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.

•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.

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.

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.

A Russian-made Gunn diode

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.

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.

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,

• "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,

• burglar alarms and equipment to detect trespassers,

• sensors to avoid derailment of trains, • remote vibration detectors, • rotational speed tachometers, • moisture content monitors