Construction and Operation of MOSFETs

There are two important types of MOSFETs: N-type units with N channels and P-type units with P channels.

N-type units with N channels are called depletion mode devices because they conduct when zero bias is applied to the gate. In the depletion mode, the electrons are conducting until they are depleted by the gate bias voltage. The drain current is depleted as a negative bias is applied to the gate.

P-type units with P channels are enhancement mode devices. In the enhancement mode, the electron flow is normally cut off until it is aided or enhanced by the bias voltage on the gate.

Although P-channel depletion MOSFETs and N-channel enhancement MOSFETs exist, they are not commonly used.

Construction and Operation of MOSFETs

Construction and Operation of Depletion MOSFETs

Figure 1 shows a cross section of an N-channel depletion MOSFET. It is formed by implanting an N channel in a P substrate. A thin insulating layer of silicon dioxide is then deposited on the channel, leaving the ends of the channel exposed to be attached to wires and act as the source and the drain.

construction and operation of  depletion mosfet
Fig. 1. N-channel depletion MOSFET.

A thin metallic layer is attached to the insulating layer over the N channel. The metallic layer serves as the gate. An additional lead is attached to the substrate.

The metal gate is insulated from the semiconductor channel so that the gate and the channel do not form a PN junction. The metal gate is used to control the conductivity of the channel as in the JFET.

The MOSFET shown in Figure 2 has an N channel. The drain is always made positive with respect to the source, as in the JFET. The majority carriers are electrons in the N channel, which allow drain current (ID) to flow from the source to the drain. The drain current is controlled by the gate-to-source bias voltage (EGS), as in the JFET.

construction and operation of mosfet
Fig. 2. N-channel depletion MOSFET with bias supply.

When the source voltage is zero, a substantial drain current flows through the device, because a large number of majority carriers (electrons) are present in the channel.

When the gate is made negative with respect to the source, the drain current decreases as the majority carriers are depleted. If the negative gate voltage is increased sufficiently, the drain current drops to zero.

One difference between MOSFETs and JFETs is that the gate of the N-channel depletion MOSFET can also be made positive with respect to the source. This cannot be done with a JFET because it would cause the gate and channel PN junction to be forward biased.

When the gate voltage of a depletion MOSFET is made positive, the silicon dioxide insulating layer prevents any current from flowing through the gate lead. The input resistance remains high, and more majority carriers (electrons) are drawn into the channel, enhancing the conductivity of the channel.

A positive gate voltage can be used to increase the MOSFET’s drain current, and a negative gate voltage can be used to decrease the drain current. Because a negative gate voltage is required to deplete the N-channel MOSFET, it is called a depletion mode device. A large amount of drain current flows when the gate voltage is equal to zero.

All depletion mode devices are considered to be normally turned on when the gate voltage is equal to zero.

An N-channel depletion MOSFET is represented by the schematic symbol shown in Figure 3.

Schematic symbol for an N-channel depletion MOSFET.
Fig. 3. Schematic symbol for an N-channel depletion MOSFET.

Note that the gate lead is separated from the source and drain leads. The arrow on the substrate lead points inward to represent an N-channel device.

Some MOSFETs are constructed with the substrate connected internally to the source lead and the separate substrate lead is not used. A properly biased N-channel depletion MOSFET is shown in Figure 4.

Properly biased N-channel depletion MOSFET.
Fig. 4. Properly biased N-channel depletion MOSFET.

Note that it is biased the same way as an N-channel JFET. The drain-to-source voltage (EDS) must always be applied so that the drain is positive with respect to the source. The gate-to-source voltage (EDS) can be applied with the polarity reversed.

The substrate is usually connected to the source, either internally or externally. In special applications, the substrate may be connected to the gate or another point within the FET circuit.

A depletion MOSFET may also be constructed as a P-channel device. P-channel devices operate in the same manner as N-channel devices. The difference is that the majority carriers are holes.

The drain lead is made negative with respect to the source, and the drain current flows in the opposite direction. The gate may be positive or negative with respect to the source. The schematic symbol for a P-channel depletion MOSFET is shown in Figure 5. The only difference between the N-channel and P-channel symbols is the direction of the arrow on the substrate lead.

Schematic symbol for a P-channel depletion MOSFET.
Fig. 5. Schematic symbol for a P-channel depletion MOSFET.

Both N-channel and P-channel depletion MOSFETs are symmetrical. The source and drain leads may be interchanged.

In special applications, the gate may be offset from the drain region to reduce capacitance between the gate and drain. When the gate is offset, the source and drain leads cannot be interchanged.

Construction and Operation of Enhancement MOSFETs

Depletion MOSFETs are devices that are normally on. That is, they conduct a substantial amount of drain current when the gate-to-source voltage is zero. This is useful in many applications. It is also useful to have a device that is normally off—that is, a device that conducts only when a suitable value of EGS is applied.

construction and operation of  enhancement mosfet
Fig. 6. P-channel enhancement MOSFET.

