The collector characteristic curve shows the behavior of the transistor. To get these curves we bias a BJT transistor with a variable power supply and change the V_BB and V­_CC. By variating these voltages, either transistor PN junctions may be forward or reverse-biased. Based on the condition of the BJT transistor junction biasing, the transistor can operate in four regions of operation.

Transistor Regions of Operation

Suppose we have an NPN transistor connected with variable power supplies as the diagram shows.

Cutoff region

When the base power supply V_BB is set to zero, the current IB becomes zero. Both of the PN junctions of the BJT transistor become reverse bias and no collector current I_C flows. In this region, the transistor behaves as an open switch. The V_CC voltage appears across the collector-emitter terminals V_CE. This region is the cutoff region. This state is ideal for applications needing a switch that completely disconnects the circuit

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Saturation region

When collector supply V_CC is set to zero, and base supply is set to produce a certain base current I_B. Both of the PN junctions become forward-biased. The base current I­_B flows through the emitter terminal because of low impedance and no current flows through collector I_C=0. This Transistor region of operation is the saturation region. In this region, both junctions are forward-biased. Collector current I_C reaches a certain largest current independent of base current IB. In the saturation region, the voltage across collector-emitter terminals V_CE is zero. This turns the BJT into a closed switch, making it indispensable in digital circuits where toggling between open and closed positions is critical.

The active or Linear region

By increasing the V­_CC the voltage across collector-emitter terminals V_­CE may increase. As the V_CE passes the 0.7v, the base-collector PN junction becomes reverse bias. And the emitter base is forward bias. This mode of operation is the active or linear region of operation in the BJT transistor characteristic curve. By increasing the V­_CE beyond 0.7v, the collector current remains constant for a given value of base current I_B. Increasing the V_CE can cause a very slight increase in I_C because of the widening of the base-collector depletion region. The following formula can calculate the active region collector current I_C.

This configuration enables the transistor to amplify signals, making it vital for audio equipment, radio transmitters, and other applications where signal boosting is required.

Breakdown region

By increasing the V_CC, the V_­CE can be increased which may increase the reverse bias of the base-collector junction. The high reverse bias may cause widening of the base-collector junction and finally break down the junction. As the BJT transistor enters the breakdown region, the collector current will increase rapidly as the curves diagram shows. A transistor is not used in the breakdown region.

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Understanding Basic BJT Parameters

When discussing Bipolar Junction Transistors (BJTs), two fundamental parameters come into play: DC Beta (β_DC) and DC Alpha (α_DC).

DC Beta (β_DC)

• Definition: DC Beta, often denoted as β_DC represents the DC current gain of a transistor. It is calculated as the ratio between the DC collector current and the DC base current.
• Significance: β_DC is crucial because it helps determine the transistor’s amplification capability—essentially how much current the transistor can control.
• Variability: It’s important to note that β_DC is not a fixed value. It varies with the collector current and the junction temperature. Typically, as the collector current increases, β_DC rises to a peak before declining if the current continues to rise. Meanwhile, with a constant collector current, an increase in junction temperature will cause β_DC to increase.

DC Alpha (α_DC)

• Definition: DC Alpha, represented as α_DC, is the ratio of the DC collector current to the DC emitter current.
• Usage: While α_DCis less commonly used in practical applications compared to β_DC, it remains an important parameter for understanding the efficiency of a BJT.
• Characteristics: The value of α_DC is always less than 1 since the collector current is slightly less than the emitter current due to recombination losses in the base.

Conclusion

• Based on transistor characteristic curves has four regions of operation
• The cutoff region works as an open switch
• The saturation region works as a closed switch
• I_C remains constant for a certain amount of I_B active region
• The Breakdown region of a transistor is not used

FAQs

Why is BJT called bipolar?

A BJT is named “bipolar” due to its operation relying on two charge carriers: holes and electrons.

What are the advantages of BJT?

Advantages of BJTs include superior performance in high-load and high-frequency conditions compared to MOSFETs. They offer higher fidelity, better gain in linear areas, and faster operation due to lower control pin capacitance than MOSFETs.