off three things to group:§ Simple SPICE NDR circuit design§ Primary Gunn diode circuit designs I found in
literature§ ALL sources I used this summer
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¡ This twin JFET subcircuit probably won’t truly behave like a GaN device
¡ Realistically building this circuit will require more details than simply R, L, and C
¡ Most likely final circuit will need filters or transmission lines of some type, will need a lot of refining
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1. Learned a ton about AC Circuits!
2. Lesson Plan: Introduction to inductors and capacitors at intro-physics level
3. Research and literature review methods ( using software like EndNote or Zotero to create citations)
4. Possible usage of SPICE in the classroom▪ Most likely not PSPICE, but online browser
versions are also available
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¡ A HUGE thanks to Jonathan Sculley¡ Dr. Doug Yoder¡ Bette Finn, ECE Research Librarian¡ NSF ¡ Dr. Leyla Conrad