Power BJT: Bipolar Junction Transistor

Power BJT Characteristics

• BJT is the first Switching Control Semiconductor Device (SCSD) to allow full control.
• Its construction and operating characteristics differ significantly from signal BJT due to voltage and current ratings requirements.
• In the cutoff region, the collector current is almost zero, and the maximum voltage between collector and emitter is denoted by VCEO (the rated voltage).

Operating Conditions

• To emulate an ideal switch, the base current in the on-state must be high enough for the operating point to lie on the hard saturation line associated with the lowest voltage drop across the device.
• Power transistors have very small reverse voltage withstanding capability.

Limitations

• BJTs cannot block negative collector–emitter voltages.
• To block negative voltage, a diode is connected in series with the collector.

Failure Modes

• Second breakdown occurs when both the collector–emitter voltage and the collector current are high, resulting in high power loss and local hot spots in the semiconductor.
• The current density increases in hot spots, leading to damage.

Key Features

• Current-Controlled Device
• Low current gain compared to signal amplifiers (typically 10)