MOSFETs are ideal for high-power applications whereas BJTs are more commonly used in low-current applications.The MOSFET has a source, drain, and gate whereas the BJT has a base, emitter, and collector. While both have three terminals, these differ.The MOSFET (voltage controlled) is a metal-oxide semiconductor whereas the BJT (current controlled) is a bipolar junction transistor.There are many differences between the MOSFET and BJT. In either case, the current’s direction in the base is the same as the collector. In the NPN, the polarity is the opposite and current flows in the emitter and out the collector. Current flows in the collector of a PNP and out of the emitter. In between these structures is a small layer of the other doping agent called the “base”. Each type has a large collector element and a large emitter element which are doped in the same way. There are two types of BJT – PNP and NPN. A BJT has three pins – the base, collector, and emitter – and two junctions: a p-junction and n-junction. The Bipolar Junction Transistor (BJT) is a current-driven device (in contrast, MOSFET is voltage-driven) that is widely used as an amplifier, oscillator, or switch, amongst other things. Understanding the difference between n- and p-type semiconductors What is BJT? Because gate terminal voltage is positive or negative, channel conductivity is decreased.Įnhancement mode: when the gate terminal’s voltage is low, the device does not conduct unless more voltage is applied to the gate terminal. This often sees MOSFETs being used in low-power devices or as building blocks to reduce power consumption.ĭepletion mode: When the gate terminal’s voltage is low, the channel exhibits maximum conductance. This level of insulation roots low power consumption and is the primary benefit of this type of transistor. The gate terminal itself is made from metal and is detached from the source and drain terminals using a metal oxide. Likewise, in n-channel MOSFETs, the source and drain terminals are made of n-type semiconductor. In p-channel MOSFETs, the source and drain terminals are made of p-type semiconductor. This means that in total, there are four different types of MOSFET. Both of these types can either be in enhancement or depletion mode (see figure 1). MOSFETs are available in two types, “ p-channel” and “ n-channel”. The voltage that is applied across the gate controls how much current flows into the drain. In a MOSFET, the drain is controlled by the voltage of the gate terminal, thus a MOSFET is a voltage-controlled device. Note that when you're switching 500 V, a 1 V drop is only 0.2% efficiency loss.Metal-Oxide- Semiconductor Field-Effect Transistor (MOSFET) is a kind of Field Effect Transistor (FET) that consists of three terminals – gate, source, and drain. It doesn't take super low resistance between C and B of a BJT to turn it on, then the BJT can handle the real load current. These can be good tradeoffs at voltages MOSFETs would have too much resistance at. You turn on the FET with a control voltage, which turns on the BJT, which does the heavy lifting. That's a FET between collector and base of a BJT. The characteristics above have given rise to the IGBT. Neither is inherently better, and each has its advantages. However, the R dson gets worse with voltage capability, so above a few 100 V, BJTs usually win.īJTs are also controlled by current, while MOSFETS by voltage. Generally, for transistors rated up to only a few 100 V, MOSFETS will dissipate less. Whichever is lower depends on the specific parts and the current. It can be around a volt for high power transistors at max current. For lowish currents (relative to the max capability), this is around 200 mV. The dissipation is therefore I 2R dson.Ī saturated BJT looks largely like a voltage source. That's what the R dson spec is all about. MOSFETs look mostly like resistors when full on. I'll assume you're asking about power dissipation when both are used as a full-on switch.
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