Understanding Diode Characteristics and Negative Resistance

The question asks about a specific type of diode where, as the voltage across it increases, the current flowing through it actually decreases. This behavior is quite unusual for most standard electronic components like resistors or typical diodes, which usually show increasing current with increasing voltage (either linearly or non-linearly).

This unique characteristic, where the differential resistance (\( dV/dI \)) is negative, is known as negative differential resistance.

Analyzing the Given Options

Let's look at the behavior of the diodes listed in the options:

• Tunnel Diode: Also known as an Esaki diode. This diode is specifically designed to utilize the quantum mechanical effect of tunneling. Its forward bias characteristic curve exhibits a region where the current increases rapidly at low forward voltage, reaches a peak current (\( I_P \)), then decreases as the voltage increases further until it reaches a valley current (\( I_V \)), after which it starts increasing again like a normal p-n junction diode. The region between the peak voltage (\( V_P \)) and the valley voltage (\( V_V \)) is the negative differential resistance region, where increasing voltage leads to decreasing current.
• Schottky Diode: This is a metal-semiconductor diode. It typically has a lower forward voltage drop and faster switching speed compared to standard p-n junction diodes. Its current-voltage characteristic is generally similar to that of a regular diode in forward bias – current increases significantly with increasing voltage above the turn-on voltage. It does not exhibit a negative differential resistance region.
• Laser Diode: This is a semiconductor device that emits coherent light when current passes through it. Electrically, it behaves somewhat like a forward-biased p-n junction, but its primary function is light emission, not exhibiting a negative differential resistance region in the way described.
• Gunn Diode: Also known as a Transferred Electron Device (TED), the Gunn diode is a bulk semiconductor device (not a junction diode) that also exhibits negative differential resistance. However, the mechanism is different from that of a tunnel diode (it's due to transferred electron effect). While it has negative resistance, the Tunnel Diode is a junction diode and is a more direct and classic example of the current-voltage characteristic described. The question asks for a "diode", and while Gunn diodes are two-terminal devices, tunnel diodes are typically classified more squarely as diodes exhibiting this specific forward bias characteristic.

Tunnel Diode's Unique Characteristic

The Tunnel Diode operates based on quantum tunneling. At low forward bias, electrons tunnel across the junction, resulting in current. As the voltage increases slightly, the tunneling increases, and so does the current. However, beyond a certain point (the peak voltage \( V_P \)), the energy bands align in such a way that the tunneling probability decreases as the voltage increases. This causes the current to drop even as the voltage rises, leading to the negative differential resistance region until the voltage reaches \( V_V \). Beyond \( V_V \), the diode starts behaving like a normal forward-biased p-n junction, with current increasing with voltage.

Based on the characteristic where current decreases as voltage increases in a specific operating region, the Tunnel Diode is the correct answer.

Revision Table: Diode Characteristics

Additional Information: Negative Differential Resistance

Negative differential resistance (NDR) is a property in some electronic components or circuits where an increase in voltage across the device terminal results in a decrease in electric current through it. This is different from negative resistance, which is sometimes used loosely to describe NDR or can refer to a device that actually sources power.

Devices exhibiting NDR are crucial in various applications:

• Oscillators: NDR can be used to build oscillators by negating the positive resistance of other components in a resonant circuit.
• Amplifiers: NDR can provide gain in amplifier circuits.
• Switching Circuits: The sharp transition in the NDR region can be used for high-speed switching.

Both Tunnel Diodes and Gunn Diodes are key semiconductor devices that demonstrate NDR, albeit through different physical mechanisms. The question specifically points to a "diode" with the characteristic in its voltage-current curve, which is the defining feature of the Tunnel Diode's forward bias performance.