Negative Resistance Devices Explained

A negative resistance device is an electronic component that exhibits a unique characteristic where, over a certain range of its operation, an increase in current through the device leads to a decrease in the voltage drop across it. This is contrary to Ohm's law, where an increase in current typically results in an increase in voltage for a positive resistance. Devices with negative resistance are crucial in various applications, including oscillators, switching circuits, and memory circuits.

Let's analyze the given options to identify which one is a negative resistance device.

Silicon Controlled Rectifier (SCR) Characteristics

The Silicon Controlled Rectifier (SCR) is a four-layer, three-junction semiconductor device belonging to the thyristor family. It is primarily used for high-power switching applications. Its voltage-current (V-I) characteristic curve demonstrates a distinct region of negative resistance.

• Forward Blocking Mode: When the anode is positive with respect to the cathode, but the voltage is below the forward breakover voltage (\(V_{BO}\)), the SCR conducts only a small leakage current.
• Forward Conduction Mode: If the forward voltage increases and reaches \(V_{BO}\), or if a gate signal is applied, the SCR switches rapidly from the OFF state to the ON state.
• Negative Resistance Region: Once the SCR turns ON, the voltage across it drops sharply to a very low value (typically 1-2 volts), while the current through it increases significantly. This rapid decrease in voltage for an increasing current is the hallmark of the negative resistance characteristic. This region occurs between the peak point (just before breakover) and the valley point (the holding current). After this region, the device behaves like a low positive resistance.

Therefore, the Silicon Controlled Rectifier is indeed a negative resistance device in its forward conduction characteristic.

Understanding Other Electronic Devices

P-N Junction Diode

A P-N junction diode is a two-terminal semiconductor device that allows current to flow predominantly in one direction (forward bias) and blocks current in the opposite direction (reverse bias).

• In forward bias, as the voltage increases beyond the cut-in voltage (e.g., 0.7V for silicon), the current increases exponentially, exhibiting positive resistance.
• In reverse bias, only a small leakage current flows until the breakdown voltage is reached.

A P-N junction diode does not typically exhibit a negative resistance region in its normal operating characteristics.

Field Effect Transistor (FET)

A Field Effect Transistor (FET) is a voltage-controlled device used for amplification and switching. It controls the current flow between the source and drain terminals by varying the voltage at the gate terminal.

• The common FET types, such as JFETs and MOSFETs, primarily operate in ohmic and saturation regions, where they exhibit positive resistance characteristics or act as current sources (high output impedance), but not negative resistance.

FETs are generally not considered negative resistance devices.

Bipolar Transistor (BJT)

A Bipolar Transistor (BJT) is a current-controlled device used for amplification and switching. It controls a large current between the collector and emitter terminals based on a small current applied to the base terminal.

• BJTs operate in cutoff, active, and saturation regions. In all these regions, their I-V characteristics do not display a negative resistance behavior where voltage decreases as current increases. They exhibit positive resistance or act as current sources.

Bipolar transistors are not classified as negative resistance devices.

Key Takeaway: Identifying Negative Resistance

Among the listed options, only the Silicon Controlled Rectifier (SCR) inherently exhibits a negative resistance region in its forward V-I characteristic curve after the forward breakover voltage is reached, making it a true negative resistance device.