Semiconductor theory

Semiconductor Theory

Electrical and Electronic Principles

© University of Wales Newport 2009 This work is licensed under a Creative Commons Attribution 2.0 License.

The following presentation is a part of the level 4 module -- Electrical and Electronic Principles. This resources is a part of the 2009/2010 Engineering (foundation degree, BEng and HN) courses from University of Wales Newport (course codes H101, H691, H620, HH37 and 001H). This resource is a part of the core modules for the full time 1 st

year undergraduate programme.

The BEng & Foundation Degrees and HNC/D in Engineering are designed to meet the needs of employers by placing the emphasis on the theoretical, practical and vocational aspects of engineering within the workplace and beyond. Engineering is becoming more high profile, and therefore more in demand as a skill set, in today’s high-tech world. This course has been designed to provide you with knowledge, skills and practical experience encountered in everyday engineering environments.

Contents Silicon Periodic Table of the Elements N-Type Impurity P-Type Impurity Creating A Diode Forward Bias Reverse Bias Diode Equation Rectification Credits

In addition to the resource below, there are supporting documents which should be used in combination with this resource. Please see:Green D C, Higher Electrical Principles, Longman 1998 Hughes E , Electrical & Electronic, Pearson Education 2002Hambly A , Electronics 2nd Edition, Pearson Education 2000Storey N, A Systems Approach, Addison-Wesley, 1998


Silicon is the most common metalloid. It is a chemical element, which has the symbol Si and atomic number 14. A tetravalent metalloid, it is less reactive than its chemical analogue carbon.Silicon is the eighth most common element in the universe by mass, but very rarely occurs as the pure free element in nature. It is more widely distributed in dusts, sands, planetoids and planets as various forms of silicon dioxide (silica) or silicates. In Earth's crust, silicon is the second most abundant element after oxygen, making up 27.7% of the crust by mass.Silicon has many industrial uses. It is the principal component of most semiconductor devices, most importantly integrated circuits or microchips. * The above text is taken from http://en.wikipedia.org/wiki/Silicon and is available under the Creative Commons Attribution-ShareAlike License.

Silicon.

Silicon is widely used in semiconductors because it remains a semiconductor at higher temperatures than the semiconductor germanium and because its native oxide is easily grown in a furnace and forms a better semiconductor/dielectric interface than any other material.

In the form of silica and silicates, silicon forms useful glasses, cements, and ceramics. It is also a constituent of silicones, a class-name for various synthetic plastic substances made of silicon, oxygen, carbon and hydrogen, often confused with silicon itself.* The above text is taken from http://en.wikipedia.org/wiki/Silicon and is available under the Creative Commons Attribution-ShareAlike License.

5B

Boron

6C

Carbon

7N

Nitrogen

13Al

Aluminium

14Si

Silicon

15P

Phosphorus

31Ga

Gallium

32Ge

Germanium

33As

Arsenic

Thew term tetravalent means that Carbon, Silicon and Germanium have four electrons in their outermost shell (orbital).

Silicon.

When pure Silicon crystallises it bonds with the four atoms in its immediate vicinity. This means that each electron is shared by two atoms and the result is shown below.

Si

Si

Si

Si

Si

Si

Si

Si

Si


Metals conduct electricity well as they have electrons which are not tied into the lattice – these are called "free electrons“. Silicon crystals look like a metal but as previously stated all the electrons are held firmly in place. This means that pure silicon is an insulator and does not allow the conduction of electricity. The sharing of electrons between atoms is called covalent bonding. Doping SiliconIf, instead of pure Silicon you introduce a small quantity of an impurity then the Silicon changes its nature.

5B

Boron

6C

Carbon

7N

Nitrogen

13Al

Aluminium

14Si

Silicon

15P

Phosphorus

31Ga

Gallium

32Ge

Germanium

33As

Arsenic

There are two types of impurities:N-type - By doping pure silicon with Group V elements such as phosphorus, extra valence electrons are added that become unbonded from individual atoms and allow the compound to be an electrically conductive n-type semiconductor. http://en.wikipedia.org/wiki/Silicon and is available under the Creative Commons Attribution-ShareAlike License.

P

Si

Si

Si

Si

Si

Si

Si

Si

The extra (fifth) electron has nothing to hold it in place and so it is free to move around and hence free to carry current.

This is called n-type as the electron is negatively changed

Note the overall material is not changed.

P-type - Doping with Group III elements, which are missing the fourth valence electron, creates "broken bonds" (holes) in the silicon lattice that are free to move. The result is an electrically conductive p-type semiconductor. http://en.wikipedia.org/wiki/Silicon and is available under the Creative Commons Attribution-ShareAlike License.

B

Si

Si

Si

Si

Si

Si

Si

Si

The newly created hole has nothing to hold it in place and so it is free to move around and hence free to carry current.

This is called p-type as the hole is positively changed

Note, again, the overall material is not changed.

A minute amount of either N-type or P-type doping (typically one part per million) turns a silicon crystal from a good insulator into a viable (but not great) conductor -- hence the name "semiconductor."

N-type and P-type silicon are not that amazing by themselves; but when you put them together, you get some very interesting behaviour at the junction.


