12.8: Switching of BJT
Switching behavior in Bipolar Junction Transistors (BJTs) is a fundamental aspect utilized in various electronic circuits, particularly for digital logic applications like switches and amplifiers. In a typical switching circuit, a BJT alternates between cut-off and saturation modes, corresponding to the "off" and "on" states, respectively, thus behaving like an ideal switch.
Cut-off Mode ("Off" State): In this state, both the emitter-base and collector-base junctions are reverse-biased. The transistor prevents current flow across its terminals, effectively rendering the device "off." This mode is utilized when no signal transmission is required, maintaining a low power state in the circuit.
Saturation Mode ("On" State): Contrary to the cut-off mode, in the saturation state, both junctions are forward-biased. This setup allows maximum current flow from the collector to the emitter. The BJT in this mode behaves like a closed switch, enabling full signal transmission through the circuit.
Switching Dynamics: The transition between the "off" and "on" states is triggered by a sudden change in the emitter-base voltage, typically initiated by a positive input current pulse. The behavior of the collector current during these transitions is crucial for effective switching. It depends on the variation of the total excess minority carrier charge stored in the base region of the transistor.
During the turn-on phase, if the base charge exceeds a certain threshold (denoted as QS), the BJT enters saturation mode. Conversely, during the turn-off phase, the collector current remains nearly constant until the stored charge reduces back to QS, causing the transistor to revert to active mode before eventually decreasing towards zero as it approaches the cut-off mode.
Understanding these transitions and the associated charge dynamics is essential for designing efficient BJTs that can rapidly switch between states with minimal loss of performance and power.
