A
Gunn diode, also known as a
transferred electron device (
TED), is a form of
diodeIn electronics a diode is a two-terminal electronic component which conducts electric current asymmetrically or unidirectionally; that is, it conducts current more easily in one direction than in the opposite direction. The term usually refers to a semiconductor diode, the most common type today,...
used in high-frequency
electronicsElectronics is a branch of science and technology that deals with the controlled flow of electrons. The ability to control electron flow is usually applied to information handling or device control. Electronics is distinct from electrical science and technology, which deals with the generation,...
. It is somewhat unusual in that it consists only of N-doped
semiconductorA semiconductor is a material that has an electrical resistivity between that of a conductor and an insulator, that is, generally in the range 10
3 Siemens/cm to 10
−8 S/cm. Devices made from semiconductor materials are the foundation of modern electronics, including radio,...
material, whereas most diodes consist of both P and N-doped regions. In the Gunn diode, three regions exist: two of them are heavily N-doped on each terminal, with a thin layer of lightly doped material in between.
A
Gunn diode, also known as a
transferred electron device (
TED), is a form of
diodeIn electronics a diode is a two-terminal electronic component which conducts electric current asymmetrically or unidirectionally; that is, it conducts current more easily in one direction than in the opposite direction. The term usually refers to a semiconductor diode, the most common type today,...
used in high-frequency
electronicsElectronics is a branch of science and technology that deals with the controlled flow of electrons. The ability to control electron flow is usually applied to information handling or device control. Electronics is distinct from electrical science and technology, which deals with the generation,...
. It is somewhat unusual in that it consists only of N-doped
semiconductorA semiconductor is a material that has an electrical resistivity between that of a conductor and an insulator, that is, generally in the range 10
3 Siemens/cm to 10
−8 S/cm. Devices made from semiconductor materials are the foundation of modern electronics, including radio,...
material, whereas most diodes consist of both P and N-doped regions. In the Gunn diode, three regions exist: two of them are heavily N-doped on each terminal, with a thin layer of lightly doped material in between. When a voltage is applied to the device, the electrical gradient will be largest across the thin middle layer. Conduction will take place as in any conductive material with current being proportional to the aplied voltage. Eventually, at higher field values, the conductive properties of the middle layer will be altered, increasing its resistivity and reducing the gradient across it, preventing further conduction and current actually starts to fall down. In practice, this means a Gunn diode has a region of
negative differential resistanceNegative resistance is a property of some electric circuits where an increase in the current entering a port, results in a decreased voltage across the same port. This is in contrast to a simple ohmic resistor, which exhibits an increase in voltage under the same conditions. Negative resistors...
.
The negative differential resistance, combined with the timing properties of the intermediate layer, allows construction of an
RFRadio frequency is a frequency, or rate of oscillation, of electromagnetic radiation within the range of about 3 Hz to 300 GHz. This range corresponds to the frequency of alternating current electrical signals used to produce and detect radio waves...
relaxation oscillatorA relaxation oscillator is an oscillator in which a capacitor is charged gradually and then discharged rapidly. It is usually implemented with a resistor or current source, a capacitor, and a "threshold" device such as a neon lamp, thyratron, diac, unijunction transistor, or Gunn diode...
simply by applying a suitable
direct currentDirect current is the undirectional flow of electric charge. Direct current is produced by such sources as batteries, thermocouples, solar cells, and commutator-type electric machines of the dynamo type. Direct current may flow in a conductor such as a wire, but can also be through...
through the device. In effect, the negative differational reisitance created by the diode will negate the real and positive resistance of an actual load and thus create a "zero" resistance circuit which will sustain oscillations indefinitely. The oscillation frequency is determined partly by the properties of the thin middle layer, but can be tuned by external factors. Gunn diodes are therefore used to build oscillators in the 10
GHzGHZ or GHz may refer to:# Gigahertz .# Greenberger-Horne-Zeilinger state - a quantum entanglement of three particles.# Galactic Habitable Zone - the region of a galaxy that is favorable to the formation of life....
and higher (THz) frequency range, where a
resonatorA resonator is a device or system that exhibits resonance or resonant behavior, that is, it naturally oscillates at some frequencies, called its resonance frequencies, with greater amplitude than at others. The oscillations in a resonator can be either electromagnetic or mechanical...
is usually added to control frequency. This resonator can be take the form of a
waveguideA waveguide is a structure which guides waves, such as electromagnetic waves or sound waves. There are different types of waveguide for each type of wave...
, microwave cavity or YIG sphere. Tuning is done mechanically, by adjusting the parameters of the resonator, or in case of YIG spheres by changing the
magnetic fieldMagnetic fields surround magnetic materials and electric currents and are detected by the force they exert on other magnetic materials and moving electric charges...
