Because there are no minority carriers in an FET. As a result, it can switch off faster since no stored charge has to be removed from the junction area. There are two kinds of unipolar transistors: JFETs and MOSFETs.
In general, JFETs are more temperature stable than bipolar transistors. Furthermore, JFETs are typically much smaller than bipolar transistors. This size difference makes them particularly suitable for use in ICs, where the size of each component is critical.
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