Negative resistance

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Negative resistance

• Negative 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 are theoretical and do not
  exist as a discrete component.
• However, some types of diodes (e.g., tunnel
  diodes) can be built that exhibit negative
  resistance in some part of their operating range.
  
• Eg a differential negative resistance
  resonant-tunneling diode.

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• Electric discharges through gasses exhibit
  negative resistance, and some chalcogenide glasses
  , organic semiconductors, and conductive
  polymers exhibit a similar region of negative
  resistance as a bulk property.
• In electronics, negative resistance devices are
  used to make bistable switching circuits,
  and electronic oscillators, particularly
  at microwave frequencies
• Tunnel diodes and Gunn diodes exhibit a negative
  resistance region in their I-V (current
  voltage) curve. They have two terminals like a
  resistor but are not linear devices.
• Unijunction transistors also have negative
  resistance properties when a circuit is built
  using other components.

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Applications

• Many electronic oscillator circuits
  use one-port negative resistance devices, such
  as magnetron tubes, tunnel diodes and Gunn
  diodes.
• In these circuits, a resonator, such as an LC
  circuit, quartz crystal, or cavity resonator, is
  connected across the negative resistance device,
  and a DC bias voltage applied.
• The negative resistance of the active device can
  be thought of as cancelling the (positive)
  effective loss resistance of the resonator,
  creating sustained oscillations.
• These circuits are frequently used for
  oscillators at microwave frequencies.
• Oscillators have also been built using the
  negative resistance region of amplifying devices
  like vacuum tubes, as in the dynatron oscillator.

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GUNN DIODE

• A Gunn diode, also known as a transferred
  electron device (TED), is a form of diode used in
  high-frequency electronics.
• Its internal construction is unlike other diodes
  in that it consists only of N-doped semiconductor 
  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.

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A Russian-made Gunn diode
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WORKING

• 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
  applied voltage.
• Eventually, at higher field values, the
  conductive properties of the middle layer will be
  altered, increasing its resistivity, preventing
  further conduction and current starts to fall.
• This means a Gunn diode has a region of negative
  differential resistance.

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APPLICATIONS

• Because of their high frequency capability, Gunn
  diodes are mainly used at microwave frequencies
  and above.
• They can produce some of the highest output power
  of any semiconductor devices at these
  frequencies.
• Their most common use is in oscillators, but they
  are also used in microwave amplifiers to amplify
  signals.

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• Gunn diode oscillators are used to generate
• microwave power for
• airborne collision avoidance radar, 
• anti-lock brakes, 
• sensors for monitoring the flow of traffic, 
• car radar detectors,
• pedestrian safety systems,
• "distance traveled" recorders, 
• motion detectors,

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• "slow-speed" sensors (to detect pedestrian and
  traffic movement up to 50 m.p.h),
• traffic signal controllers,
• automatic door openers,
• automatic traffic gates,
• process control equipment to monitor throughput,

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• burglar alarms and equipment to detect
  trespassers,
• sensors to avoid derailment of trains,
• remote vibration detectors,
• rotational speed tachometers,
• moisture content monitors

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