Electronics Tutorial 1 - MOSFET

Semi-Conduction

While the models of semi-conduction are NOT on the syllabus, it is worthwhile for us to think about it so that we can understand the action of semi-conductor components.  These models are simple.  More detailed models are available, but need a good understanding of quantum theory and are beyond the level we need.

All transistors depend on semiconductors.  The most common semiconductor is silicon.  The resistivity of pure silicon is high at 0.1 W m (compared with 49 × 10-8  W m for constantan), but it can be reduced by the addition of certain impurities such as aluminium or phosphorous.  The addition of impurities is called doping.

Silicon is from Group 4 in the periodic table, so has 4 outer shell electrons.  The atoms share electrons in covalent bonds with 4 other atoms.  If we put in a Group 5 atom like phosphorus, it will share electrons with 4 silicon atoms, but since it has 5 atoms in the outer shell, there is a free, unpaired electron.  This electron involved in conduction.  The idea is modelled in the picture below:

This free electron is available for conduction.  Since it is negatively charged, the semiconductor doped with a Group 5 metal is called an n-type semiconductor.  Arsenic and Antimony are also used as dopants.

If the silicon is doped with a Group 3 metal like aluminium, there is an electron vacancy.  This is because Group 3 elements have 3 outer shell electrons.  This is shown below:

The vacancy or hole be replaced by an electron, and the empty space can thus move about.  Since it's a deficiency of an electron, it results in a positive charge.   Therefore the semiconductor doped with a Group 3 metal is called a p-type material.  Holes move from positive to negative as a conventional current.

Conduction Band Model

The conduction mechanism can be explained using the conduction band model.  Remember that electrons in the quantum world occupy energy levels.  These are like the rungs of a ladder.  This states that there are bands of energy levels to which electrons can be raised.  There are lots of rungs close together in the energy ladder.  Normally the electrons occupy the valence band, i.e. their place in the outer shell.  If the right amounts of energy is applied, the electrons can be raised into the conduction bands.

In metallic conduction, the conduction band and the valence band overlap:

Since the two bands overlap, many electrons can move into the conduction band, making metals good conductors.

In semi-conduction, the conduction band and valence band are separated by an energy gap.  For silicon this is 1.09 eV.  For germanium, the gap is 0.72 eV.  At 0 K no electrons can cross it:

However at high temperatures like 300 K, some electrons can cross the energy gap.

If the semiconductor is doped, then more electrons can cross the gap.

Depletion Zone

If a piece of n-type material is placed next to a piece of p-type material, a p-n junction,  the free electrons are attracted by the holes to fill them.  This leaves a region, the depletion zone, where there are no spare charge carriers.

The depletion zone acts as an insulator.  If a positive voltage that's sufficient to overcome the Coulomb repulsion within the depletion zone, electrons will cross to the p-type material.  The depletion zone will disappear.  This is forward bias.   If the positive voltage is applied to the n-type material (reverse bias), the depletion zone will increase, preventing conduction.

MOSFET

The most familiar transistor is the bipolar junction transistor (BJT) which is not on the syllabus, so it will not be considered here.  If you want to familiarise yourself with it, please click HERE to see it in my Access Electronics Notes.

The Metal Oxide Semi-conductor Field Effect Transistor (MOSFET) is quite different to the BJT.  Here is the symbol for the MOSFET:

The terminals are:

• Gate - a voltage at the gate turns on the MOSFET.

• Source - this is where the electrons come in.  The source is the negative terminal;

• Drain - this is where the electrons leave.  This is connected to the positive.

In some MOSFETs there is a fourth terminal, the body (B) which is connected to the 0 V line.  In most MOSFETs it's connected to the source.

As electronics  uses conventional currents (flowing from positive to negative), the current flows from drain to source.

 What do the letters G, D, and S stand for on a MOSFET?

The general characteristics for a MOSFET are:

• The input (gate) resistance is very high, about 1012 W.

• The output resistance is about the same as a bipolar transistor.  The actual value depends on the type.  For a signal MOSFET it would be in the range 10 to 50 kW, while in a power MOSFET it would be somewhat lower.

• The switch-on voltage is between 1.0 and 2.5 V.

The MOSFET is a voltage controlled device.  This means that a voltage alone is needed to turn on the MOSFET.  The current needed is about 1 pA (1 × 10-12 A).  The picture below shows the way a motor can be turned on by dipping two 4 mm plugs into a cup of coffee.

How a MOSFET Works

The n-channel enhancement mode MOSFET is made up like this:

The oxide layer is an insulator and ensures that there is NO direct connection between the gate and the n-type material. This is why the MOSFET is a voltage controlled device.  Current cannot flow from the gate to the n-type material.

When the gate-source voltage is zero very low, there is a depletion zone like this:

The depletion zone acts as an insulator.  Therefore no current flows.

