Negative differential resistance in polymer tunnel diodes ...
Negative resistance
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Transcript of Negative resistance
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Negative 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 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 generatemicrowave 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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