Which component allows an electric current to be switched on or off when a small electric current is applied to its base?

Which component allows an electric current to be switched on or off when a small electric current is applied to its base?

• A. Diode
• B. Capacitor
• C. Transistor
• D. Resistor

Answer: (C)

A transistor acts as a controllable switch: a small current applied to the base controls a much larger current that can flow from the collector to the emitter. When the base-emitter junction is forward biased (about 0.6–0.7 V for silicon), a little base current allows a much larger collector-emitter current, so the device can turn on or off as needed. This current gain is what makes it ideal for switching. The diode conducts current in one direction but has no separate base control to switch a larger current on or off. The capacitor responds to voltage changes by charging or discharging, not by being turned on or off with a base current. The resistor simply limits or dissipates current in a passive, non-switching way.

A transistor acts as a controllable switch: a small current applied to the base controls a much larger current that can flow from the collector to the emitter. When the base-emitter junction is forward biased (about 0.6–0.7 V for silicon), a little base current allows a much larger collector-emitter current, so the device can turn on or off as needed. This current gain is what makes it ideal for switching.

The diode conducts current in one direction but has no separate base control to switch a larger current on or off. The capacitor responds to voltage changes by charging or discharging, not by being turned on or off with a base current. The resistor simply limits or dissipates current in a passive, non-switching way.