The MOSFET

The
Transistor
as a
Digital Device
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A transistor can be used as an analogue device or a DIGITAL device.
It has been covered as an analogue device in audio amplifiers all over the web and in text books. But its main use in recent times has been as a digital device.
In other words we are talking about its use in DIGITAL CIRCUITS where it is either ON or OFF and is consuming little or no current.
This mode of operation has taken over electronics designs because the two states have fixed and rigid values. There is no "half-on" or "one quarter off."
And because it is either ON or OFF, the condition is 100% reliable, predictable and copyable.
When we talk about this mode of operation, we also find there is a better component than the ordinary transistor. It is called the Field Effect Transistor (FET) because it has a characteristic of having a very small voltage across it when a large current flows. A common transistor has a voltage of about 0.3v to 0.8v or more across the collector-emitter terminals and even though this voltage is very small, when a high current flows, the transistor will get very hot.
If we replace it with a FET, the device stays cool or even cold. The voltage across it will be only a few millivolts to a few hundred millivolts.
This is a great advantage as no heat-fin may be needed.
But it is important to drive both devices with a what we say is a high amplitude signal. This is especially needed for a FET as the gate needs a few volts for it to operate and a slightly higher voltage to make sure it is turned ON.
An ordinary transistor needs just 0.7v.
When we have a high amplitude signal, a digital-design can be easily converted to a FET design.
All you have to know is the base of an ordinary transistor is now the gate of a FET and the input voltage needs to be more than about 2.5v.
I say "about" because the family of FETs has a wide range of gate voltages and you need to refer to a specification-sheet for the exact value.
Here is the circuit for a common-emitter stage and a common-source stage:

The common transistor and FET transistor

Provided the common transistor has a base resistor to prevent the base being pulled higher than 0.7v, either device will work in the project you are designing.

The normal transistor MUST have a base resistor

In a Digital Stage, the input waveform can be any amplitude and any shape - with exceptions, as mentioned below.
If an ordinary transistor (commonly called a Bi-Polar Junction Transistor - BJT) is used, the transistor will turn ON fully when the signal is above 0.7v and turn OFF when the signal is below 0.4v. A resistor on the input line is needed to "match up" the signal and prevent the input of the transistor being damaged as the base must not be forced to rise above 0.7v.
If a FET is used, it will turn ON when the signal is above about 1v but the signal must be more than 2.5v to 4v to turn ON fully. The gate can be supplied with a signal as high as 18v but the additional voltage is not needed.
That's the only differences and similarities.
The input signal can be a square-wave, sine-wave, triangle wave, pulse or noise and the transistor or FET will only respond when the signal is above or below the values mentioned above.
However the rise and fall of the signal must be as fast as possible to prevent the device getting hot.
The device is basically in a "resting" or turned-OFF state and the signal turns it ON.
In other words the device is not "self-biased" or "pre-biased" or partially turned ON in any way.
For a transistor, the signal needs to be able to supply a small current as the transistor is current-driven device.
For a FET, this current is microscopic as it is voltage-driven.

This type of stage is called a DIGITAL STAGE.
It has a number of advantages.
By turning the stage ON and OFF very quickly, a LOAD such as a motor or solenoid or LED can be activated.
If the ON-time is reduced, the motor slows down and the LED dims. During this cycling the transistor stays completely cold.
This is called PULSE WIDTH MODULATION or FREQUENCY MODULATION and is the basis of an electric car accelerating and flashing LED road-signs.
The ON-time and OFF-time is called the mark-space ratio and this is controlled by a microcomputer.
For a common transistor, the base MUST be fed with sufficient current to produce saturation. It must be fully-turned-ON and for a motor the collector current must be about 5 times the operating current.
For a FET transistor the voltage on the gate must be a few volts higher than the specified gate voltage.
The waveform must be digital. This means it must rise and fall very quickly. It is called a square wave but the HIGH-time and LOW-time can be different durations and as these change the output controls the speed of the motor.
The only time when the transistor will start to get hot is during a slow rise or fall and that's why the control signal needs to be monitored and viewed to see its characteristics.
All these things only apply when a high current is being delivered. Small current don't heat the device.
In addition, when a motor or solenoid is turned off rapidly, it will produce a back EMF that can be much higher than the supply voltage and this can destroy the transistor (a transistor gets destroyed much faster with a spike, than excess current).
These spikes can be removed with a capacitor an-or a diode across the motor or coil.
It is called SNUBBING or QUENCHING and components are available for high current - high voltage applications.
Here are the sort of signals that can be processed by a digital stage:

These signals cannot be reproduced accurately by a digital stage:

The rise and fall time is too long and the audio may never turn off the stage. The FET may get very hot.

When we say the "rise time" is too long, we mean the time taken for the signal to rise from point A to point B: And this applies to the full amplitude of the signal.

This is a slow-rise wave

A transistor and FET can be interchanged (theoretically)

FETs are used for high current motors
The transistor may not be suitable

The transistor may not be suitable

Transistors are limited to a gain of about 100 when powering a motor from a microcontroller.
The output current from a micro is 25mA maximum and a 470R resistor will deliver 20mA to the base of the transistor. This gives a theoretical 2Amp capability to drive a motor. We have already mentioned a motor may require up to 5 times the running current to get it to start revolving and so a transistor driver has a number of limitations.

However a FET is voltage-controlled and if the FET will fully turn ON at 5v gate voltage, you can supply any current up to the capability of the FET. Some FETs will deliver 35 amps or 55 amps. They are also called MOSFET - Metal Oxide Field Effect Transistor
When doing all your calculations, you need to know the voltage that will be developed across the Drain-Source junction.
The specification sheet states the junction will develop a resistance of say 30milliohm for a current up to 3 amps and increase to 70milliohms for 5 amp.
You need to convert this to milliwatts of heat generated by using the formula Power = I x I x R
When 1 amp is flowing the MOSFET will dissipate 70milliwatts. When 5 amp flows, the dissipation will be 5 x 5 x 0.07 = 1.75 watts.
A transistor will dissipate more than 0.6 x 5 = 3watts when 5 amps flows.
Any device starts to feel hot when 500milliwatts is dissipated. So both will need to be heatsinked
There is nothing you can do about these losses. They are called "characteristics."
Gate voltage may be specified as 2.5v, but if you provide say 4.5v, the On-resistance will drop 50% for a high current.
Gate voltage can be as high as 12v without any damage.
A MOSFET will operate very similar to an ordinary transistor when the input voltage is in the "Gate Voltage Range." This may be 0v to 1.5v or 2.5v but this discussion is centered around DIGITAL APPLICATIONS, where the rise and fall is rapid and the gate voltage will be 0v to 5v.