Negative resistance

1. 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 ohmicresistor, 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.

2. 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 microwavefrequencies • 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.

3. 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.

4. 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.

5. A Russian-made Gunn diode

6. 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.

7. 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.

8. 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,

9. "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,

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