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.
•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