Why N-Channel Enhancement MOSFETs Suit Power Inverters
D — N-channel enhancement-mode MOSFET. Among the listed device types, it combines normally-off gate control with the low conduction resistance commonly sought in efficient power switching.
Why choose an N-channel device?
In a silicon N-channel MOSFET, electrons carry channel current. Their mobility is higher than that of holes used in a P-channel device. With comparable technology, die size and voltage rating, this generally permits lower on-state resistance, RDS(on). Lower resistance reduces conduction loss at a given current.
This is a comparison principle, not a guarantee that every N-channel part has lower loss than every P-channel part. Actual inverter performance depends on the selected device and its operating conditions.
Why enhancement mode?
An enhancement-mode MOSFET has no intentionally conducting channel at VGS = 0 and is normally off. A suitable positive gate-to-source voltage turns an N-channel device on. This makes its channel convenient to command with switching pulses.
A depletion-mode MOSFET is normally on at zero gate-to-source voltage and needs the appropriate gate bias to turn its channel off. It is therefore not the usual choice for the normally-off inverter switch sought in this MCQ.
What determines switching efficiency?
MOSFETs are majority-carrier switches, but switching speed is also affected by gate charge, capacitances and gate-driver capability. Low RDS(on) helps conduction efficiency; gate charge and switching losses must also be considered.
For example, reducing on-resistance from 20 mΩ to 10 mΩ halves conduction loss for the same channel-current waveform. This does not automatically halve total inverter loss.
Useful Formula
Pcond ≈ ID,rms² × RDS(on), using the MOSFET current RMS over the full switching period. Gate-drive power ≈ Qg × Vdrive × fs.
Why the Other Options Are Not the Answer
- A — N-channel depletion mode: offers an N-channel but has normally-on channel behaviour at zero gate bias.
- B — P-channel enhancement mode: offers normally-off control, but generally has a higher resistance trade-off for comparable silicon device size and voltage rating.
- C — P-channel depletion mode: combines normally-on behaviour with the P-channel resistance trade-off.
Remember for the Exam
Use the gate voltage at which the datasheet specifies RDS(on). Threshold voltage indicates the onset of small channel current, not full enhancement.
Common mistake: Do not assume zero switching loss or use the gate-threshold voltage as the required full-load drive voltage.
Reference: Texas Instruments SLPA021: power MOSFET selection and gate drive
