When is a MOSFET more appropriate as a switch than a BJT?

Question

In my experimentation, I've used only BJTs as switches (for turning on and off things like LEDs and such) for my MCU outputs. I've been repeatedly told, however, that N-channel enhancement-mode MOSFETs are a better choice for switches (see here and here, for examples), but I'm not sure I understand why. I do know that a MOSFET wastes no current on the gate, where a BJT's base does, but this is not an issue for me, as I'm not running on batteries. A MOSFET also requires no resistor in series with the gate, but generally DOES require a pull-down resistor so the gate doesn't float when the MCU is rebooted (right?). No reduction in parts count, then.

There doesn't seem to be a great surplus of logic-level MOSFETs that can switch the current that cheap BJTs can (~600-800mA for a 2N2222, for example), and the ones that do exist (TN0702, for example) are hard to find and significantly more expensive.

When is a MOSFET more appropriate than a BJT? Why am I continually being told that I should be using MOSFETs?

Answer 1

BJTs are much more suitable than MOSFETs for driving low-power LEDs and similar devices from MCUs. MOSFETs are better for high-power applications because they can switch faster than BJTs, enabling them to use smaller inductors in switch-mode supplies, which increases efficiency.

Answer 2

When is a MOSFET more appropriate as a switch than a BJT?

Answer: 1) a MOSFET is better than a BJT when:

1. When you need really low power.
   1. MOSFETs are voltage-controlled. So, you can just charge their Gate once and now you have no more current draw, and they stay on. BJT transistors, on the other hand, are current-controlled, so to keep them on you have to keep sourcing (for NPN) or sinking (for PNP) current through their Base to Emitter channel. This makes MOSFETs ideally-suited to low-power applications, because you can make them draw a lot less power, especially in steady-state (ex: always ON) scenarios.
2. When your switching frequencies aren't too high.
   1. MOSFETs start losing their efficiency gains the faster you switch them, because:
      1. Charging and discharging their Gate capacitances repeatedly is like charging and discharging a tiny little battery repeatedly, and that takes power and current, especially since you are likely discharging that tiny little charge to GND, which is just dumping it and converting it into heat instead of recovering it.
      2. The high gate capacitances can involve rather large (up to hundreds of mA, for example, for a TO-220-sized part) momentary input and output currents, and power losses are proportional to the square of the current (P = I^2 * R). This means each time you double the current you quadruple the power losses and heat generation in a part. High Gate capacitances on MOSFETs with high-speed switching means you must have large Gate drivers and very high drive currents to a MOSFET (ex: +/-500mA), as opposed to the low drive currents to a BJT (ex: 50mA). So, faster switching frequencies means more losses in driving the Gate of a MOSFET, as opposed to driving the Base of a BJT.
      3. Rapid switching of the Gate also significantly increases losses through the primary Drain to Source channel because the faster your switching frequency, the more time (or times per second, however you want to think about it) you spend in the Ohmic region of the transistor, which is the region between fully ON and fully OFF, where R_DS (resistance from Drain to Source) is high, and hence, so are losses and heat production.
      4. So, in summary: the faster your switching frequency, the more MOSFET transistors lose their efficiency gains they otherwise naturally have over BJT transistors, and the more BJT transistors begin to be appealing from a "low power" stand-point.
   2. Also (see the book reference, quotes, and example problem below!) BJT transistors can switch a touch faster than MOSFETs (ex: 15.3 GHz vs 9.7 GHz in "Example G.3" below).
3. When your power and current requirements ARE a dominating factor.
   1. For any given component package size, my personal experience in searching for parts indicates the best BJT transistors can only drive about 1/10 as much current as the best MOSFET transistors. So, MOSFETs excel at driving high currents and high powers.
   2. Example: a TIP120 NPN BJT Darlington transistor can only drive about 5A continuous current, whereas the IRLB8721 N-Channel Logic-Level MOSFET, in the same physical TO-220 package, can drive as much as 62A.
   3. Additionally, and this is really important!: MOSFETs can be placed in parallel to increase a circuit's current-capability. Ex: if a given MOSFET can drive 10A, then putting 10 of them in parallel can drive 10A/MOSFET x 10 MOSFETs = 100A. Putting BJT transistors in parallel, however, is NOT recommended unless you have active or passive (ex: using power resistors) load balancing for each BJT transistor in parallel, as BJT transistors are diodic in nature, and hence act more like diodes when placed in parallel: the one with the smallest diodic voltage drop, VCE, from Collector to Emitter, will end up passing the largest current, possibly destroying it. So, you'd have to add a load-balancing mechanism: Ex: a tiny-resistance, but huge power, power resistor in series with each BJT transistor/resistor pair in parallel. Again, MOSFETs do NOT have this limitation, and hence are ideal for placing in parallel to increase current limits of any given design.
4. When you need to etch transistors into integrated circuits.
   1. Apparently, based on the quote below, as well as numerous other sources, MOSFETs are easier to miniaturise and etch into ICs (chips), so most computer chips are MOSFET-based.
5. [I need to find a source for this--please post a comment if you have one] When voltage spike robustness is not your primary concern.
   1. If I recall correctly, BJT transistors are more resistant to having their voltage ratings momentarily exceeded than are MOSFETs.
6. When you need a giant (high power) diode!
   1. MOSFETs have a built-in and natural body diode, which is sometimes even specified and rated in a MOSFET's datasheet. This diode can frequently handle very large currents, and can be very useful. For an N-channel MOSFET (NMOS), for instance, which can switch current from Drain to Source, the body diode goes in the opposite direction, pointing from Source to Drain. So, feel free to take advantage of this body diode when necessary, or just use the MOSFET as a diode directly.
   2. Here's a quick Google search for "mosfet body diode" and "mosfet diode", and a brief article: DigiKey: The Significance of the Intrinsic Body Diodes Inside MOSFETs.
   3. Beware, however, due to this body diode, MOSFETs can NOT naturally block, switch, or control currents in the opposite direction (from Source to Drain for an N-Channel, or from Drain to Source for a P-Channel), so to switch AC current with a MOSFET you'd need to place two MOSFETs back-to-back so their diodes work together to block or allow the current, as appropriately, in conjunction with any active switching you might do to control the MOSFET.

