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
¡ 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
§ 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
LC Circuit
Capacitor starts fully charged, energy stored in electric field
Capacitor discharges, and current creates magnetic field in Inductor
Inductor keeps current going, and re-charges capacitor in opposite direction
Capacitor discharges again in opposite direction, and cycle repeats!
Frequency of oscillation:
Of course, real physical elements have nonzero resistance, meaning loss of energy over time
¡ 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”
¡ Exhibits “negative resistance” over a certain voltage range
¡ Acts as its own source of amplification and feedback!
¡ 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
¡ 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.
¡ 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
¡ 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/
¡ 50.75 GHz
¡ 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
¡ 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
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
¡ A HUGE thanks to Jonathan Sculley¡ Dr. Doug Yoder¡ Bette Finn, ECE Research Librarian¡ NSF ¡ Dr. Leyla Conrad