.
Gallium arsenide Gunn diodes are made for frequencies up to 200 GHz, gallium nitride materials can reach up to 3 terahertz.
The Gunn diode is named for the physicist
J.B. GunnJohn Battiscombe Gunn was an Egyptian-born US/British physicist.While working for IBM, in 1963 he invented the Gunn diode based upon the Cyril Hilsum-Ridley-Watkins effect. He is a member of the National Academy of Engineering, and received the 1969 IEEE Morris N. Liebmann Memorial Award.-...
who, in 1963, produced the first device based upon the theoretical calculations of
Cyril HilsumCyril Hilsum CBE FRS FREng HonFInstP is a British physicist and academic.-Life:He entered Raine's Foundation School in 1936 as the middle of three brothers, leaving in 1943 after being accepted into University College London, where he did his Bsc. In 1945 he joined the Royal Naval Scientific...
.
Microscopic view
GaAs has a third band above the conduction band. The gap is indirect, so a
phononIn physics, a phonon is a quantized mode of vibration occurring in a rigid crystal lattice, such as the atomic lattice of a solid. The study of phonons is an important part of solid state physics, because phonons play a major role in many of the physical properties of solids, including a material's...
is needed or created to deliver the impulse for the transition. The energy stems from the kinetic energy of ballistic electrons. They either start out in a high-energy Fermi-Dirac region and are ensured a sufficiently long mean free path by applying a strong electric field, or they are injected by a cathode with the right energy. For the latter, the cathode material has to be chosen carefully; chemical reactions at the interface need to be controlled during fabrication and additional monoatomic layers of other materials inserted. In either case, with forward voltage applied, the Fermi level in the cathode is the same as the third band, and reflections of ballistic electrons starting around the Fermi level are minimized by matching the density of states and using the additional interface layers to let the reflected waves interfere destructively. In GaAs the drift velocity in the third band is lower than in the usual conduction band, so with a small increase in the forward voltage, more and more electrons can reach the third band and current decreases. This creates a region of negative incremental resistance in the voltage/current relationship.
Multiple Gunn diodes in a series circuit are unstable, because if one diode has a slightly higher voltage drop across it, it will conduct less current, and the voltage drop will rise further. In fact, even a single diode is internally unstable, and will develop small slices of low conductivity and high field strength which move from the cathode to the anode. It is not possible to balance the population in both bands, so there will always be thin slices of high field strength in a general background of low field strength. So in practice, with a small increase in forward voltage, a slice is created at the cathode, resistance increases, the slice takes off, and when it reaches the anode a new slice is created at the cathode to keep the total voltage constant. If the voltage is lowered, any existing slice is quenched and resistance decreases again.
Applications
- Negative resistance behaviour can be used to amplify
- Common use is a high frequency and high power signal source
A
bias teeA bias tee is a kind of multiplexer which has 3 ports arranged in the shape of a T and where frequencies from 1 MHz to 30 GHz pass horizontally through the T and lower frequencies take a 90° turn. It is used to bias transistors and diodes...
is needed to isolate the bias current from the high frequency oscillations. Since this is a single-port device, there is no isolation between input and output.
Radio Amateur Use
By virtue of their low voltage operation, Gunn diodes can serve as microwave frequency generators for very low powered (few-milliwatt) microwave transmitters. In the late 1970s they were being used by some radio amateurs in Britain. Designs for transmitters were published in journals. They typically consisted simply of an approximately 3 inch waveguide into which the diode was mounted. A low voltage (less than 12 volt) direct current power supply that could be modulated appropriately was used to drive the diode. The waveguide was blocked at one end to form a resonant cavity and the other end ideally fed a parabolic dish.
See also
- tunnel diode
A tunnel diode or Esaki diode is a type of semiconductor diode which is capable of very fast operation, well into the microwave frequency region, by using quantum mechanical effects....
is also fast
- Avalanche diode
An avalanche diode is a diode that is designed to go through avalanche breakdown at a specified reverse bias voltage and conduct as a type of voltage reference....
is slow
- Zener diode
A Zener diode is a type of diode that permits current in the forward direction like a normal diode, but also in the reverse direction if the voltage is larger than the breakdown voltage known as "Zener knee voltage" or "Zener voltage"...
is a combination of the above two for temperature compensation
- ARPES
Angle resolved photoemission spectroscopy , also known as ARUPS , is a direct experimental technique to observe the distribution of the electrons in the reciprocal space of solids...
allows to find materials suitable for Gunn diodes
External links