Bipolar Junction Transistors (BJTs) have a varied and extensive history of applications in the field of electronics. They are utilized in a multitude of applications for amplification and as a switching mechanism. In power electronics, BJTs are sought after for their ability to handle high power, voltage, and current ratings, low turn on/off times, low voltage drop, and minimal leakage current in blocking mode. They are predominantly employed for switching purposes in power electronics.

BJT Structure

A bipolar junction transistor (BJT) is a semiconductor device that consists of three layers and three terminals, with alternating p-type and n-type layers. The three terminals are the emitter, base, and collector. There are two types of BJTs: NPN and PNP, which are distinguished by the arrangement of their layers. In an NPN BJT, the emitter and collector are made of n-type material, while the base is made of p-type material. Conversely, in a PNP BJT, the emitter and collector are made of p-type material, and the base is made of n-type material. A BJT has two junctions: a collector-base junction (CBJ) and a base-emitter junction (BEJ).

Figure 8. NPN Transistor structure and symbol

Figure 9. PNP Transistor structure and symbol

BJT Operation and Characteristics

The V-I characteristics of an NPN BJT are depicted in Figure 10, where the base current (IB) is plotted against the base-emitter voltage (VBE). The output V-I characteristics of an NPN BJT are illustrated in Figure 11, where the collector current (IC) is plotted against the collector-emitter voltage (VCE). For the PNP BJT, the polarities in the curves are reversed.

Figure 10. NPN BJT input V-I characteristics

Figure 11. NPN BJT output V-I characteristics

Bipolar junction transistors (BJTs) exhibit three distinct modes of operation, namely cut-off, active, and saturation, which are determined by the biasing conditions of the junctions.

In the cut-off mode, both the collector-base junction (CBJ) and base-emitter junction (BEJ) are reverse-biased, resulting in the BJT being turned off.

In the active mode, the BEJ is forward-biased while the CBJ is reverse biased, causing the BJT to function as an amplifier, with the base current being amplified and the collector-emitter voltage decreasing as the base current increases.

In the saturation mode, both the BEJ and CBJ are forward-biased, leading to the BJT acting as a closed switch, with the base current being sufficiently high to cause the collector-emitter voltage to be low.

Figure 12. BJT transfer characteristics

In BJT designs, the switching characteristics hold significant importance. During transient switching conditions, the capacitances of BJTs play a crucial role in influencing the turn-on and turn-off times of the transistors. The internal capacitances of BJTs cause a delay in the immediate turn-on of the transistor upon application of the base voltage. This delay time is dependent on the time taken for the BEJ to become forward-biased due to its capacitance, while the rise time is determined by the time taken for the collector current to reach its steady-state value. Similarly, the turn-off time is determined by the reverse-biased BEJ capacitance. These factors are critical as they determine the switching losses. To ensure optimal operation, BJTs should be operated with appropriate drive circuitry and ample base current to facilitate quick turn-on. Additionally, it is essential to keep bipolar junction transistors (BJTs) properly saturated to minimize conduction losses.

BJTs Applications

BJTs are frequently employed in a multitude of power electronics applications and they include:

Flyback Converters

Flyback converters are devices used for power conversion that facilitate both AC to DC and DC to DC conversion, while simultaneously providing galvanic isolation between the input and output. The split inductor employed in these converters forms a transformer that offers the necessary isolation. Functionally, the flyback converter is akin to the buck-boost converter, but it confers the added benefit of isolation.

Chopper Drives:

Chopper drives can be connected between a DC motor and a fixed-voltage DC input to vary the armature voltage. By varying the duty cycle through the control of BJTs' switching, it is possible to regulate the power flow and speed of motors. DC choppers can also be used to take advantage of the regenerative braking of motors, which returns energy to the source. This makes them an attractive option for modern transportation systems.

Other Applications:

BJTs are also used in switched-mode power supplies (SMPS), inverters, diverse converters such as forward converters, and audio devices.