Introduction:
The MOSFET (which stands for Metal Oxide Semiconductor Field Effect Transistor) is used in many microcontrollers. The MOSFET is best compared with a regular transistor, as both the FET and the transistor have three terminals that enable them to control currents. The difference between the FET and the regular transistor is that the FET only needs a voltage to switch, while the transistor requires current. Therefore, the FET is controlled without power, which benefits the minimal heat development in a microcontroller.
The image shows a MOSFET. The three pins are the connections “gate”, “drain”, and “source”.
MOS Transistor as a Switch:
For the N-MOS transistor, the gate must be positive to turn the FET on. The P-MOS transistor is not yet described on this page.
The left connection is called the gate (g), the top is the drain (d), and the bottom is the source (s).
When a positive voltage is applied to the gate, a high concentration of electrons is formed directly under the gate insulation due to the electric field. This creates an n-channel between the drain and source, allowing direct conduction between them. The arrow in the symbol indicates the direction of electron flow. In the n-MOS, the arrow points towards the channel.
The gate is also referred to as the control electrode. Compared to the regular transistor, the drain corresponds most closely to the collector and the source to the emitter. Normally, there is no conduction possible between the drain and the source due to an np-pn junction, similar to two diodes with their cathodes connected.
The diagram features a battery, switch, LED, and MOSFET. When the switch is closed, a voltage is applied to the gate, creating conduction between the drain and source, allowing current to flow. Because current flows through the resistor and LED, the LED will illuminate.
In this example, the gate is controlled by the manual switch. In reality, the gate is controlled by an ECU. The drain is connected to the negative terminal of an actuator; in the diagram, the LED represents the actuator. The source is connected to the battery ground.
MOS Transistor Characteristics:
Like the regular transistor, the MOSFET also has characteristics. With these characteristics, it can be determined what the gate voltage should be to control the actuator with the MOSFET.
In the image below, on the left is a diagram with a 5 Watt lamp controlled by the MOSFET. On the right are the MOSFET’s characteristics. The vertical axis (the Y-axis) of the characteristics shows the current through the drain. The horizontal axis (the X-axis) shows the voltage difference between the drain and the source.
If the transistor conducts because the ECU supplies the gate with a supply voltage, a current will flow, and the lamp will illuminate. The voltage measured with the voltmeter in this situation amounts to 12 volts. The 5 Watt lamp allows a current of 0.42 Amperes (420 mA) through the drain.
With the known voltage of 12 volts and current of 420 mA, these two intersections can be plotted in the characteristics. A line can be drawn between these two points, which is the load line. Using this load line, it can be determined the minimum gate voltage required for the MOSFET to conduct. To ensure the MOSFET is fully conducting, the gate voltage is always taken higher than necessary. Think of it like the factor 1.5 Ibk for the normal transistor.
The ideal gate voltage, as read from the characteristics, is 5.5 volts.0The higher the current through the drain, the higher the gate voltage must be to allow the MOSFET to conduct.
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