When is a MOSFET more appropriate as a switch than a BJT?
Answer: 1) a MOSFET is better than a BJT when:
- When you need really low power.
- 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.
- When your switching frequencies aren't too high.
- MOSFETs start losing their efficiency gains the faster you switch them, because:
- 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.
- 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.
- 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.
- 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.
- 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).
- When your power and current requirements ARE a dominating factor.
- 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.
- 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.
- 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.
- When you need to etch transistors into integrated circuits.
- 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.
- [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.
- If I recall correctly, BJT transistors are more resistant to having their voltage ratings momentarily exceeded than are MOSFETs.
- When you need a giant (high power) diode!
- 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.
- 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.
- 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).
- You need higher switching frequencies.
- See above.
- (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:
- 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.
- You need to make an op-amp.
- 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.
- [Results may vary] You care about cost and availability a lot.
- 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.
- 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.
- [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.
- You want simplicity in design.
- 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.
- 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.
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
- [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".