Figure 6 shows a MOSFET that functions as a normally off device. It is similar to a depletion MOSFET, but it does not have a conducting channel. Instead, the source and drain regions are diffused separately into the substrate.

The figure shows an N-type substrate and P-type source and drain regions. The opposite arrangement could also be used. The lead arrangements are the same as with a depletion MOSFET.

A P-channel enhancement MOSFET must be biased so that the drain is negative with respect to the source.

When only the drain-to-source voltage (EDS) is applied, a drain current does not flow. This is because there is no conducting channel between the source and drain.

When the gate is made negative with respect to the source, holes are drawn toward the gate, where they gather to create a P-type channel that allows current to flow from the drain to the source.

When the negative gate voltage is increased, the size of the channel increases, allowing even more current to flow. An increase in gate voltage tends to enhance the drain current.

The gate of a P-channel enhancement MOSFET can be made positive with respect to the source without affecting the operation. The MOSFET’s drain current is zero and cannot be reduced with the application of a positive gate voltage.

 Schematic symbol for a P-channel enhancement MOSFET.
Fig. 7. Schematic symbol for a P-channel enhancement MOSFET.

The schematic symbol for a P-channel enhancement MOSFET is shown in Figure 7. It is the same as that for a P-channel depletion MOSFET except that a broken line is used to interconnect the source, drain, and substrate region. This indicates the normally off condition. The arrow points outward to indicate a P channel.

A properly biased P-channel enhancement MOSFET is shown in Figure 8. Notice that EDS makes the MOSFET’s drain negative with respect to the source. EGS also makes the gate negative with respect to the source.

Properly biased P-channel enhancement MOSFET.
Fig. 8. Properly biased P-channel enhancement MOSFET.

Only when EGS increases from zero volts and applies a negative voltage to the gate does a substantial amount of drain current flow. The substrate is normally connected to the source, but in special applications the substrate and source may be at different potentials.

N-channel enhancement MOSFETs may also be constructed. These devices operate with a positive gate voltage so that electrons are attracted toward the gate to form an N-type channel. Otherwise, these devices function like P-channel devices.

Schematic symbol for an N-channel enhancement MOSFET
Fig. 9. Schematic symbol for an N-channel enhancement MOSFET.

Figure 9 shows the schematic symbol for an N-channel enhancement MOSFET. It is similar to the P-channel device except that the arrow points inward to identify the N channel. Figure 10 shows a properly biased N-channel enhancement MOSFET.

Properly biased N-channel enhancement MOSFET.
Fig. 10. Properly biased N-channel enhancement MOSFET.

MOSFETs are usually symmetrical, like JFETs. Therefore the source and drain can usually be reversed or interchanged.

MOSFET Safety Precautions

Certain safety precautions must be observed when handling and using MOSFETs. It is important to check the manufacturer’s specification sheet for maximum rating of EGS.

To avoid damage to the device, MOSFETs are usually shipped with the leads shorted together. Shorting techniques include wrapping leads with a shorting wire, inserting the device into a shorting ring, pressing the device into conductive foam, taping several devices together, shipping in antistatic tubes, and wrapping the device in metal foil.

Newer MOSFETs are protected with zener diodes electrically connected between the gate and source internally. The diodes protect against static discharges and in-circuit transients and eliminate the need for external shorting devices.

In electronics, a transient is a temporary component of current existing in a circuit during adjustment to a load change, voltage source difference, or line impulse. If the following procedures are followed, unprotected MOSFETs can be handled safely:

  • Prior to installation into a circuit, the leads should be kept shorted together.
  • The hand used to handle the device should be grounded with a metallic wrist band.
  • The soldering iron tip should be grounded.
  • A MOSFET should never be inserted or removed from its circuit when the power is on.

Testing MOSFETS with Ohmmeter

The forward and reverse resistance should be checked with a low-voltage ohmmeter on its highest scale. MOSFETs have extremely high input resistance because of the insulated gate.

The meter should register an infinite resistance in both the forward- and reverse-resistance test between the gate and source or drain. A lower reading indicates a breakdown of the insulation between the gate and source or drain.

Summary

MOSFETs (insulated gate FETs) isolate the metal gate from the channel with a thin oxide layer. Depletion mode MOSFETs are usually N-channel devices and are classified as normally on. Enhancement mode MOSFETs are usually P-channel devices and are normally off.

One difference between JFETs and MOSFETs is that the gate can be made positive or negative on MOSFETs.

The source and drain leads can be interchanged on most MOSFETs because the devices are symmetrical.

MOSFETs must be handled carefully to avoid rupture of the thin oxide layer separating the metal gate from the channel. Electrostatic charges from fingers can damage a MOSFET.

Prior to use, keep the leads of a MOSFET shorted together. Wear a grounded metallic wrist strap when handling MOSFETs. Use a grounded soldering iron when soldering MOSFETs into a circuit and make sure the power to the circuit is off.

MOSFETs can be tested using a commercial transistor tester or an ohmmeter.

Leave a Comment

Your email address will not be published. Required fields are marked *