2N7002 vs BSS138: MOSFET Selection Guide

Open almost any consumer electronic product built in the last 25 years — a phone charger, a Wi-Fi router, an LED strip controller, a fitness tracker, a drone flight controller, a smart-plug — and you will find at least one, usually several, SOT-23 N-channel MOSFETs doing low-power switching or level-shifting duties. Nine times out of ten, those MOSFETs are either a 2N7002 or a BSS138.

The two parts occupy the same ecological niche: small-signal, logic-level, N-channel enhancement-mode MOSFETs in the industry-standard SOT-23 package, priced under $0.05 in volume, and fully second-sourced across every major discrete semiconductor vendor. They are the "555 timer" of the MOSFET world — not the best at anything specific, but good enough at everything general that designers default to them without a second thought.

That default is usually correct, but not always. The two parts differ in voltage rating, current capacity, on-resistance and gate threshold in ways that matter for specific circuits. Picking the wrong one produces designs that work on the bench but fail in the field — overheating under load, failing to turn on reliably at 3.3 V gate drive, or breaking down under inductive kickback.

This guide lays out the real differences, the application boundaries, and the decision rules so you can spec the right part on the first pass.

The numbers, taken from Nexperia (2N7002) and onsemi (BSS138) datasheets at 25 °C unless noted:

The headline takeaways:

2N7002 wins on voltage and current. 60 V vs 50 V gives you 20% more margin on 24 V and 48 V rails where inductive kickback can push transient voltages well above nominal. 300 mA vs 200 mA continuous gives you 50% more headroom for load switching.

BSS138 wins on gate threshold and low-voltage drive. Vgs(th) of 0.8-1.5 V vs 1.0-2.5 V means BSS138 turns on more reliably at 1.8 V and 2.5 V gate drive. At Vgs=2.5 V, BSS138 shows slightly lower Rds(on) than 2N7002 — counterintuitive given the 10 V spec, but real.

BSS138 switches faster. Lower gate charge (1.0 nC vs 1.5 nC) and lower input capacitance (30 pF vs 50 pF) mean faster turn-on/turn-off with the same gate driver. For PWM above 100 kHz, this matters.

Neither is a power MOSFET. Both are limited to a few hundred milliamps continuous. For anything above 500 mA, you need to step up to SI2302 (2.8 A), AO3400 (5.7 A), or a larger package entirely.

The single most common use of 2N7002 and BSS138 is bidirectional level shifting on open-drain buses — primarily I²C, but also 1-Wire, SMBus, and some interrupt lines. The classic circuit is two MOSFETs per signal line, with source connected to the low-voltage side, drain to the high-voltage side, gate to the low-voltage rail, and pull-up resistors on both sides.

How it works: When the low-voltage side pulls the line low, the MOSFET's body diode is forward-biased from the high-voltage side's pull-up, so the high-voltage side also goes low. When the low-voltage side releases, the MOSFET turns on (Vgs = Vlow > Vth) and actively pulls the high-voltage side low. When the high-voltage side pulls low, the MOSFET's gate-source voltage goes negative relative to the high-side rail, turning it off; the low-voltage side's pull-up brings it low through the body diode. It is asymmetric but works for open-drain buses.

Which MOSFET to choose for level shifting:

For 3.3 V ↔ 5 V I²C at 100-400 kHz: either works. 2N7002 is the traditional choice because its 60 V rating gives margin if the 5 V rail spikes. BSS138 works equally well and switches slightly faster. Pick whichever your CM has in stock.

For 1.8 V ↔ 3.3 V I²C: BSS138 is strongly preferred. Its Vgs(th) max of 1.5 V guarantees turn-on at Vgs=1.8 V with 0.3 V of overdrive. 2N7002's Vgs(th) max of 2.5 V means at Vgs=1.8 V it may not turn on at all — you are below the guaranteed threshold. This is a real failure mode: boards that work on the bench with one batch of 2N7002 fail with the next batch because the threshold distribution shifted.

For 1.2 V ↔ 1.8 V or lower: neither part is suitable. Vgs(th) is too high. Use a dedicated level translator like TXS0102, TXB0104, or NVT4857 (NXP) which are designed for sub-1.8 V operation.

