Topic 2.6.3 Enhancement Mode N-Channel MOSFET S

Topic 2.6.3 – enhancement mode n-channel MOSFET’s

Learning Objectives:

At the end of this topic you will be able to;

describe the switching action of n-channel MOSFET’s;

recognise that MOSFET’s have a very high input resistance;

perform calculations using ;

understand that decreases from a very high value to a very low value as is increased and is at a minimum value of at saturation;

perform calculations on MOSFET switching circuits;

compare the performance of MOSFET and transistor switches.

MOSFET’s.

In our previous topic we looked the use of the NPN transistor as an electronic switch. This device was essentially current controlled, requiring a small base current to enable a much larger collector current to flow. Our work revealed a potentially disastrous problem with the loading effect of the transistor circuit. Unless great care was taken in the design of the sensing circuit it was possible that the transistor switch would not work as it was intended.

In this topic we are going to investigate the operation of a different type of transistorwhich will overcome this loading problem. The transistor is called a MOSFET, which stands for Metal Oxide Semiconductor Field Effect Transistor, which is a bit of a mouthful, so we will simply refer to it as a MOSFET. There are many different types of MOSFET, available but we will be concentrating only on one type in this course, which is then-channel enhancement MOSFET. You will not be asked about any other version in the examination.

The Field Effect Transistor

Detailed knowledge of how a MOSFET is constructed are not required, the important thing is that the Field Effect Transistor, or simply FET, uses the voltage that is applied to the input terminal to control the output current, since their operation relies on the electric field (hence the name field effect) generated by the input voltage. This then makes the Field Effect Transistor a VOLTAGE operated device.

The Field Effect Transistor has very similar properties to those of the NPN Transistor i.e., high efficiency, instant operation, robust and cheap. However they can be made much smaller than an equivalent NPN transistor and along with their low power consumption and dissipation make them ideal for use in integrated circuits such as the CMOS range of chips.

The Field Effect Transistor has one major advantage over its standard NPN transistor cousins, in that their input impedance is very high, (Thousands of Ohms) making them very sensitive to input signals, but this high sensitivity also means that they can be easily damaged by static electricity.

Enhancement-mode MOSFET

The symbol, and picture for an n-channel enhancement mode MOSFET is shown below.

The leads for this type of transistor are labelled as Gate (G), Drain (D) and Source (S).

The Enhancement-mode MOSFEThas the property of being normally "OFF" when the gate bias voltage is equal to zero.

A drain current will only flow when a gate voltage (VGS) is applied to the gate terminal. This positive voltage reduces the overall resistance of the device allowing current to flow between the Drain (D) and Source (S). Increasing this positive gate voltage will cause an increase in the drain current, ID through the channel. The MOSFET, can also saturate when VGS is increased sufficiently, when this occurs the resistance of the MOSFET reaches its lowest value and will be written in data sheets as .

The transfer characteristic of the MOSFET is similar to that of the NPN transistor, with one major difference, the linear region is very small, making it very unlikely that the MOSFET will operate in this region, as shown below.

The exact voltages at which cut-off ends and saturation ends are functions of the device itself and therefore the above characteristic is given only for illustrative and comparative purposes to the transistor characteristic.

There only two formulae we need in order to design MOSFET circuits. First is the transconductance formula, which relates the Drain current to the input voltage VGS. The transconductance of a MOSFET is given the symbol gM, and defined as

Secondly, the formula for the power dissipated in a MOSFET when it is saturated is defined as

Enhancement-mode MOSFET's make excellent electronics switches due to their low "ON" resistance and extremely high "OFF" resistance and extremely high gate resistance. Enhancement-mode MOSFET's are used in integrated circuits to produce CMOS type Logic Gates and power switching circuits as they can handle large currents and can be driven directly by digital logic levels.

So let us look at how the MOSFET is used in a circuit.

Example : The following circuit shows a MOSFETbeing used to switch on a high powered lamp from a light sensing circuit.

An extract from the datasheet for the MOSFET is shown below:

VDS/V (max) / VGS /V (max) / ID /A (max) / PTOT /W (max) / gM /S (typical) / /Ω
50 / 15 / 8 / 50 / 1.3 / 0.24

(a) Calculate the minimum value of voltage from the light sensing sub-system to allow the load to operate at it’s rated power.

(b) Calculate the power dissipated in the MOSFET when the lamp is operating at full power.

Solution : (a)First calculate the current needed by the load to operate at full power.

The minimum value of VGS can now be calculated using the transconductance formula as shown below.

