Higher switching losses but lower conduction losses
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Switching losses refer to the energy dissipated during the transition of a device from on-state to off-state or vice versa. Conduction losses, on the other hand, refer to the energy dissipated during the on-state operation of a device.

BJT (Bipolar Junction Transistor) and power MOSFET (Metal-Oxide-Semiconductor Field-Effect Transistor) are two commonly used power devices in electronic circuits. Each of them has different characteristics and performance parameters, which determine their switching and conduction losses.

Switching Losses:
Switching losses in power devices occur during the transition between on-state and off-state. These losses are mainly due to the charging and discharging of the internal capacitances of the device, as well as the voltage and current spikes that occur during the switching process.

- BJT: When a BJT switches from off-state to on-state, there is a significant amount of charge stored in the base region that needs to be removed before the transistor fully turns on. This leads to a longer turn-on time and higher switching losses compared to MOSFETs. Therefore, a BJT has higher switching losses.
- MOSFET: Power MOSFETs have a very low gate-to-source capacitance, which allows for faster turn-on and turn-off times. This results in lower switching losses compared to BJTs.

Conduction Losses:
Conduction losses occur during the on-state operation of a power device when it conducts current from the source to the drain (for MOSFET) or from the collector to the emitter (for BJT).

- BJT: Bipolar junction transistors have a relatively higher on-state voltage drop (VCE(sat)) compared to MOSFETs. This leads to higher conduction losses in BJTs.
- MOSFET: Power MOSFETs have a lower on-state resistance (RDS(on)), resulting in lower conduction losses compared to BJTs.

Conclusion:
Based on the above comparisons, it can be concluded that a BJT has higher switching losses but lower conduction losses compared to a power MOSFET. However, it is important to note that the specific characteristics and performance parameters of each device can vary depending on the specific model and technology used. Therefore, it is always recommended to consult the datasheets and specifications of the devices for accurate information regarding their switching and conduction losses.