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. That said, a low value gate resistor must be included, just to safeguard the device from sudden voltage spikes and transients.Metal-Oxide- Semiconductor Field-Effect Transistor (MOSFET) is a kind of Field Effect Transistor (FET) that consists of three terminals – gate, source, and drain. Hence, a base resistor may be crucial for a BJT but a resistor for the gate of the MOSFET may be immaterial. With MOSFETs it's just the opposite, you can switch them ON with any voltage between 3 V and 15 V, with current as low as 1 to 5 mA. However, the base voltage for a BJT does not matter much, as it can be as low as 0.6 to 1 volts for a satisfactory switching of the connected load. This implies that the base resistor in a BJT plays an important role and must be correctly calculated to ensure that the load is optimally switched ON. Meaning its base current needs to be increased proportionately with increase in its collector load current. BJT Base Resistor vs MOSFET Gate ResistorĪs explained above, a the base switching of a BJT is current dependent. The N-type MOSFETs can be turned ON or activated by applying a specified positive voltage at their gates while P-type MOSFETs will require just the opposite that is a negative voltage to get turn ON. Enhancement type is more frequently used and is the prevalent one. enhancement mode type and depletion mode type. This advantage also allows us to connect many MOSFETs in parallel even with very low current inputs (in mA). The main difference between a BJT and a MOSFET is that, a BJT operation depends on current and needs to be proportionately increased with the load, whereas a mosfet depends on voltage.īut here the MOSFET gets an edge over a BJT, because voltage can be easily manipulated and achieved to the required degrees without much trouble, in contrast increasing current means greater power that's to be delivered, which results in bad efficiency, bulkier configurations etc.Īnother big advantage of a MOSFET against the BJT is it's high input resistance, which makes it possible to be integrated with any logic IC directly, no matter how big the load may be that's being switched by the device. However even though these two counterparts may look similar with their functions and style, these two components are completely different with their characteristics and configurations. Though even Bipolar Junction Transistors (BJTs) can not be ignored due to there immense flexibility and low cost, MOSFETs also have certainly become hugely popular as far as switching heavy loads is concerned and due to the high efficiency associated with these components. Until MOSFETs arrived in the field of electronics, transistors or BJTs to be precise ruled the power switching circuits and applications. In this post we discuss the method of correctly replacing a BJT with a MOSFET, without affecting the final outcome of the circuit.

Transistor vs mosfet