'Collector drift region' is introduced in Power BJT to

The structure of a power transistor is as shown:

• We can clearly observe that a power transistor is a vertically oriented four-layer structure of alternating p-type and n-type.
• The vertical structure is preferred because it maximizes the cross-sectional area and through which the current in the device is flowing. This also minimizes on-state resistance and thus power dissipation in the transistor.
• The doping of the emitter layer and collector layer is quite large typically 1019 cm-3. A special layer called the collector drift region (n-) has a light doping level of 1014 cm-3.
• The thickness of the drift region determines the breakdown voltage of the transistor. More the thickness of the collector drift region more will be the breakdown voltage (The transistor will block large voltage during OFF state)
• Small base thickness is preferred in order to have good amplification capabilities. However, if the base thickness is small the breakdown voltage capability of the transistor is
   compromised.

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A conventional transistor consists of thin p-layer sandwiched between two n layers and vice versa to form a three-terminal device with terminals named as Emitter, Base, and Collector as shown: