# MOSFET vs BJT: Choosing Between Voltage and Current Control

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Technical comparison between **MOSFET** and **BJT (Bipolar)** by [Novapart](https://novapart.co).

## Overview

The honest answer to "MOSFET or BJT" is that the question is usually settled by the current you are switching and by what is driving the gate, not by any general superiority of one technology.

A MOSFET is controlled by voltage. Put enough volts between gate and source and the channel conducts; the gate itself draws no steady current because it is a capacitor separated from the channel by an oxide layer. A BJT is controlled by current. To hold a bipolar in saturation you must keep pushing base current, roughly the collector current divided by the transistor's gain. That single distinction propagates into almost every other difference between the two.

Start with conduction loss, because it is what usually decides power designs. A saturated bipolar drops a roughly fixed voltage, its V_CE(sat), typically somewhere between 0.2 V and 1 V depending on the device and how hard you drive the base. That drop does not care how large the transistor is. Switch 10 A through a BJT at 0.5 V and you are dissipating 5 W, and the only way to reduce it is a better device or more base drive. A MOSFET instead behaves as a resistor, its R_DS(on), so the loss is I²·R. That is worse at very high currents in a small device, but R_DS(on) is something you can buy your way out of: modern parts reach single-digit milliohms, and at 10 A through 5 mΩ the loss is half a watt. This is why power switching moved to MOSFETs and never moved back.

Drive is where the bipolar answers. To turn on a logic-level MOSFET you need V_GS above threshold with margin, and if the load sits between the transistor and ground with the switch above it, you need a gate voltage above the supply rail, which means a bootstrap capacitor or an isolated supply. A BJT switching the same load low-side needs a resistor from a microcontroller pin. For a relay coil, an LED string or a small fan, the bipolar solution is smaller, cheaper and has fewer ways to go wrong.

Thermal behaviour is the difference designers discover late. R_DS(on) rises with temperature, so a MOSFET that gets hotter than its neighbour conducts less and the current redistributes. Paralleling MOSFETs is therefore mostly safe. V_BE falls with temperature, so a hot bipolar takes more of the current, gets hotter still, and takes even more. Paralleled bipolars need emitter ballast resistors, and bipolars in linear operation need genuine thermal design rather than a rule of thumb.

Two places where the bipolar is not merely adequate but better. First, deep saturation at low current: a small-signal NPN driven hard can sit at a few tens of millivolts, below what a comparable MOSFET's R_DS(on) would give you at the same current. Second, anything analogue. The exponential relationship between base voltage and collector current is predictable across decades, and two bipolars on the same die match to within a millivolt. Current mirrors, bandgap references, differential pairs and log amplifiers are all built on that property, and MOSFETs do not match it.

Cost matters at the signal level and stops mattering at the power level. A general-purpose NPN in SOT-23 is the cheapest active component in the catalogue, and for a job that involves driving one LED there is no argument for anything else. Above a few amps the comparison inverts, because the heatsink and the efficiency loss you avoid with a MOSFET cost more than the part.

## Comparison Parameters

| Parameter | MOSFET | BJT (Bipolar) | Recommendation |
|---|---|---|---|
| Control variable | Voltage (V_GS). Gate draws no steady current. | Current (I_B). Base current flows continuously while on. | left |
| Steady-state drive power | Essentially zero once the gate is charged. | I_C / hFE, continuous. At 5 A and hFE 50 that is 100 mA. | left |
| Drive complexity | Needs V_GS above threshold. High-side switching needs a bootstrap or isolated supply. | A resistor from a logic pin often suffices. | right |
| On-state loss at high current | I²·R_DS(on). Falls as you pay for a lower R_DS(on). | I·V_CE(sat), typically 0.2–1 V. Does not scale down with size. | left |
| On-state loss at low current | Negligible, but the part costs more than it needs to. | V_CE(sat) dominates, yet at tens of mA the absolute loss is tiny. | None |
| Switching speed | Limited by gate charge and driver strength. Fast with a real driver. | Limited by stored base charge; slow to come out of saturation. | left |
| Thermal behaviour | Positive R_DS(on) tempco, so paralleled devices share current. | Negative V_BE tempco invites thermal runaway; paralleling needs emitter resistors. | left |
| Saturation voltage at very low current | R_DS(on) applies at any current, so V_DS is proportional. | V_CE(sat) can reach a few tens of mV in deep saturation. | right |
| Analogue / linear use | Usable, but transconductance varies and matching is poor. | Predictable exponential transfer, excellent matching on one die. | right |
| ESD sensitivity | Thin gate oxide; needs handling care and often a gate clamp. | Robust. A junction is far harder to damage than an oxide. | right |
| Unit cost at signal level | Small-signal MOSFETs cost more than their bipolar equivalents. | A general-purpose NPN is the cheapest active device there is. | right |
| Catalogue depth at Novapart | 24,938 active single MOSFETs, around half in stock. | 6,022 active bipolar transistors. | left |

