Negative resistance Negative resistance is a property of

Negative resistance ØNegative 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. ØThis 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 Ndoped 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 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,

• "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