SOT-23 MOSFET Cross Reference Guide: AO3400A, SI2302 and 2N7002

When a MOSFET becomes expensive, hard to source, or simply disappears from the approved vendor list, the first search is usually obvious:

"What can replace it?"

That search can become misleading very quickly.

A part with the same SOT-23 package is not automatically a replacement. A higher current rating does not guarantee better performance, and a lower RDS(on) does not help if the VDS rating is too low for the circuit.

For engineers and procurement teams, a useful MOSFET cross reference should start with the actual job of the device, then work back to the part number.

One warning before using any cross-reference table: the values below are reference data, not a blanket statement that every listed device is pin-to-pin interchangeable. Exact manufacturer, suffix, package drawing, test condition, and datasheet revision should be checked before approving a substitute.

Part Number	Package	VDS	RDS(on)	Replacement Note
AO3400A	SOT-23	30V	Low mΩ	Reference part
SI2302DS	SOT-23	20V	Low mΩ	Check voltage margin
2N7002	SOT-23	60V	Ω class	Small-signal use
IRF540N	TO-220	100V	Low mΩ	Not pin-to-pin

**The AO3400A 5.7A figure is a datasheet rating under specified conditions; it should not be treated as the continuous current capability of every PCB.

*Current ratings depend on thermal conditions and manufacturer specifications.

Is the Same SOT-23 Footprint Enough?

No.

This is probably the first thing I check in a MOSFET replacement guide.

SOT-23 tells you the package family, but not necessarily the pin assignment or the electrical behavior.

Before changing a part, compare the actual:

· Gate pin

· Drain pin

· Source pin

· Body diode orientation

· Package dimensions

· PCB footprint

This becomes especially important when a buyer is looking for an AO3400A drop in replacement.

A candidate can match the footprint and still have a different pin configuration. That is not a drop-in replacement.

The same issue appears with parts such as 2N7002. A 2N7002 pin for pin alternative has to be verified from the exact manufacturer's drawing rather than assumed from the "2N7002" name.

Electrical Matching Comes Before the Price

Once the package is confirmed, I move to the electrical side.

For a practical SOT-23 MOSFET alternative, I normally compare four main values first:

VDS, ID, RDS(on), and VGS.

VDS needs enough margin above the maximum voltage the MOSFET can actually see, including transients.

ID needs to cover the real load, but the headline current figure is not enough. Thermal conditions matter.

RDS(on) should be checked at the gate voltage the circuit actually provides.

That last point is easy to miss.

A MOSFET may have a very good RDS(on) number at 10V VGS but perform very differently when driven directly from a 3.3V MCU.

For a low RDS(on) SMD MOSFET replacement, I would rather see a clear RDS(on) specification at 2.5V or 4.5V than a very impressive number measured only at 10V.

Why 2N7002 Is Not a General Replacement

2N7002 appears frequently in replacement searches because it is common, inexpensive, and available in SOT-23.

But it belongs to a different electrical class from a low-resistance load-switching device.

That makes it useful for:

· Level shifting

· Small signal switching

· Logic interfaces

It is much less suitable when the original circuit depends on low conduction loss at hundreds of milliamps or more.

For example, a MOSFET with several ohms of RDS(on) can dissipate noticeable power even at modest current. A low-resistance device may dissipate only a small fraction of that heat.

So when someone asks me for an AO3400A equivalent N-channel SOT-23, I first ask what the MOSFET is actually switching.

SI2302: Closer, But Check the Voltage Margin

SI2302-type devices are more interesting when the application is a low-voltage load switch.

However, a commonly referenced SI2302DS is a 20V device. That can be fine on a 5V rail, but it deserves a much closer look on a 12V or 24V design where transients are possible.

This is a good example of a problem with automated MOSFET cross reference search tables.

A search engine may show two devices with similar RDS(on), similar package, and similar current numbers. The voltage rating can still make one unusable.

A lower-cost substitute is only useful if it keeps enough operating margin.

What About Large Parts Such as IRF540N?

The IRF540N is a completely different example.

It uses a TO-220 package and offers much higher current and voltage capability than these small SOT-23 devices.

Could it replace a small MOSFET electrically?

Sometimes, perhaps.

Can it replace one on the same PCB?

Usually not.

The package is larger, the gate characteristics are different, and the PCB would normally need to change.

This is why "equivalent" and "drop-in" should not be treated as the same term.

Three Checks I Use Before Approving a Replacement

1. Pinout and Package

Confirm the exact package drawing and SOT-23 package footprint.

2. Electrical Parameters

Check:

· VDS

· ID

· RDS(on) at actual VGS

· Gate charge

· VGS(th)

Do not use VGS(th) as proof that a MOSFET will be fully enhanced. It only describes the threshold test condition.

3. Thermal Conditions

A lower RDS(on) part can reduce conduction loss, but the PCB still needs to remove the heat.

For higher-current applications, look at the package thermal resistance, copper area, and the actual operating temperature.

This is where a MOSFET replacement connects directly with PCB thermal design.

Can a Cross Reference Also Reduce BOM Cost?

Yes, but I would not start with the cheapest quotation.

For an AO3400A substitute for cost reduction, the comparison should include:

· Unit price

· Availability

· MOQ

· Package

· Electrical margin

· Qualification effort

· Expected production volume

A cheaper MOSFET that requires PCB changes or creates more heat may cost more in the end.

For an AO3400A second source sourcing guide, I would also ask whether samples are available from the proposed manufacturer and whether the part can be tested on the existing board before the BOM is changed.

My View on MOSFET Cross Reference

I would not publish a fixed list saying:

AO3400A = SI2302 = 2N7002

That is too broad and can lead to the wrong replacement.

2N7002 is useful when the circuit needs a small-signal MOSFET. A low-RDS(on) device is more appropriate for load switching. A 20V part may work on a 5V rail but may be unsuitable on a higher-voltage system.

The part number is only the starting point.

For a real MOSFET replacement guide, the circuit, gate voltage, load, transient environment, package and sourcing requirements all need to be considered together.

Need Help Finding a MOSFET Replacement?

If you are looking for an AO3400A, SI2302, 2N7002, or another SOT-23 MOSFET alternative, send us the exact MPN, electrical requirements, or BOM. We can help check pin compatibility, electrical specifications, available alternatives, and sourcing options for your application.

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