What is a MOSFET: Basics & Tutorial

- the MOSFET, Metal Oxide Semiconductor field effect transistor offers many advantages, particularly in terms of high input impedance and overall performance.

FETs of all types are widely used electronics components today. Of all the types of FET, the MOSFET is possibly the most widely used.

Even though MOSFETs have been in use for many years, these electronics components are still a very important element in today's electronics scene. Not only are MOSFETs found in many circuits as discrete components, but they also form the basis of most of today's integrated circuits.

MOSFETs provide many advantages. In particular they offer a very high input impedance and they are able to be used in very low current circuits. This is particularly important for integrated circuit technology where power limitations are a major consideration.

MOSFET history

The term MOSFET stands for Metal Oxide Semiconductor Field Effect Transistor, and the name gives a clue to its construction.

The devices had been known about for several years but only became important in mid and late 1960s. Initially semiconductor research had focussed in developing the bipolar transistor, and problems had been experienced in fabricating MOSFETs because process problems, particularly with the insulating oxide layers.

Now the technology is one of the most widely used semiconductor techniques, having become one of the principle elements in integrated circuit technology today. Their performance has enabled power consumptions in ICs to be reduced. This has reduced amount of heat being dissipated and enabled the large ICs we take for granted today to become a reality. As a result of this the MOSFET is the most widely used form of transistor in existence today.

MOSFET symbol

There a several MOSFET circuit symbols that are used. Some MOSFET symbols are equivalents of each other, while others indicate more detail about the MOSFET itself.

As there are several varieties of MOSFET, the symbols used to indicate them need to be different.

MOSFET circuit symbols for enhancement P-channel and P-channel types .
MOSFET symbols for N-channel and P-channel types (enhancement mode)

MOSFET symbol used above generally indicates that the device has a bulk substrate - this is indicated by the arrow on the central area of the substrate.

MOSFET circuit symbols for enhancement P-channel and P-channel types for devices that have no bulk substrate.
MOSFET symbols for N-channel & P-channel types (no bulk substrate)

It can be seen from the MOSFET circuit symbols above that there are two common circuit symbols for a MOSFET with no bulk substrate. Both are widely used.

MOSFET circuit symbols for depletion P-channel and P-channel types.
MOSFET symbols for N-channel & P-channel types (depletion mode)

MOSFET basics

The MOSFET provides some key features for circuit designers in terms of their overall performance.

Key MOSFET Features
Feature Details
Gate construction gate is physically insulated from the channel by an oxide layer. Voltages applied to the gate control the conductivity of the channel as a result of the electric field induced capacitively across the insulating dielectric layer.
N / P channel Both N-channel and P-channel variants are available
Enhancement / depletion Both enhancement and depletion types are available. As the name suggests the depletion mode MOSFET acts by depleting or removing the current carriers from the channel, whereas the enhancement type increases the number of carriers according to the gate voltage.

The two main types of MOSFET are N-channel and P-channel. Each has different features:

Comparison of the key features of N-channel and P-channel MOSFETs
Parameter N-channel P-channel
Source / drain material N-Type P-Type
Channel material P-Type N-Type
Threshold voltage Vth negative doping dependent
Substrate material P-Type P-Type
Inversion layer carriers Electrons Holes

In view of the structure of the MOSFET - its gate is insulated from the channel by a thin oxide layer and this means that it can be damaged by static if it is not handled in the correct way, or the circuit does not protect it adequately.

Like other forms of FET, the current flowing in the channel of the MOSFET is controlled by the voltage present on the gate. As such MOSFETs are widely used in applications such as switches and also amplifiers. They are also able to consume very low levels of current and as a result they are widely used in microprocessors, logic integrated circuits and the like. CMOS integrated circuits used MOSFET technology.

Also like other forms of FET, the MOSFET is available in depletion mode and enhancement mode variants. The enhancement mode is what may be termed normally OFF, i..e when the VGS gate source voltage is zero and requires a gate voltage to turn it on, whereas the other form, deletion mode devices are normally ON when VGS is zero.

There are basically three regions in which MOSFETs can operate:

  • Cut-off region:   In this region of the MOSFET is in a non-conducting state, i.e. turned OFF - channel current IDS = 0. The gate voltage VGS is less than the threshold voltage required for conduction.
  • Linear region:   In this linear region the channel is conducting and controlled by the gate voltage. For the MOSFET to be in this state the VGS must be greater than the threshold voltage and also the voltage across the channel, VDS must be greater than VGS.
  • Saturation region:   In this region the MOSFET is turned hard on. The voltage drop for a MOSFET is typically lower than that of a bipolar transistor and as a result power MOSFETs are widely used for switching large currents.

    Switching for Different Types of MOSFET
    MOSFET type VGS +ve VGS 0 VGS -ve
    N-Channel Enhancement ON OFF OFF
    N-Channel Depletion ON ON OFF
    P-Channel Enhancement OFF OFF ON
    P-Channel Depletion OFF ON ON

MOSFET structure

As already implied the key factor of the MOSFET is the fact that the gate is insulated from the channel by a thin oxide layer. This forms one of the key elements of its structure.

For an N-channel device the current flow is carried by electrons and in the diagram below it can be seen that the drain and source are formed using N+ regions which provide good conductivity for these regions.

In some structures the N+ regions are formed using ion implantation after the gate area has been formed. In this way, they are self-aligned to the gate.

The gate to source and gate to drain overlap are required to ensure there is a continuous channel. Also the device is often symmetrical and therefore source and drain can be interchanged. On some higher power designs this may not always be the case.

The structure of a typical enhancement mode MOSFET showing the different regions within the device.
N channel enhancement mode MOSFET structure

It can be seen from the diagram that the substrate is the opposite type to the channel, i.e. P-type rather than N-type, etc. This is done to achieve source and drain isolation.

The oxide over the channel is normally grown thermally as this ensure good interfacing with the substrate and the most common gate material is polysilicon, although some metals and silicides can be used.

The depletion mode has a slightly different structure. For this a separate N-type channel is set up within the substrate.

The structure of a typical depletion mode MOSFET showing the different regions within the device.
N channel depletion mode MOSFET structure

P-channel FETs are not as widely used. The main reason for this is that the holes do not have as high a level of mobility as electrons, and therefore the performance is not as high. However they are often required for use in complementary circuits, and it is mainly for this reason that they are manufactured or incorporated into ICs.

MOSFET circuit design

The MOSFET follows the same basic circuit design principles that are used for all forms of FET. They are essentially high impedance voltage devices and as such they are treated in a slightly different way to bipolar transistors that are current devices.

Note on FET circuit design:

FETs can be used in a whole variety of circuits. Like the bipolar transistor, there are basic circuits. These include the common source, common drain and common gate. These form the basis of FET circuits.

Click on the link for further information about FET circuit design

There are many different circuits in which MOSFETs can be used, from low power amplifiers to high power switching applications. In all these circuit areas FETs can be used and they offer high levels of performance.

By Ian Poole

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