Two of the most common three terminal semiconductor nowadays are MOSFET and BJT. MOSFET is metal oxide semiconductor field effect transistor. It is a variant of FET. BJT on the other hand is bipolar junction transistor. It is worth to discuss MOSFET and BJT comparison to understand which part to use. Read below comparisons.
1. Application of MOSFET and BJT
Both MOSFET and BJT are good to use as electronic switches. They are both useful as relay drivers, signal inverters, power switch to name few.
Both are also famous in signal amplifiers. However, BJT is very much famous compared to MOSFETs.
On the other hand, MOSFET is widely used as a switch because of easier circuit requirement and analysis required.
Some MOSFET applications:
2. Comparing MOSFET and BJT in terms of Construction
In terms of construction, MOSFET and BJT comparison is very different.
MOSFET is either depletion or enhancement per construction. Enhancement type is the most dominant.
MOSFET is also either N channel (NMOS) or P channel (PMOS). This is something related to the dominant charge carriers. NMOS is commonly used as low side driver while PMOS is common in high side driving.
MOSFET is a three terminal semiconductor device. It has gate (G), drain (D), and source (S). Below are circuit symbols for the MOSFET.
BJT is either a NPN or PNP in terms of dominant charge carriers. NPN is common in low side driving while PNP is famous for high side driving. BJT has three terminals as with the MOSFET. It has base (B), collector (C) and emitter (E). Below are the electronic symbols for NPN (left) and PNP (right) versions.
3. MOSFET and BJT Comparison in Terms of Operation
MOSFET is popular as a switch in which it will operate in cut-off and saturation. In the same manner, BJT as well. At saturation, MOSFET drain to source voltage drop is ideally zero. Also, the same manner to BJT; the voltage across collector-emitter is ideally zero.
Both devices are useful as signal amplifier. In this operation mode, both devices will operate in the linear region. At this operation, MOSFET drain-source voltage is not zero but there is a value depending to the circuit biasing. This is also the case for a BJT, there is a voltage drop across collector-emitter.
MOSFET is a voltage-controlled device while BJT is a current controlled device. Voltage controlled means that to set the MOSFET operation, it needs to have adequate voltage across gate to source. On the other hand, current controlled means that to set the BJT operation, it needed an amount of current on the base.
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4. How to Turn ON MOSFET and BJT
To turn on a MOSFET, it needs to apply voltage on the gate to source. To drive it to saturation, the applied voltage level must be far above the gate to source threshold voltage (VGSth).
To turn on a BJT, first is to ensure that the VBE requirement is met. BJT VBE is around 0.7V typical. Actually, some parts have higher VBE requirements like below table.
Once the VBE requirement is satisfied, current will then flow to the base. To make the circuit to work as a switch, the base current must be big enough to drive the BJT into saturation. If the purpose is to make the circuit as amplifier, then a small amount of current will do (not letting the device to saturate).
5. Current Rating
MOSFET is rated via drain current while BJT is rated via collector current.
With the same package, in most cases, MOSFETs can handle big currents compared to BJT.
6. Voltage Rating
MOSFET is rated by drain to source voltage or VDSS. It is also rated by gate to source voltage (VGSS).
BJT rating is in terms of VCEO or collector-emitter voltage (with open base). There is also a rating in terms of VEBO or emitter-base voltage (with open collector). Some devices (per datasheet) also providing rating for VCBO or collector-base voltage (with open emitter).
Both devices are comparable in terms of voltage rating.
7. MOSFET and BJT Static Power Dissipation/Power Loss
MOSFET power dissipation or power loss is due to the drain to source on state resistance. It has no power loss on the gate ideally when the circuit is design only for on/off switch.
Pdiss_MOSFET = Idrain X Idrain X RDSon
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BJT on the other hand has losses on both the base and the collector-emitter. The power loss on the base is due to the VBE and the base current. The power loss in the collector-emitter is due to the voltage drop and the collector current.
Pdiss_BJT = Ibase X VBE + Icollector X VCE
Comparing the loss of MOSFET due to the RDSon and the loss of BJT due to VCE saturation voltage, MOSFET is very small. This is because RDSon value nowadays is reaching very small value. On the other hand, VCEsat value of BJT is high.
8. Comparing Switching Losses of MOSFET and BJT
In applications like switching converters and power supplies, the MOSFET will have power loss on the gate that is caused by the gate charge or input capacitance. It has also losses due to the output capacitance (COSS) and due to rise and fall times. These losses are directly proportional to the switching frequency.
Ploss_MOSFET_switching = Ploss_gatecharge + Ploss_COSS + Ploss_rise_fall
By looking at the BJT datasheet, there is no information about input capacitance, output capacitance or the rise time and fall time. In other words, the switching losses of a BJT is could be negligible. Despite of this, MOSFET is still overall has the advantage. The added losses due to switching is not enough to offset the loss of the BJT due to higher VCEsat.
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9. Which is Better in Switching Speed?
MOSFET has parasitic elements defined in the datasheet. These are input capacitance, gate charge, output capacitance, and rise and fall times to name some. These parasitic elements introduce a delay on the device switching speed somehow.
On the other hand, BJT do not have these parameters defined in the datasheet. So, it could be assumed that these are negligible. Thus, BJT will be faster than MOSFET.
The switching delays involve are in nanoseconds or microsecond range. So, even MOSFET is slower, this switching delay is still fast enough for most applications where MOSFETs are used.
10. Heat Stability
MOSFET is more stable with respect to heat. It is only the RDSon that is dependent to heat. However, for BJT, the VBE and VCE are both affected by heat.
11. Circuit Complexity
Both are easy to design and use in a circuit. However, for beginners, I think MOSFET is simple to understand. You only need to satisfy the gate threshold voltage with margin and the MOSFET will work as a switch. For BJT on the other hand, there are several configurations like self-bias, emitter bias and voltage divider bias. These requires understanding to properly bias the circuit.
All of the above details are may not covering everything in MOSFET and BJT comparison. However, the basic ideas that can help someone to select which part to use were being covered. Feel free to put your comments and inputs below.
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