MOSFET

Why Use MOSFET?

Metal-Oxid Semiconductor Field-Effect Transistor (MOSFET) has several advantages compared to a bipolar transistor:

  • Compatibility with integrated circuit fabrication technologies;
  • Very high input resistance;
  • Lower power consumption;
  • Better thermal stability;
  • Low sensitivity to radiation.

Where Are MOSFETs Used?

Where Are MOSFETs Used?

What is a MOSFET ?

A MOSFET is a Metal-Oxide Semiconductor Field-Effect Transistor. It is the electronic version of a switch.

Whenever it is switched on it allows a current to flow. In the switched off mode, however, it stops the current from flowing.

Its function is exactly the same as that of a manual switch we use for switching lights on and off. But instead of being operated by our hands, the switch is controlled by electricity.

MOSFET Switch

The switch we use to turn a light on and off is: big, slow, and impacts on a great current.

The transistor or "electronic switch" is in contrast usually small, fast, and controls a smaller current.

Switching is one of the most important functions to consider when it comes to producing computers. Almost all microprocessors, memory and support chips use MOS switches.

MOS Structure

Metal-Oxide Semiconductors (MOS) structure consists of a piece of metal on an oxide that is placed on a semiconductor (silicon).

The MOSFET is sometimes referred to as an IGFET (which is an Insulated-Gate FET) because the gate is insulated from the substrate by silicon dioxide SiO2 (i.e. quartz).

Terminals

The metal-oxide semiconductor FET or MOSFET is basically a four-terminal device. It has a source - S,a gate - G, a drain - D and a terminal to the substrate (body - B).

The flow of the current between the source and the drain depends on the voltages that are applied to these terminals. A MOSFET is essentially a voltage-controlled solid-state switch.

Types of MOSFETs

There are two types of MOSFETs - depletion- and enhancement- mode MOSFETs.

A channel marks the main difference between the two types (the so-called conducting "path"). It is located between the source and drain.  With the depletion-mode MOSFET it is formed in the doping process during fabrication. A depletion MOSFET is said to be "on" when the gate voltage is zero.

Construction

The construction of an enhancement MOSFET is similar to that of the depletion-mode MOSFET with the exception that there is no channel between the source and drain. An enhancement MOSFET is considered to be "off" when the gate voltage is zero.

n-Channel and p-Channel MOSFETs

Both types of MOSFET devices have either an n-channel or a p-channel depending on their added impurities. Note that the MOSFET is a unipolar device. Its operation depends on only one type of charge, either electrons or holes, but never both.

In this tutorial we shall focus on the n-channel device.The reason for this is that the action of a p-channel MOSFET is merely complementary since it has all voltages and currents simply reversed.

Schematic Symbols

The schematic symbol for an enhancement MOSFET has a broken channel line. This is supposed to indicate that there is no conducting channel between the source and the drain when the gate voltage is zero.

In the schematic symbol for a depletion MOSFET the thin continuous vertical line next to the right of the gate is supposed to represent the existing channel.

Symbols of n-channel & p-chanel MOSFETs

For n-channel devices the arrow on the p substrate points inward. The schematic symbol for the p-channel MOSFET is similar except that the arrow points outward.

Symbols of Three-Terminal MOSFETs

In some applications, a voltage can be applied to the substrate for additional control of the drain current. In most applications, however, the substrate is connected to the source.

Usually the manufacturer connects the substrate internally to the source. This results in a three-terminal device.

Mode of Operation

Transistor action in the MOSFET is principally due to the gate voltage UGS controlling the output current. Since the gate is insulated from the substrate the gate current IG is essentially zero. Thus virtually no input power is needed to control the output current.

Because the number of charge carriers in the channel is either enhanced or depleted by the gate voltage, MOSFETs are accordingly referred to as enhancement- or depletion- mode devices.

Enhancement-mode MOSFET

This figure shows the regular biasing polarities in an enchancement mode n-type device.

When the gate voltage is zero the enhancement MOSFET is "off " since there is no conducting path between the source and the drain. If, however, a sufficiently high positive voltage is applied to the gate, electrons will be attracted to the surface under the gate. They will accordingly form a shallow n-type inversion layer channel connecting the source with the drain.

Threshold Voltage

Here we can see the regular biasing polarities for an n-type enhancement mode device

Electrons flowing from the source to the drain can pass through the narrow channel whenever the voltage difference is established. The electron flow is equivalent to the current referred to as the drain current ID. The more positive the gate voltage is, the more electrons will be in the channel and the greater the drain current will be too.

The minimum UGS that creates the inversion layer to enable UDS is called the threshold voltage UT (Uth).

Depletion-mode MOSFET

Here we can see the regular biasing polarities of an n-type depletion-mode device.

The depletion MOSFET is "on" even when the gate is shorted to the source because the channel had already been formed when the device was fabricated.

If the gate voltage is negative, it will drive the electrons from the channel. Therefore the more negative the gate voltage is the smaller the drain current will be.

Threshold Voltage

Here we can see the regular biasing polarities of an n-type depletion-mode device.

If the gate voltage is sufficiently negative, the device will be switched off. However, if a positive voltage should be applied, the device will operate in the enhancement mode. This device can therefore function in two different ways depending on whether the input is positive or negative.

Depletion n-channel transistors have negative threshold voltages UT. They are contrary to enhancement n-channel devices that have positive thresholds.

Output Characteristics

This figure illustrates a typical set of output characteristics for an n-channel MOSFET, which correspond to the drain current ID in comparison to the drain-source voltage UDS. For a given value of UGS, when UDS increases the drain current will at first rapidly increase, but further on it will level off and tend to remain at an almost constant value.

Here the pinchoff voltage UP is the voltage where the drain curve will change from an almost vertical to an almost horizontal direction.

Pinchoff Voltage

The pinchoff voltage divides the two major operating regions of the MOSFET: the ohmic and the current-source (or saturation) region. The pinchoff voltage can be calculated for any UGS with UP = UGS - UT.

The ohmic region is the region where UDS is lower than UP. In the ohmic region a MOSFET functions like a small resistor. When UDS is greater than UP the MOSFET will function as a current source.

MOSFET Ratings

All electronic devices must operate within the prescribed limits of voltage, current, and power (as given in data sheets).

The maximum drain current IDmax is specified to limit the internal power dissipation within the semiconductor PD = UDSID less than the maximum power PDmax.

If the value of the drain voltage UDS becomes too large, then a reverse breakdown will occur. This will happen due to a rapid rise in the drain current ID. Normally, operations must be avoided in the region of the avalanche breakdown. Hence UDS must always be lower than UDSmax.

Oxide Breakdown

If the value of UGS in a MOSFET becomes too large, then the insulating layer will break down.

This is a destructive breakdown. After that the device will no longer function properly.
Problems of this kind can occur in a MOSFET that is not connected to any circuit. Due to a high input resistance static, electric charges can produce a voltage that is high enough to break the insulator and destroy the device.

MOSFET Protection

To prevent the accumulation of static charges MOSFETs are packed with a wire that shorts all leads. The shorting wire should not be removed until the MOSFET is connected to its circuit.

Protection with Zener Diode

Some MOSFETs are fabricated with Zener diodes that are connected between the gate and the source to prevent this static electricity problem.

The Zener diode functions here as a great impedance. It only conducts when its breakdown voltage is reached. Therefore no gate at the source voltage can become too large anymore.