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What is negative resistance and what causes it to occur ?

Negative resistance is a phenomenon where the current through a device decreases as the voltage across it increases. This behavior is contrary to the normal positive resistance observed in most conventional electronic components. Understanding negative resistance involves delving into the underlying mechanisms and the specific types of devices that exhibit this peculiar characteristic.

1. Definition of Negative Resistance:

a. Opposite Behavior:

  • In a component exhibiting negative resistance, an increase in voltage leads to a decrease in current, contrasting with the typical positive relationship observed in ohmic resistors.

b. Regions of Operation:

  • Negative resistance is often observed within specific voltage or current ranges, and it is essential to consider the device’s operating conditions.

2. Types of Devices Exhibiting Negative Resistance:

a. Tunnel Diodes:

  • Mechanism:
    • Negative resistance is prominently seen in tunnel diodes, where quantum tunneling plays a crucial role.
    • As the voltage across the diode increases, electrons “tunnel” through the potential barrier, causing a reduction in current.

b. Gas Discharge Tubes:

  • Mechanism:
    • Certain gas discharge tubes, like neon lamps, can exhibit negative resistance behavior under specific conditions.
    • As the voltage increases, the gas becomes ionized, resulting in a sudden decrease in resistance.

c. Gunn Diodes:

  • Mechanism:
    • Gunn diodes, based on the Gunn effect in semiconductors like gallium arsenide, display negative differential resistance.
    • The drift velocity of charge carriers in the material decreases with increasing electric field, leading to negative resistance.

d. Avalanche Diodes:

  • Mechanism:
    • Avalanche diodes can exhibit negative resistance during the avalanche breakdown process.
    • As carriers gain energy and collide with other atoms, a cascade effect occurs, causing a decrease in resistance.

e. Active Devices in Certain Configurations:

  • Negative Resistance Amplifiers:
    • In certain feedback configurations, active devices like transistors can create negative resistance conditions.
    • This can be utilized to design negative resistance amplifiers for specific applications.

3. Causes of Negative Resistance:

a. Quantum Tunneling:

  • In devices like tunnel diodes, negative resistance arises due to quantum tunneling, allowing electrons to traverse a potential barrier.

b. Carrier Mobility and Drift:

  • In semiconductor devices like Gunn diodes, negative resistance results from the interplay between carrier mobility and the electric field, leading to a decrease in drift velocity.

c. Avalanche Breakdown:

  • In devices undergoing avalanche breakdown, such as avalanche diodes, negative resistance is observed during the rapid increase in current due to impact ionization.

d. Gas Ionization:

  • In gas discharge tubes, negative resistance occurs as the gas undergoes ionization under the influence of an increasing electric field.

4. Applications and Considerations:

a. Oscillators:

  • Negative resistance devices are utilized in oscillator circuits, where the negative resistance region supports sustained oscillations.

b. Amplifiers:

  • Negative resistance characteristics can be harnessed for specific amplifier applications.

c. Frequency Generation:

  • Gunn diodes, exhibiting negative differential resistance, are commonly used in microwave frequency generation.

d. Voltage-Controlled Oscillators (VCOs):

  • Devices with negative resistance properties are valuable in the design of voltage-controlled oscillators used in communication systems.

5. Challenges and Considerations:

a. Operating Conditions:

  • Negative resistance is often limited to specific voltage or current ranges, and devices must be operated within these conditions.

b. Temperature Sensitivity:

  • The behavior of devices exhibiting negative resistance can be sensitive to temperature variations.

Conclusion:

Negative resistance is a fascinating and unconventional phenomenon observed in specific electronic components under certain conditions. Understanding the mechanisms, types of devices, and applications associated with negative resistance is crucial for utilizing this behavior in designing specialized circuits and electronic systems.

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