The gate voltage makes an electric field across the region of n-material (called the n-channel).  The positive voltage repels the holes and attracts electrons to the n-channel.  The depletion layer is reduced, so that more electrons are available for the n-channel.  Electrons can travel from the source to the drain.  Remember that conventional current flows from the drain to the source.

The current in the picture above is less than the maximum current because the n-channel is narrower than it could be.  If we increase the gate voltage a bit more, the depletion zone below the gate is reduced even further.

The n-channel is now fully open so that the MOSFET conducts the biggest current it can.  Note that the depletion zone is still there.  Current does not flow through through the p-type material.  The depletion zone can be thought of as like a rubber diaphragm in a water valve.  The water can get past it when the valve is open, but not round it.

The n-channel starts to open with a gate voltage of about 1.7 V.  It is fully open with a gate voltage of about 2.5 V to 3.0 V.  Once fully open the current cannot increase, unless the supply voltage is increased.  The MOSFET is saturated.

This type of MOSFET is said to have an n-channel enhancement mode.  It acts as a normally open switch (i.e. OFF).  This is the one you need to know about.

There are three other types of MOSFET.

• p-channel enhancement type - the drain is negative compared to the source.  It acts as a normally open switch.

• n-channel depletion type - It acts as a normally open switch (ON) and turns off with an increase in gate voltage.

• p-channel depletion type.

You are not expected to describe these types in the exam.

 In the Exam This description on MOSFET action may well form a six-mark essay question.     Think the points through in a logical way:    Gate voltage = 0;    Gate voltage just as the current flows;    Full current flow.

Characteristics of a MOSFET

The characteristics of a MOSFET can be measured in a circuit like this:

We can make the following measurements:

• The gate - source voltage, VGS: the voltage between the gate and the source that controls the MOSFET;

• The drain-source voltage, VDS: the voltage between the drain (+) and the source (-).

• The drain source current, IDS: the current through the MOSFET;

• The threshold voltage, Vth: the gate source voltage that just creates a conducting path in the n-channel.

The results of this experiment are shown on the graph:

 Write down what is happening at points A to E in the graph. Calculate the transconductance of this MOSFET at point C.  Give your answer to an appropriate number of significant figures and give the correct units.  What is the equivalent resistance?

The characteristics of the MOSFET were explored at a constant drain-source voltage.   This is often different to the gate-source voltage, and may be from a separate supply, as was the case in this experiment.   We can work out what the voltage was given our answer from Question 3 and knowing that there was a 150 W current limiting resistor.

 Calculate the drain-source voltage using data from the graph, circuit, and your answer to Question 3.

The MOSFET can be used as an amplifier or a switch.  The amplifier is not on the syllabus.

The circuit below acts as a switch.

When the switch S1 is pressed, the LED D1 turns on.  The MOSFET is known as a driver.

 Explain in terms of what you know about MOSFET why D1 shines when S1 is switched on.

MOSFETs are used in many digital chips.  We will look at digital electronics later

Power MOSFETs can carry a heavy current , for example 10 A.  The MOSFET is still turned on by a gate-source voltage of about 2 V, and the gate current is negligible.  This contrasts to bipolar transistors that need a driver transistor to drive the power transistor.  Power MOSFETs need to have a heat-sink to prevent them from overheating and going into thermal runaway.  If a motor or any other inductive component is being turned on by a MOSFET, a reverse biased diode is needed in parallel with the motor.  This prevents damage from a reverse voltage spike.

The MOSFET can take the place of a relay.  It has the advantage of having no moving parts, nor are there any inductive effects from a coil.  It can last a lot longer than an electromechanical relay, which has a finite number of operations.  The MOSFET can act at high frequency as well.

If you are using an electronic circuit using a high voltage, for example 230 V AC mains, it may be preferable to use a relay for improved isolation for the electronic circuit from the high voltage circuit.

 In the circuit above, the MOSFET has a resistance of 22 W.  The motor takes a current of 0.50 A and has a power of 6.5 W. Calculate the supply voltage to this circuit.

The switch on voltage is determined by the voltage divider made by R1 and R2.  The voltage divider equation is:

 The threshold voltage of the MOSFET is 2.0 V.  (a)  What is the component R2? (b) If R1 has a value of 150 kW, calculate the value of R2 that will give a gate-source voltage of 2.0 V

 In the Exam You may well be asked about the input circuit for the MOSFET.  It will involve a voltage divider.    Although the MOSFET amplifier is NOT on the syllabus, you may possibly be asked to do a voltage divider calculation on the output stage.    If one of the resistors is a transducer, you may be asked to lift a resistance from a graph, having been given a value for, say, light level.

MOSFETS need to be handled with care.  They are stored in conductive packages to prevent static electricity building up.  A high voltage can punch a hole through the thin oxide layer in the gate.  That renders the component useless.  Therefore you need to wear an earthing strap before handling a MOSFET.