Creating a DiodeWhen you connect electrically N-type and P-type silicon as shown below, you find that the following happens and this produces a useful effect.

p type n typeNegative Positive

The electrons in the N-type will move across the junction to recombine with the holes in the P-type. This would continue until the regions were depleted of carries (holes or electrons). There are though two factors which limit the number which cross the junction.

The N-type is loosing electrons therefore it becomes positively charged which attracts the electrons and reduces the chance of them crossing the barrier.

The P-type is gaining electrons making it negatively

charged. This repels the electrons wishing to cross into the P-type.

If we were able to look at the charge along the PN device we would see:

P-type N-type Semiconductor Theory

The value of potential at which the system stabilises is about 0.6 volts for Silicon.

For Germanium (which was the first semiconductor material used for electronic devices) the value is about 0.2 volts.

Let us look at what happens if we now place a voltage across the PN junction.


Forward Bias.

V

p n

The voltage source will attempt to drive electrons around the circuit anticlockwise (opposite direction to conventional current flow).

If V is less than the junction potential (Vj) for Si, electrons flowing into the N-type will see the negative barrier and therefore current flow will be extremely small.

V

Vj

e e e e

As V increases the barrier gets smaller and when V = Vj the barrier disappears and current flow increases.

Any increase in V above Vj will result in damage to the junction as the barrier becomes inverted and draws electrons across it. If we do use this device we need to ensure that we limit the current flowing through it.

V

Vj

e e e e


Reverse Bias.

V

p n

With the supply voltage reversed the Junction potential is modified as follows:

V

Vj

e e e e

With the increased barrier there will in theory be no flow of current. In practice there will exist within any crystal structure doped or not carriers of both types (holes and electrons).


These are due to temperature and are called Thermally Generated Hole Electron Pairs. As the temperature rises electrons within the lattice are given more energy and it is possible for them to break free. This produces a hole in the lattice and a free electron. The number of these is small but increases with temperature.

Si

Si

Si

Si

Si

Si

Si

Si

Si


These opposite polarity carriers mean that there will be a small current flow and this is referred to as the Reverse Leakage Current (Io).In N-type Silicon –

The majority carriers are electrons The minority carriers are holes

In P-type Silicon – The majority carriers are holes The minority carriers are electrons

The PN junction is referred to as a diode and its symbol reflects its operation:

Shows that current flows in this direction

Shows current does not flow in this direction


The P-type side is called The N-type side is calledthe Anode (A) the Cathode (K)


Diode equationThere exists a relationship between the voltage applied to the diode and the current flowing through it. It has the following form:

1KTVq

o eII

whereI is the current flowing through the diodeIo is the reverse leakage current (typically 1 x 10-

10 A)V is the applied voltageq is the charge on an electron 1.602 x 10-19 CK is Boltzmann’s Constant 1.38 x 10-23 JK-1

T is the absolute temperature (ºC + 273)Semiconductor Theory

For room temperature the constants can be combined to give a single value:When temperature is 20ºC.

This is normally quoted as 40.Therefore the equation becomes:

Determine for the following values of V

6239.KT

q

140 Vo eII

Ve40

V

0.05 7.39

0.1 54.6

0.2 2980

0.3 162754

0.4 8886110

Ve40 From the table we can see that as long as V is greater than 0.1 the –1 in the equation can be ignored. The equation therefore becomes: Vo eII 40


If Io = 1 x 10-10A then determine the how the current varies with applied voltage.

V I

0.05 739pA

0.1 5.46nA

0.2 298nA

0.3 16.3μA

0.4 889μA

0.5 48.5mA

0.6 2.65A

0.7 145A

The graph shows this as a plot of Current I against Applied Voltage V.

0

1

2

3

4

5

6

7

8

9

10

0.5 0.55 0.6 0.65

Voltage

Cur

rent

You can see that up to about 0.6 volts the current is relatively small whilst above 0.6 the current increases rapidly.

Rectification.This is the process of converting an A.C. input into a D.C. output.A.C. – This is a signal that periodically changes polarity. Examples include the mains voltage and signals generated from acoustic sources.

AC mains signal

-400

-200

0

200

400

time

volta

ge

D.C. – This type of signal never changes polarity and so it will be either positive or negative. Batteries will generate D.C. outputs.Semiconductor Theory

There are three basic rectifier circuits:Half Wave Rectification

VpLOAD

The diode will remove the negative half cycle leaving only the positive. Though the voltage is fluctuating it is D.C.

Half Wave Rextifi ed

time

volt

age

Vp

Full Wave Rectification

VpLOAD

The diodes will remove the negative half cycles from the two waveforms, which are 180º out of phase leaving only the positive half cycles. These are then summed to give the output.Full Wave Rectifi ed

time

volt

age

Vp/2

This produces a smoother output but with a reduced voltage.

Bridge Rectification

Vp

LOAD

The diodes direct the flow to the load differently depending upon the polarity of the input A.C.

If the input is + on the top, diodes 2 and 3 conduct.

+

-+

-

If + on the bottom diodes 1 and 4 conduct.

This ensures that the polarity at the load never changes.Semiconductor Theory

Bridge Rectified

time

volta

ge

Vp

Note that at any moment in time two diodes are conducting which means that the output peak is actually Vp – 1.2v (2 x 0.6v)



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