2) So, here's a few cases you might still choose a BJT over a MOSFET:

(More pertinent reasons in bold--this is somewhat subjective).

1. You need higher switching frequencies.
   1. See above.
   2. (Although this is rarely ever an issue I think since MOSFETs can be switched so fast these days anyway). Someone with a lot of real-world, high-frequency design experience feel free to chime in, but based on the textbook below, BJTs are faster:
      1. Example: a certain NPN BJT transistor reached 15.3 GHz with a Collector current, I_C, of 1 mA, as opposed to a comparable NMOS transistor (N-channel MOSFET) which only reached a transition frequency of 9.7 GHz at a Drain current, I_D, of 1 mA.
2. You need to make an op-amp.
   1. The textbook I cite farther below says BJTs are good for this (being used to make op-amps) here (emphasis added):

      It can thus be seen that each of the two transistor types has its own distinct and unique advantages: Bipolar technology has been extremely useful in the design of very-high-quality general-purpose circuit building blocks, such as op amps.

3. [Results may vary] You care about cost and availability a lot.
   1. When choosing parts, sometimes many parts work for a given design objective, and BJTs may be cheaper at times. If they are, use them. With BJTs having been around much longer than MOSFETs, my somewhat-limited, subjective experience buying parts shows BJTs are really cheap and have more surplus and inexpensive options to choose from, especially when searching for through-hole (THT) parts for easy hand-soldering.
   2. However, your experience may vary, perhaps even based on where in the world you are located (I don't know for sure). Modern-day searches from modern-day reputable suppliers, such as DigiKey, show the opposite to be true, and MOSFETs win again. A search on DigiKey in Oct. 2020 shows 37808 results for MOSFETs, with 11537 of them being THT, and only 18974 results for BJTs, with 8849 of them being THT.
   3. [Much more-relevant] the Gate driver ICs and circuits frequently required to drive MOSFETs (see just below) can add cost to your MOSFET-based design.
4. You want simplicity in design.
   1. All BJTs are effectively "logic level" (this isn't really a concept for BJTs, but bear with me), because they are current-driven, NOT voltage driven. Contrast this to MOSFETs, where most require a V_GS, or Gate to Source Voltage, of 10V~12V to fully turn ON. Creating the circuitry to drive a MOSFET Gate with these high voltages when using a 3.3V or 5V microcontroller is a pain in the butt, especially for newcomers. You may need more transistors, push-pull circuits/half-H-bridges, charge pumps, expensive Gate driver ICs, etc., just to turn on the stinking thing. Contrast this to a BJT where all you need is one resistor and your 3.3V microcontroller can turn it on just fine, especially if it's a Darlington BJT transistor so it has a huge Hfe gain (of around 500~1000 or more) and can be turned on with super low (<1~10 mA) currents.
   2. So, designs can get much more complicated to properly drive a MOSFET transistor as a switch instead of a simple BJT transistor as a switch. The solution then is to use "logic-level" MOSFETs, which means they are designed to have their Gates controlled with microcontroller "logic levels", such as 3.3V or 5V. The problem, however, is: logic-level MOSFETs are more rare still, and have fewer options to choose from, they are much more expensive, relatively speaking, and they still may have high Gate capacitances to overcome when trying to do high-speed switching. This means even with logic-level MOSFETs you still may need to go right back to a more-complicated design to get a push-pull Gate driver circuit/half-H-bridge, or a high-current, expensive, Gate driver IC in order to enable high-speed switching of the logic-level MOSFET.

