The Three Workhorses of Power Switching
Almost every power circuit – a motor driver, a power supply, a DC-DC converter – turns on and off using one of three semiconductor switches: the bipolar junction transistor (BJT), the MOSFET, or the insulated-gate bipolar transistor (IGBT). Each is optimized for a different balance of voltage, current, switching speed, and driver complexity. Understanding the trade space prevents the classic error of over-specifying (and over-paying) or under-specifying (and overheating).
BJT: Current-Controlled and Cheap
The BJT is a current-controlled device: a base current sets a larger collector current. It is inexpensive and robust, but it requires continuous base drive, which wastes power, and its switching speed is limited by stored charge. In new designs the BJT survives mainly in low-cost, low-frequency, and legacy applications; elsewhere the MOSFET or IGBT has replaced it.
MOSFET: Voltage-Controlled and Fast
The MOSFET is a voltage-controlled device: a gate voltage creates a conducting channel with essentially no steady-state gate current. That makes driving it simple and efficient. MOSFETs excel at high switching frequency because they switch quickly and have no minority-carrier storage delay. Their conduction loss rises with the square of current (I-squared-R, set by on-resistance Rds(on)), so their domain is lower-voltage and/or higher-frequency operation. Device choice within the family is covered in our MOSFET selection guide.
IGBT: The High-Voltage Compromise
The IGBT combines a MOSFET-like, voltage-controlled gate with a bipolar output stage. The result is low conduction loss at high voltage and current – better than a MOSFET of comparable rating at high voltages – at the cost of slower switching and a turn-off tail current. IGBTs dominate high-power motor drives, inverters, induction heating, and traction, where voltages run into the hundreds of volts and frequency stays moderate.
Comparing the Trade-Offs
Drive requirements
MOSFET and IGBT gates are voltage-driven and need only a gate driver with charge/discharge capability; the BJT needs controlled base current. Gate drive is therefore a major selection factor – and gate resistors and drivers materially affect EMI and switching loss.
Conduction loss
MOSFET: resistive (I-squared-R), rises steeply with current. IGBT and BJT: roughly a fixed voltage drop, more favorable at very high current and high voltage.
Switching loss
MOSFET: fastest, best for high frequency. IGBT: moderate, with a tail current. BJT: slowest, limited by stored charge.
Cost and ruggedness
BJTs are cheapest per ampere at low frequency; MOSFETs are cost-effective at low voltage; IGBTs are economical in high-voltage, high-power modules. All three benefit from proper thermal design and, where inductive loads are switched, from clamp or snubber networks.
Where Each Wins
Use a MOSFET for low-voltage, high-frequency switching such as synchronous converters, load switches, and motor drives under roughly 100 V. Use an IGBT for high-voltage, high-current, moderate-frequency work such as industrial inverters and welding. Use a BJT where cost dominates and frequency is low, or where a legacy design demands it. Related switching components – such as optoisolated gate drive and digital transistors – are covered in our optocoupler guide and digital transistor article.
Getting the Selection Right
Confirm voltage rating with margin (including inductive spikes), current rating at the real ambient, gate-drive compatibility, thermal path, and switching frequency. Then verify against the datasheet’s switching and thermal curves, not just the headline numbers.
By Technology sources BJTs, MOSFETs, and IGBTs from traceable channels with cross-references to alternates. Share your voltage, current, and switching-frequency requirements at sales@by-techs.com for matched devices, pricing, and lead time.