How to use MOSFET transistors as switch

This article is a quick reference on how to use MOSFET transistors as a switch. Please refer to this article if you want to know how to use a BJT transistor as a switch.

There are two main types of transistors: Bipolar Junction Transistors (BJT) and Field Effect Transistors (FET). The FET transistor group contains two subgroups: Junction Field Effect Transistor (JFET) and Metal Oxide Semiconductor Field Effect Transistor (MOSFET).

The discrete MOSFET transistors have three terminals: Gate (G), Drain (D), and Source (S). There is a fourth terminal named Body (B) or Substrate (Sub) that most of the time is connected to the Source or common ground to eliminate some junction parasitic effects.

There are two types of MOSFET transistors: Enhancement and Depletion type.

Depletion Mode MOSFET

The MOSFET Depletion type has the conductive channel already formed, so it acts as a closed switch. To turn the device OFF (or to open the switch), you need to apply a voltage between Gate and Source - V_GS. The voltage applied at the gate will push the charge carriers from the channel, basically temporarily destroying it. The MOSFET Depletion type transistors are further divided by the channel charge carrier type: N-Channel (or NMOS) and P-Channel (or PMOS) transistors.

Figure 1. Depletion Mode MOSFET Symbols.

Figure 1 shows the Depletion Mode MOSFET schematic symbols. As you can see, a continuous line represents the conductive channel between Drain and Source. The arrow for the N-Channel type points inside, and for the P-Channel, the arrow points outside.

Enhancement Mode MOSFET

The MOSFET Enhancement type has no initial conductive channel formed yet, the device is turned off, and it acts as an open switch. To turn on the device (or close the switch) you need to apply a voltage between the Gate and Source - V_GS. This will attract the charge carriers close to Drain and Source terminals and create the conductive channel, which will allow the current to flow.

As for the MOSFET Depletion type, based on charge carrier type, there are two types of MOSFET Enhancement transistors: N-channel and P-channel.

Figure 2. Enhancement Mode MOSFET Symbols

Figure 2 illustrates the Enhancement Mode MOSFET schematic symbols. A dashed line represents the conductive channel between Drain and Source. As for Depletion type, the arrow for the N-Channel type points inside, and outside for the P-Channel.

Because Enhancement MOSFET type transistors are more common, we won’t cover below the Depletion type.

MOSFET Enhancement type N-Channel as switch

Similar to BJT, MOSFET transistors have three working regions: Cut-Off, Linear (or Ohmic), and Saturation region. Depending on which region the transistor operates, it can act as an open or closed single-pole single-throw (SPST) switch.

For N-Channel MOSFETs, the Source terminal is connected to the common ground, and the load is connected to the power source. See Figures 3 and 4. Because the gate is very sensible, connect a pull-down resistor R_PD between the gate and the common ground to avoid fluctuations that uncontrollable turns the device ON and OFF.

In the Cut-Off region, the resistance between Drain and Source terminal R_DS is very high and no current flows I_D=0A. The voltage drop between Drain and Source V_DS is close to the power source voltage Vcc. The device is considered being OFF and acts as an open switch (Figure 3). To set the transistor in the Cut-Off region, the voltage drop between Gate and Source should be less than the Threshold Voltage V_TH value.

Figure 3. N-Channel MOSFET transistor schematic acting as an open switch.

In the Linear (and Saturation) region, the RDS resistance is very low, and the I_D is at its maximum value. The V_DS is close to 0V. The device is considered being ON and acts as a closed switch (Figure 4). To set the transistor in the Linear region, the V_GS should be greater than V_TH voltage, and the V_DS should be close to V_GS voltage.

Figure 4. N-Channel MOSFET transistor schematic acting as a closed switch.

If V_DS is higher than V_GS voltage drop, the transistors will go in Saturation region. In the Saturation region, increasing V_DS won’t have any effect on the I_D, but the extra voltage will contribute to the power dissipation because of the R_DS. So you may want to avoid the Saturation region.

MOSFET Enhancement type P-Channel as switch

For the P-Channel MOSFET, the Source terminal is connected to the circuit power source, and the load is connected close to the circuit’s common ground, see Figures 5 and 6. Use a pull-up resistor R_PU to connect the Gate to the voltage source to avoid any fluctuations that uncontrollable turns the device ON and OFF.

For the P-channel MOSFET transistor to be in the Cut-Off region, you connect the gate to the positive voltage source (Figure 5).

Figure 5. P-Channel MOSFET transistor schematic acting as an open switch.

To turn on a P-channel MOSFET device, you need to apply 0 or a negative voltage at the gate (Figure 6).

Figure 6. P-Channel MOSFET transistor schematic acting as a closed switch.

MOSFET characteristic to look up for

Threshold voltage V_TH - is the voltage value at which the current starts to flow through the transistors. For switching applications, make sure this value is below 2V.

Max Drain-Source current I_D - Make sure the max I_D current at VGS at 5V or 3.3V satisfies your requirement. The datasheets usually specify this value in a table or you need to derive from the current-voltage characteristic diagram.

The Drain Source resistance when the device is on R_DS(ON) - should be as low as possible. The lower the V_GS, the higher is R_DS(ON). However, you need to make sure that at 5V V_GS (or 3.3V) the R_DS(ON) is low enough. As the V_DS and/or I_D increases, the R_DS causes the transistor to heat and lose power. R_DS also depends on the transistor temperature. If the transistor temperature increases, the R_DS increases as well, leading to more power dissipation and more temperature increase.

Although the MOSFET transistors are voltage driven, they require a current to charge/discharge the Gate-Source parasitic capacitance. Usually, an MCU pin doesn’t provide enough current, (and depending on MOSFET used), it may take longer to switch the transistor ON and OFF. So, at a higher switching frequency, the switching circuit may not work as expected.

Summary

Because of their low R_DS, the MOSFET transistors are often used as switch devices in electronic circuits. By controlling the Gate-Source voltage, you can set the transistor into a Cut-Off or Linear region. The V_GS value depends on the transistor mode type (Depletion or Enhancement) and the channel type (N-Channel or P-Channel).

In the Cut-Off region, the transistor acts as an open switch, no current flows through the Drain and we consider it to be turned “OFF”.

In the Linear region, the transistor acts as a close switch, the current through the Drain, and we consider it to be turned “ON”.