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This book (ISBN-13: 978-0199339136) Microelectronic Circuits (The Oxford Series in Electrical and Computer Engineering), 7th Edition, by Adel S. Sedra and Kenneth C. Smith, in "Appendix G: COMPARISON OF THE MOSFET AND THE BJT" (view online here), provides some additional insight (emphasis added):

G.4 Combining MOS and Bipolar Transistors—BiCMOS Circuits

From the discussion above it should be evident that the BJT has the advantage over the MOSFET of a much higher transconductance (gm) at the same value of dc bias current. Thus, in addition to realizing higher voltage gains per amplifier stage, bipolar transistor amplifiers have superior high-frequency performance compared to their MOS counterparts.

On the other hand, the practically infinite input resistance at the gate of a MOSFET makes it possible to design amplifiers with extremely high input resistances and an almost zero input bias current. Also, as mentioned earlier, the MOSFET provides an excellent implementation of a switch, a fact that has made CMOS technology capable of realizing a host of analog circuit functions that are not possible with bipolar transistors.

It can thus be seen that each of the two transistor types has its own distinct and unique advantages: Bipolar technology has been extremely useful in the design of very-high-quality general-purpose circuit building blocks, such as op amps. On the other hand, CMOS, with its very high packing density and its suitability for both digital and analog circuits, has become the technology of choice for the implementation of very-large-scale integrated circuits. Nevertheless, the performance of CMOS circuits can be improved if the designer has available (on the same chip) bipolar transistors that can be employed in functions that require their high gm and excellent current-driving capability. A technology that allows the fabrication of high-quality bipolar transistors on the same chip as CMOS circuits is aptly called BiCMOS. At appropriate locations throughout this book we present interesting and useful BiCMOS circuit blocks.

This answer repeats this: Are BJTs used in modern integrated circuits to the same extent as MOSFETs?.

In the "Appendix G" of the textbook quoted above, you can also refer to "Example G.3". In this example, they show an NPN BJT transistor reaching a transition frequency, f_T as high as 15.3 GHz with a Collector current, I_C, of 1 mA. This is contrasted to the NMOS transistor (N-channel MOSFET) reaching a transition frequency of only 9.7 GHz at a Drain current, I_D, of 1 mA.

Additional study and help for using transistors, whether BJTs or MOSFETs

1. [my answer] Switching a Solenoid Using Arduino's 5V Output? - here I present a full, detailed tutorial on how to read an NPN BJT transistor datasheet, pull out the necessary values, and calculate gains, currents, and required resistors and other components to drive a solenoid or relay or other inductive load, including with necessary snubber diode to eliminate harmful back-EMF voltages and currents and "ringing".

Answer 3

BJT's waste some current whenever they're switched on, regardless of whether the load is drawing anything. In a battery-powered device, using a BJT to power something whose load is highly variable but is often low will end up wasting a lot of energy. If a BJT is used to power something with a predictable current draw, though (like a LED), this problem isn't as bad; one can simply set the base-emitter current to be a small fraction of the LED current.

