In a transistor, the _______ region is the widest and the _______ region is the thinnest of all.

• In a transistor, the collector region is the widest, while the base region is the thinnest.
• This structure allows for efficient operation of the transistor, where the base controls the current flow between the emitter and collector regions.
• The Collector region is kept the largest so as to be able to fast dissipate the heat produced due to the collector current flowing through it.
• The large area is also required to accommodate the wide space charge region due to reverse biased CB junction.
• The emitter region is heavily doped, so not much of the space charge width is in the emitter region.
• Also, the EB junction is forward biased which further eliminates the need to have a large emitter area.

Additional Information 

Notes: Transistor has three regions:

Emitter:

• Highly doped
• medium area

Collector:

• Moderate doping
• largest area

Base:

• lightest doping
• lightest area