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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.

Side-by-side datasheet comparison of 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:

Side-by-side comparison of gate-charge/capacitance characteristics

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?

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  • \$\begingroup\$ 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. \$\endgroup\$
    – Designalog
    Commented Dec 14, 2022 at 9:17

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These MOSFETs differ in Id (Drain current) significantly. Namely, AO3400 offers way higher Id, therefore, it naturally has larger gate area (A). The higher the A, the higher the gate capacitance (Cg), typically it's just proportional to A (one can approximate MOSFET gate capacitance with two parallel plates capacitor model).

The process of turning the MOSFET ON is about charging the Cg with Qg. Now since Cg is very different between these two parts, it will take different time to charge that gate capacitance using the same current, e.g. it will take several times longer to charge AO3400's Cg than that of a counterpart using the same current and that difference is expressed in difference in Qg between the parts.

However, these two MOSFETs have different gate resistance (Rg) values. The gate resistance is a some sort of series resistance to Cg, which actually limits the gate current and hence the time needed to charge Cg. Note Rg of AO3400 is few times lower, this allows for faster charging of Cg (even though it's larger than that of BS138) and hence shorter turn-on rise times.

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