Introduction:
A power transistor is a key component in electronic devices that amplifies and controls the flow of electrical current. It is commonly used in applications such as power amplifiers, voltage regulators, and switching circuits. Understanding the characteristics and specifications of power transistors is crucial for designing and implementing efficient and reliable electronic systems.

Explanation:

1. Three-terminal device:
A power transistor is a three-terminal device, meaning it has three connection points: the emitter (E), the base (B), and the collector (C). These terminals are responsible for controlling the flow of current through the transistor. The base-emitter junction controls the current flow, and the collector-emitter junction handles the amplified current.

2. Bipolar device:
A power transistor is a bipolar device, which means it utilizes both types of charge carriers: electrons and holes. Bipolar transistors are constructed using two types of semiconductor materials, namely P-type and N-type. The two commonly used types of bipolar transistors are NPN and PNP.

3. Current controlled:
The operation of a power transistor is primarily controlled by the current flowing through its base-emitter junction. By varying the base current, the transistor can be turned on or off, allowing or blocking the flow of current through the collector-emitter junction. Therefore, a power transistor is considered a current-controlled device.

4. Incorrect options:
a) Two-terminal: A power transistor is not a two-terminal device since it has three terminals (emitter, base, and collector).
b) Unipolar: A power transistor is not a unipolar device since it utilizes both electrons and holes as charge carriers.
c) Voltage controlled: While voltage can have an impact on the operation of a power transistor, it is primarily controlled by the current flowing through the base-emitter junction.

Conclusion:
In conclusion, a power transistor is a three-terminal, bipolar, and current-controlled device. Understanding these characteristics is essential for effectively implementing power transistors in electronic circuits.