1. Minority vs. Majority Carriers:
• BJT (Bipolar Junction Transistor): This is a "minority carrier" device. To switch it OFF, the minority carriers stored in the base region must be removed (storage time), which significantly slows down the process.
• MOSFET (Metal-Oxide-Semiconductor FET): This is a "majority carrier" device. It operates by creating an electric field that attracts majority carriers to form a channel.
2. The Advantage of the MOSFET: Because MOSFETs do not involve the storage and removal of minority charge carriers, they can switch between ON and OFF states much faster than BJTs. Their speed is primarily limited only by the time it takes to charge and discharge the gate-to-source capacitance.
3. JFETs and Triodes:
• JFETs: While also majority carrier devices, they generally have higher internal capacitances and lower current capabilities than MOSFETs, making them less ideal for high-speed power switching.
• Triodes: These are vacuum tubes; while fast, they are physically bulky and inefficient compared to modern solid-state MOSFET technology.
Consequently, MOSFETs are the standard choice for high-frequency applications like switching power supplies and high-speed digital logic.