(b)The power dissipated in the MOSFET is given by IDsquared multiplied by rDS(ON)

Hopefully you can see that the circuit calculations relating to the MOSFET are much more straightforward than those for an NPN transistor.

The last question here illustrates the need to have as a low value, because the power dissipated in the MOSFET is reliant on this value. If this value was high then the power dissipated in the MOSFET would be excessive.Even with it having a low value the power dissipated in a MOSFET is significant, and they usually run hot, and need to be mounted on heatsinks.

They are capable however of handling much larger currents than the NPN transistor, and are therefore most suited to switching high powered loads like motors and solenoids.

Comparision between NPNTransistors and MOSFET’s

Transistors are "Current Operated Devices" where a much smaller Base current causes a larger Collector to Emitter current, which themselves are nearly equal, to flow.
A transistor can also be used as an electronic switch to control devices such as lamps, motors and solenoids etc.
The NPN transistor requires the Base to be more positive than the Emitter.

Field Effect Transistors, or FET's are "Voltage Operated Devices"
FET's have very high input resistances so very little or no current (MOSFET types) flows into the input terminal making them ideal for use as electronic switches.
The high input impedance makes the design of the sensing sub-system easier, since we do not have to worry about loading effects.
The input impedance of the MOSFETmeans that static electricity can easily damage MOSFET devices so care needs to be taken when handling them.
They can be used as ideal switches due to their very high channel "OFF" resistance, low "ON" resistance.

There now follows a number of examination style questions which will allow you to practice some numerical problems.

Examination Style Questions.

1.The circuit below shows a MOSFET being used to interface a CMOS logic system to a lamp rated at 24V, 12A.

An extract from the datasheet for the MOSFET is shown below:

VGS /V (max) / ID /A (max) / PTOT /W (max) / gM /S (typical) / /Ω
15 / 15 / 90 / 1.2 / 0.15

(a)Calculate the minimum value of VGS required to enable the lamp to operate at its rated current.

………………………………………………………………………………………………

[2]

(b)Calculate the power dissipated in the MOSFET when the lamp is operating at its rated current.

………………………………………………………………………………………………

[2]

(c)The maximum output current available from the logic system is 30mA. Explain why an NPN transistor would be unsuitable for this application.

………………………………………………………………………………………………

[2]

2.The circuit below shows a MOSFET being used as a transducer driver to interface a logic system to a lamp rated at 18V, 9A.

An extract from the datasheet for the MOSFET is shown below:

VGS /V (max) / ID /A (max) / gM /S (typical) / /Ω
10 / 16 / 1.5 / 0.2

(a)Calculate the minimum value of VGS required to enable the lamp to operate at its rated current.

………………………………………………………………………………………………

[2]

(b)Why is it important for the MOSFET to have a small value of ?

………………………………………………………………………………………………

[1]

3.The following circuit is setup to check some parameters of a MOSFET.

The following results were obtained with the lamp bright and the MOSFET saturated.

VIN /V / VOUT/V / I2 /A
3 / 0.6 / 1.5

(a)Estimate the value of I1.

………………………………………………………………………………………………

[1]

(b)Use the results to calculate the value of .

………………………………………………………………………………………………

[2]

(c)What is the main disadvantage of a MOSFET compared with a NPN transistor switch.

………………………………………………………………………………………………

[1]

4.The circuit below shows a MOSFET being used to interface a 24V, 5A.load to a logic system.

An extract from the datasheet for the MOSFET is shown below:

VDS /V (max) / VGS /V (max) / ID /A (max) / PTOT /W (max) / gM /S (typical) / /Ω
50 / 12 / 10 / 80 / 1.6 / 0.3

(a)Calculate the minimum value of VGS required to enable the lamp to operate at its rated current.

………………………………………………………………………………………………

[2]

(b)What is the advantage of the MOSFET having a small value of .

………………………………………………………………………………………………

[1]

(c)Explain why a MOSFET interface is the most suitable interface between a CMOS logic system and a high-power load.

………………………………………………………………………………………………

[2]

Self Evaluation Review

Learning Objectives / My personal review of these objectives:
 /  / 
describe the switching action of n-channel MOSFET’s;
recognise that MOSFET’s have a very high input resistance;
perform calculations using ;
understand that decreases from a very high value to a very low value as is increased and is at a minimum value of at saturation;
perform calculations on MOSFET switching circuits;
compare the performance of MOSFET and transistor switches.

Targets:1.………………………………………………………………………………………………………………

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