For SPI level shifting (push-pull, not open-drain): the MOSFET trick does not work correctly. Use TXS0108E, TXB0108, or 74LVC2T45 instead. SPI is push-pull and the asymmetric MOSFET circuit causes contention and slow rise times.

For UART level shifting (push-pull TX, open-drain-ish RX): the MOSFET circuit works for the RX line but not reliably for TX. Use a dedicated translator or a resistor divider for TX, MOSFET for RX.

Pull-up resistor selection: For I²C at 400 kHz, use 2.2 kΩ on the low-voltage side and 4.7 kΩ on the high-voltage side. For 100 kHz, 4.7 kΩ and 10 kΩ. Lower values give faster rise times but waste current; higher values save power but slow the bus. Total bus capacitance (including the MOSFET's ~50 pF) must stay under 400 pF per I²C spec.

The second most common use is low-side load switching: MCU GPIO drives the gate, MOSFET switches a load (LED, small relay, sensor power, motor) between ground and the load's negative terminal.

Thermal design rule: Power dissipation in the MOSFET is Id² × Rds(on). For SOT-23, junction-to-ambient thermal resistance is about 250 °C/W (no copper pour) or 150 °C/W (with 1 in² copper pour). Maximum junction temperature is 150 °C. At 25 °C ambient with no copper, you can dissipate (150-25)/250 = 0.5 W. With copper pour, 0.83 W.

Practical current limits at Vgs=3.3 V:

For 2N7002 (Rds(on) ≈ 4 Ω at Vgs=3.3 V): - 100 mA → 40 mW → fine, no heatsink - 200 mA → 160 mW → fine with small copper pour - 300 mA → 360 mW → needs 1 in² copper pour, junction rises 54-90 °C - 400 mA → 640 mW → marginal even with copper, junction near 100 °C - 500 mA → 1 W → thermal failure without heatsink

For BSS138 (Rds(on) ≈ 5 Ω at Vgs=3.3 V): - 100 mA → 50 mW → fine - 150 mA → 112 mW → fine with copper - 200 mA → 200 mW → needs copper pour - 250 mA → 312 mW → marginal - 300 mA → 450 mW → thermal failure risk

The takeaway: at 3.3 V gate drive, neither part should be pushed above 200 mA continuous without careful thermal design. If your load draws more than 200 mA, step up to SI2302 (Rds(on) 65 mΩ at Vgs=4.5 V, ~80 mΩ at Vgs=3.3 V — 50× better than 2N7002) or AO3400 (Rds(on) 40 mΩ at Vgs=4.5 V).

Inductive loads (relays, solenoids, motors): Always add a flyback diode (1N4148 for small loads, SS34 for larger) across the inductor. Without it, the collapsing magnetic field generates a voltage spike that exceeds the MOSFET's Vds rating and destroys it. The 2N7002's 60 V rating gives more margin here than BSS138's 50 V, which matters on 24 V relay coils where kickback can reach 40-50 V.

Gate drive circuit: Always include a 100 Ω series gate resistor (damps ringing from PCB trace inductance) and a 10-100 kΩ gate-to-source pull-down (keeps MOSFET off during MCU reset or deep sleep when the GPIO is high-Z). These two components cost $0.002 and prevent 90% of "why does my load turn on randomly at boot" bugs.

Both 2N7002 and BSS138 are fully second-sourced across every major discrete semiconductor vendor. The cross-reference table:

2N7002 equivalents: - Nexperia 2N7002, 2N7002AK (AEC-Q101 qualified) - onsemi 2N7002LT1G, NTR2101P (P-channel variant) - Diodes Inc 2N7002-7-F, 2N7002Q (automotive) - Vishay SI2302BDS (enhanced variant, lower Rds(on)) - STMicroelectronics STN7002 - Infineon 2N7002 - Toshiba 2SK1117 (Japanese equivalent) - ROHM RQ1L020GNTC1 - Asian clones: CJ2302 (Jiangsu Changjiang), AO2302 (Alpha & Omega), IRLML2502 (Infineon logic-level)

