Small-Signal MOSFET and Diode Buyer's Guide 2026: 2N7002 vs BSS138 vs AO3400
Small-signal MOSFETs and rectifier diodes are the cheapest parts on your BOM and the most common cause of a board that works on the bench and fails in the field. This guide compares the four SOT-23 MOSFETs you will meet in almost every design - 2N7002, BSS138, AO3400 and AO3401 - together with the three rectifier diodes that cover 90% of general-purpose jobs: 1N4007, SS34 and 1N5819.
N-channel small-signal MOSFETs compared
| Part | Vds | Current | Rds(on) | Vgs(th) | Best for |
|---|---|---|---|---|---|
| 2N7002 | 60V | 115mA cont. / 300mA pulse | ~1.2-2 ohm at 10V | 1.0-2.5V | General level shifting, small loads, legacy designs |
| BSS138 | 50V | 200-220mA | ~3.5 ohm at 10V | 0.8-1.5V | I2C level shifting at 1.8V / 3.3V rails |
| AO3400A | 30V | 5.7A | ~28m ohm at 10V | 0.5-1.4V | High-current load switches, LED strings, small fans |
| SI2302 | 20V | 2.5A | ~60m ohm at 4.5V | 0.65-1.2V | Logic-level load switch on 3.3V rails |
P-channel and complementary pairs
| Part | Vds | Current | Rds(on) | Typical use |
|---|---|---|---|---|
| AO3401A | -30V | -4A | ~50-60m ohm at -4.5V | High-side switching, reverse-polarity protection, load disconnect |
| BSS84 | -50V | -130mA | ~10 ohm at -5V | Small high-side signal switching, level shifting |
Designers pairing an N-channel low-side switch with a P-channel high-side switch usually keep the same package and gate-drive scheme, which is why AO3400 and AO3401 are so often specified together.
Rectifier diodes: standard vs Schottky
| Part | Type | Vrrm | Current | Vf (typ.) | Package | Typical use |
|---|---|---|---|---|---|---|
| 1N4007 | Standard | 1000V | 1A | ~1.0V | DO-41 | Mains rectification, general purpose, reverse protection |
| SS34 | Schottky | 40V | 3A | ~0.5V | SMA | Buck converters, freewheeling, ORing rails |
| 1N5819 | Schottky | 40V | 1A | ~0.45V | DO-41 | Low-voltage rectification, reverse-polarity protection |
The rule of thumb: above about 100V you need a standard or ultrafast rectifier (1N4007 family); below about 40V and above 100kHz, a Schottky (SS34, 1N5819) saves both loss and heat. Schottky diodes leak more in reverse and have lower surge tolerance, so they are a poor choice for mains-facing circuits.
How to choose in five steps
1. Set the voltage margin. Pick Vds / Vrrm at least 1.5-2x the highest voltage the part will ever see, including transients.
2. Check gate drive at your logic level. A 3.3V MCU cannot fully enhance a MOSFET whose Vgs(th) is 2.5V. For 1.8V/3.3V logic, prefer BSS138, AO3400A or SI2302.
3. Size the current for Rds(on), not the headline rating. The useful number is the Rds(on) specified at your actual gate voltage, not at 10V.
4. Budget the thermal path. In SOT-23 the copper area under the drain tab sets your real power limit - not the datasheet current.
5. Add the small parts. A 10-100 ohm gate resistor, a gate pull-down, and a snubber or freewheeling diode prevent most field failures.
Common mistakes that kill discretes
Under-driving the gate. Running a MOSFET in its linear region turns it into a heater.
Ignoring inrush. Capacitive loads dump a large current spike through the device at power-up.
Skipping the freewheeling diode across inductive loads such as relays, solenoids and motors.
No derating. A 1A diode in a 40C ambient enclosure is not a 1A diode.
Buying unverified stock. Counterfeit and re-marked SOT-23 parts are common in the open market - always buy from a supplier who can trace the lot.
Sourcing small-signal discretes without surprises
The few cents you save on a grey-market reel can cost you an entire production run. Before you buy, check these four things:
Traceability. Genuine parts ship with manufacturer packaging, date codes and lot information.
Consistency. A supplier who can hold the same die for repeat orders saves you re-qualification.
Mixed-BOM support. One shipment should be able to cover a full kit of MOSFETs, diodes and passives.
Fast quotes. For prototypes and urgent builds, a 24-hour quote matters more than a catalogue price.
FAQ
Can I replace a 2N7002 with a BSS138?
Usually yes - both are SOT-23 N-channel MOSFETs and BSS138 has a lower gate threshold, which makes it better for 3.3V logic. Check the higher Rds(on) if you are switching more than about 100mA.
Is AO3400 a drop-in upgrade for 2N7002?
Electrically it is much stronger (5.7A vs 115mA), but it is a 30V part - never use it where the 60V rating of a 2N7002 is required.
When should I use SS34 instead of 1N4007?
Use SS34 for low-voltage, high-frequency switching where the lower forward drop and fast recovery cut losses. Keep 1N4007 for mains-level voltages where its 1000V rating and low reverse leakage matter.
Why do my MOSFETs get hot even though the current is low?
Almost always gate under-drive or a slow gate transition. Confirm Vgs at the actual logic level, add a gate resistor, and check the switching waveform.
Related parts in stock at Homehta
Related small-signal parts we supply every week:2N7002 |BSS138 |BSS84 |AO3400 |AO3401 |SI2302 |1N4007 |SS34 |1N5819- genuine stock, wholesale pricing, RFQ online.
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