Cut-off State
When the voltage applied across the transistor's emitter junction is lower than the turn-on voltage of the PN junction, the base current drops to zero; consequently, both the collector current and the emitter current become zero. At this point, the transistor loses its current amplification capability. The region between the collector and the emitter effectively acts as an open switch-this constitutes the transistor's cut-off state. A defining characteristic of a switching transistor in the cut-off state is that both the emitter junction and the collector junction are reverse-biased.
Conduction State
When the voltage applied across the transistor's emitter junction exceeds the PN junction's turn-on voltage, and as the base current increases to a certain threshold, the collector current ceases to increase in proportion to the base current. Instead, it stabilizes around a specific, relatively constant value. At this juncture, the transistor again loses its current amplification capability, and the voltage drop between the collector and the emitter becomes very small. The region between the collector and the emitter effectively acts as a closed switch-this constitutes the transistor's conduction state. A defining characteristic of a switching transistor in the *saturated* conduction state is that both the emitter junction and the collector junction are forward-biased. In contrast, a transistor operating in the *amplification* state is characterized by a forward-biased emitter junction and a reverse-biased collector junction. This principle-using a voltmeter to measure the voltage values across the emitter and collector junctions-is precisely how one can determine the transistor's current operating status. Switching transistors function specifically by leveraging these inherent switching characteristics of the transistor.
Operating Modes
Transistors come in a wide variety of types, and different models are designed for distinct applications. Most transistors feature either plastic or metal packaging. Regarding the physical appearance of common transistors, the electrode marked with an arrow is the emitter; if the arrow points outward, it indicates an NPN-type transistor, whereas if the arrow points inward, it indicates a PNP-type transistor. In practical terms, the direction in which the arrow points signifies the direction of the conventional current flow.