Answer 4

A good N-channel MOSFET will have a very low \$R_{ds(on)}\$ (drain-source equivalent resistance) when properly biased, which means that it behaves very much like an actual switch when turned on. You will find that the voltage across the MOSFET when on will be lower than the \$V_{ce(sat)}\$(collector-emitter saturation voltage) of a BJT.

A 2N2222 has \$V_{ce(sat)}\$ from \$ 0.4V - 1V \$ depending on biasing current.

A VN2222 MOSFET has a maximum \$R_{ds(on)}\$ of \$ 1.25 \Omega\$.

You can see that the VN2222 will dissipate much less across the drain-source.

Also, as previously explained, the MOSFET is a transconductance device - voltage on the gate allows current through the device. Since the gate is high-impedance to the source, you do not require constant gate current to bias the device on - you need only overcome the inherent capacitance to get the gate charged up then the gate consumption becomes miniscule.

Answer 5

BJT's are more suitable in some situations because they are often cheaper. I can buy TO92 BJT's for 0.8p each but MOSFET's don't start until 2p each - it might not sound like much but it can make a big difference if you're dealing with a cost sensitive product with many of these.

Answer 6

FET devices having almost no input current (gate current) are the best choice for the LEDs driven by the micro-controller as micro-controller doesn't need to provide much current through its die, keeping itself cool (less heat dissipation on chip) while LED current is almost all driven through the external FET channel. Yes, it is also true that the Ron of the typical FET devices are very low keeping low voltage drop across FET which is advantageous for low power application.

However, there is some disadvantage when it comes to noise immunity at the gate of the MOSFET, which may not be the case for the BJTs. Any potential (noise) applied at gate of the MOSFET will make the channel conduct upto some extent. It is not highly (but still adequate) to use the Mosfet to drive the relay coils with low Vt (threshold). In that case, if your Microcontroller is driving the FET, you might want to get a FET with higher Vt (threshold).

Answer 7

MOSFETs are more robust for high current requirements. For example 15A rated Mosfet can pass 60A (f.e. IRL530) of current for a short period. 15A rated BJT can pass 20A pulses only. Also Mosfets have better thermal junction to case resistance even if it has smaller die.

Answer 8

Which one is better depends on the application. Here are some considerations, but its by no means exhaustive.

A MOSFET also requires no resistor in series with the gate, but generally DOES require a pull-down resistor so the gate doesn't float when the MCU is rebooted (right?). No reduction in parts count, then.

Well that depends. If you are OK with the LED turning on for a few ms when the micro starts up, then you can omit the resistor sometimes and save the space/cost.

Other factors...

• For driving inductive loads the built in body diode of a MOSFET allows you to save on parts count. Whereas a BJT would probably require a diode in parallel.

• Many power BJTs that have very long turn off times (like several us), so you couldn't even switch them fast if you tried. For high speed switching MOSFET's can be better in that case.
  
  If you were to go to one of the major manufacturer websites (like Analog Devices) and look at their DC-DC converter controller chips, nearly all of them use MOSFETs rather than BJTs. Making a converter that runs BJTs at several hundred kHz usually isn't practical.

• BJTs can have less power dissipation in high voltage applications.
  
  The drain-source-on-resistance for a particular MOSFET size and technology is proportional to the blocking voltage. At low voltages MOSFET's can have drain-source resistances even below 1mOhm. But at higher voltages (like > 1000V) most MOSFETs have drain-source resistances of several hundred mOhms, if not several ohms.
  
  For bipolar transistors (and IGBTs), VCE-sat typically starts out as a good fraction of a volt but increases slower than linearly with voltage rating (up to a point anyways).
  
  If you consider a constant current, and you look at similar parts with higher and higher voltage ratings there comes a point where the voltage drop across a MOSFET becomes larger than the drop across a bipolar transistor. In those higher voltage applications a BJT or IGBT can be more efficient.

• BJTs can have less power dissipation for high current applications.
  
  The drain-source channel in a MOSFET behaves like a resistor, and the power dissipation increases like I^2 * R. Additionally R increases with temperature, making power dissipation get even worse as the part gets hot.
  
  For a BJT, VCE-sat does increase with increasing collector current, but can often have a pretty flat slope. Additionally VCE-sat typically decreases with temperature, so as the part gets hot it actually starts dissipating less power.