Construction

A lightly doped region called base is sandwiched between two regions called the emitter and collector respectively. The collector handles large quantities of current, hence its dopant concentration is the highest. The emitter's dopant concentration is slightly lesser, but its area is larger to provide for more current than the collector. The collector region should be heavily doped because electron-hole pairs recombine in that region, while the emitter is not such a region. We can have two varieties in this kind of transistor.

NPN Transistor

An NPN transistor is made by joining one positive-type semiconductor in between two negative-type semiconductors. Here a lightly doped p-type semiconductor (semiconductor with more holes than electrons) is sandwiched between two well-doped n-type regions. It is like two pn-junctions facing away. An IEEE symbol for the NPN transistor is shown here. The arrow between the base and emitter is in the same direction as current flowing between the base-emitter junction. Power dissipated in the transistor is

${\displaystyle P=V_{CE}I_{C}}$, where ${\displaystyle V_{CE}}$ is the voltage between the collector and the emitter and ${\displaystyle I_{C}}$ is the collector current.

PNP Transistor

A PNP transistor is made by sandwiching a negative-type semiconductor in between two positive-type semiconductors.

Amplifier

A transistor conducts current when the base voltage is greater than BE junction's voltage. Current is non zero.

${\displaystyle I_{B}\neq 0}$

${\displaystyle I_{C}=\alpha I_{E}}$

${\displaystyle I_{C}=\beta I_{B}}$

Switch

A transistor conducts current when the base voltage is greater than BE junction's voltage. Current is non zero. This corresponds to a closed switch. A transistor does not conduct current when the base voltage is less than BE junction's voltage. Current is zero. This correspond to an open switch.