## Verdict & Recommendation

Switching more than about an amp, or switching it fast, or switching it often: use a MOSFET. The I²R loss falls as you pay for a lower R_DS(on), the gate takes no steady power, and paralleling is straightforward. Budget for a proper gate driver and for a bootstrap arrangement if the switch is high-side.

Switching a relay, an LED, a small solenoid or a buzzer from a microcontroller pin: use a BJT. One resistor and a cheap NPN do the job, and the base current you are spending is irrelevant at that scale.

Building anything analogue that depends on matching or on predictable transconductance, such as a current mirror or a differential pair: use bipolars, and put them on the same die if you can.

Where neither is obviously right, at a few hundred milliamps of DC load, the deciding factor is usually what else the board needs. If a gate driver is already present for another reason, extend it. If the only control available is a 3.3 V logic pin and the load is modest, the bipolar wins on total cost and part count.

## Relevant In-Stock Components

| SKU | Name | Manufacturer | Price | Stock |
|---|---|---|---|---|
| [BCV47,215](https://novapart.co/products/BCV47,215/bipolar-bjt-single-transistor-npn-60-v-500-ma-250) | Bipolar (BJT) Single Transistor, NPN, 60 V, 500 mA, 250 mW, SOT-23, Surface Mount | NEXPERIA | €0.0840 | 1000+ |
| [2N7002W-7-F](https://novapart.co/products/2N7002W-7-F/power-mosfet-n-channel-60-v-115-ma-75-ohm-sot-323) | Power MOSFET, N Channel, 60 V, 115 mA, 7.5 ohm, SOT-323, Surface Mount | DIODES INC. | €0.0580 | 1000+ |
| [BSS138](https://novapart.co/products/BSS138/power-mosfet-n-channel-50-v-220-ma-35-ohm-sot-23) | Power MOSFET, N Channel, 50 V, 220 mA, 3.5 ohm, SOT-23, Surface Mount | ONSEMI | €0.0440 | 1000+ |
| [KSC1845FTA](https://novapart.co/products/KSC1845FTA/bipolar-bjt-single-transistor-npn-120-v-50-ma-500) | Bipolar (BJT) Single Transistor, NPN, 120 V, 50 mA, 500 mW, TO-226AA, Through Hole | ONSEMI | €0.0850 | 1000+ |
| [DMG1012T-7](https://novapart.co/products/DMG1012T-7/power-mosfet-n-channel-20-v-630-ma-04-ohm-sot-523) | Power MOSFET, N Channel, 20 V, 630 mA, 0.4 ohm, SOT-523, Surface Mount | DIODES INC. | €0.1080 | 1000+ |
| [RN1104MFV,L3F(CT](https://novapart.co/products/RN1104MFV,L3F(CT/bipolar-pre-biased-digital-transistor-npn-50-v-100) | Bipolar Pre-Biased / Digital Transistor, NPN, 50 V, 100 mA, 47 kohm, 47 kohm | TOSHIBA | €0.0210 | 1000+ |
| [RN2402,LXHF(T](https://novapart.co/products/RN2402,LXHF(T/bipolar-pre-biased-digital-transistor-pnp-50-v-100) | Bipolar Pre-Biased / Digital Transistor, PNP, 50 V, 100 mA, 10 kohm, 10 kohm | TOSHIBA | €0.0470 | 1000+ |
| [2N7002LT1G](https://novapart.co/products/2N7002LT1G/power-mosfet-n-channel-60-v-115-ma-75-ohm-sot-23) | Power MOSFET, N Channel, 60 V, 115 mA, 7.5 ohm, SOT-23, Surface Mount | ONSEMI | €0.0600 | 1000+ |
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