# Interpreting MOSFET parameters - capacitance and turn-on/off time

I'm checking out affordable MOSFETs for hobby projects and struggling to figure out how each MOSFET parameters compare. Can someone enlighten me on how I can interpret these numbers?

I'm comparing two N-CH MOSFETs: AO3400 and BSS138.

Both have low V_GS threshold which allows uC to drive them directly, and newer AO3400 seems to handle more current (I_d(on)) with its lower R_DS(on). But that's not a deciding factor for this case.

I'm confused on a point that AO3400 has far higher capacitance while having faster turn-on/off speed. This contradicts with my understanding that MOSFET with higher capacitance takes more time to fully turn on/off.

I do see test conditions are different and both used different target V_DS (AO3400: 15 V, BS138: 30 V), as both have different breakdown voltage. In this case, is there any way for me to do a better apples-to-apples performance comparison (or educated guess) using datasheet figures/plots?

When I check other plots in datasheets, I found this:

I'm wondering if following thoughts are in the right direction:

At V_GS=5 V, gate charges are AO3400:~7 nC and BS138:~1 nC. With difference in gate resistance (AO3400:3 Ω, BS138:9 Ω), charge time (= reflects to various delay/rise/fall time) of AO3400 is probably around twice that of BS138.

Is this good as an educated guess?

Also, I'm still not sure how can I interpret huge difference in capacitance. In the above plots, input capacitance are roughly AO3400:~650 pF and BS138:~35 pF at V_DS=5 V. What kinds of other impact I may have other than time performance?

• Your understanding is fine, but the other factor you missed is that having an smaller VDS, and thus, and smaller supply, makes the charging of your load faster. This is the reason why going to smaller nodes is attractive, in principle; the supply diminishes and thus, there will be faster rise times achieved. Commented Dec 14, 2022 at 9:17