BSS138 equivalents: - onsemi BSS138LT1G, BSS138HT1G (automotive) - Nexperia BSS138, BSS138AK - Diodes Inc BSS138-7-F, BSS138Q - Vishay SI2302BDS-T1-GE3 (functionally equivalent, better specs) - STMicroelectronics STBSS138 - Asian clones: CJ2302, AO3400 (higher current variant)

Pricing reality. In 10k reel quantities, 2N7002 and BSS138 from major vendors run $0.015-0.040 per unit. Asian clones run $0.005-0.015. The $0.02-0.03 savings per unit is real money at 1M+ volumes, but the clones often have wider parameter spreads (Vgs(th) distribution can be 2-3× wider), lower reliability under thermal cycling, and inconsistent die-to-die performance. For consumer products with short life expectations, clones are acceptable. For industrial, automotive or medical, stick with major vendors.

Counterfeit risk. Low-to-moderate for both. Because the parts are so cheap, counterfeiters usually don't bother remarking genuine 2N7002 as BSS138 or vice versa — the margin is too thin. The common fraud is selling Asian clones as major-vendor parts at major-vendor prices. Detection: measure Vgs(th) distribution on a 30-pc sample. Genuine Nexperia 2N7002 shows a tight distribution (1.0-2.0 V); clones show a wide spread (0.5-3.0 V) because the die process is not controlled as tightly.

Automotive qualification. If your design targets automotive, spec the AEC-Q101 qualified variants: Nexperia 2N7002AK, onsemi BSS138HT1G, Diodes 2N7002Q. These undergo additional qualification testing (thermal cycling, HAST, ESD-HBM/CDM) and come with PPAP documentation.

2N7002 and BSS138 cover a wide middle ground, but they are not universal. Know when to step up or down:

Step up to SI2302 / AO3400 when: - Load current exceeds 200 mA continuous - You are switching a 12 V rail with meaningful current - PWM frequency exceeds 200 kHz (lower gate charge matters) - You need automotive AEC-Q101 qualification with better thermal performance

SI2302 (Vishay): 20 V, 2.8 A, 65 mΩ at Vgs=4.5 V, SOT-23. AO3400 (Alpha & Omega): 30 V, 5.7 A, 40 mΩ at Vgs=4.5 V, SOT-23. Both are logic-level and cost only slightly more than 2N7002 in volume.

Step up to IRLML2502 / Si2304 when: - Load current exceeds 3 A - You need a P-channel device for high-side switching

IRLML2502 (Infineon): 20 V, 4 A, 45 mΩ, SOT-23. Si2304 (Vishay): 20 V, 4 A N-channel, SOT-23.

Step down to dedicated level translators when: - You are level-shifting below 1.8 V - You are shifting push-pull buses (SPI, UART TX) - You need guaranteed timing and rise/fall characteristics

TXS0108E (TI): 8-bit bidirectional, 1.2-5.5 V, auto-direction sensing. TXB0104 (TI): 4-bit, similar. NVT4857 (NXP): 2-bit, designed specifically for I²C. 74LVC2T45 (Nexperia): 2-bit, fixed direction, very fast.

Step down to load switches when: - You need controlled inrush current (hot-swap, USB power gating) - You need over-current or over-temperature protection - You want to eliminate the gate-drive resistor network entirely

TPS22918 (TI): 5.5 V, 2 A, integrated load switch with controlled rise time. MIC94040 (Microchip): 1.7-5.5 V, 2 A, tiny SOT-23-5. SY6280 (Silergy): 2.5-5.5 V, 3 A, with current limit.

Step up to power MOSFETs when: - You are switching above 60 V - You are switching above 3 A continuous - You need synchronous rectification in a DC-DC converter

For 60-100 V and 10-30 A: IRF540N (TO-220), IRLZ44N (logic-level TO-220), SiR422DP (PowerPAK SO-8). For above 100 V: Infineon CoolMOS, ST STW series.

The SOT-23 MOSFET family is deep and well-structured. Once you know where 2N7002 and BSS138 sit in it, picking the right part for any given application becomes a mechanical exercise in matching voltage, current, gate drive and package constraints.