2 Fundamental Solid-state Principles

Semiconductors Fundamental Solid-State Principles

Introduction A semiconductor is a material that is neither a

good conductor nor a good insulator, but rather, lies in between the two. In their purest form, semiconductors have few applications in electronics. However, when the characteristics of a pure semiconductor are altered through a process known as doping, many useful electronic devices can be developed.

Outline 1. atomic theory 2. doping 3. pn junction 4. bias

1. Atomic theory Atom Bohr model

Atomic Theory Valence shell outermost shell Determines the conductivity of the atom Can contain up to 8 e

Atomic theory 1 valence e-  perfect conductor 8 valence e-  insulator 4 valence e-  semiconductor  Conductivity decreases with an increase in the

number of valence electrons.

Semiconductors Silicon (Si) - 14 Germanium (Ge) - 32 Carbon (C) – 6  Carbon is used primarily in the production of

resistors and potentiometers. Silicon is commonly used than germanium because it is more tolerant of heat.

Charge and conduction  The net charge of an atom is zero.  Positive net charge  Negative net charge  Fundamental laws  Electrons travel in orbital shells  Each orbital shell relates to a specific energy range

(Which orbital electrons have higher energy levels?)  For an electron to jump from its shell to one having a higher energy level, it must absorb enough energy to make up the difference between the energy levels of the two shells.  If an electron absorbs enough energy to jump from one shell to another, it will eventually give up the energy it absorbed and return to a lower-energy shell. 

Energy gaps  Energy gap – difference between energy levels of any two orbital

shells  For conductors, semiconductors, and insulators, the valence to conduction-band energy gaps are approximately 0.4, 1.1, and 1.8 eV.  The higher the energy gap, the harder it is to cause conduction. (Why?)  Excited state

Covalent bonding  Covalent bonding – a means of holding atoms together by

sharing valence electrons.

 The atoms are held together, forming a solid substance.  The atoms are electrically stable, because their valence shells

are complete.  The completed valence shells cause the silicon to act as an insulator.  Thus, intrinsic (pure) silicon or germanium is a very poor conductor.

Why is silicon more often used than germanium? At room temperature, a silicon crystal has

fewer free electrons than a germanium. Hence, silicon is a better insulator ( desirable characteristic).  How about carbon?

Conduction Hole  a gap in the covalent bond Electron-hole pair  a free electron and its

matching valence band hole Recombination  when a free electron returns to the valence shell Lifetime  the time from electron-hole pair generation to recombination

Conduction versus Temperature At room temperature, thermal energy (heat)

causes the constant creation of electron-hole pairs, with their subsequent recombination. A semiconductor always has some number of free electrons even when no voltage is applied to the element. temperature, free electrons -273 degree Celsius  absolute zero !!!Conductivity in a semiconductor varies directly with temperature.

SECTION REVIEW 1.What are the 3 particles that make up the atom? 2.What is the relationship between the number of valence electrons and the conductivity of a given element? 3.How many valence e- are there in a conductor? An insulator? A semiconductor? 4.What 3 semiconductor elements are most commonly used in electronics? 5.What are the relationships between electrons and orbital shells ? 6.What is an energy gap? What are the energy gap values for insulators, semiconductors, and conductors? 7.What forms of energy are given off by an electron that is falling into the valence band from the

SECTION REVIEW 8. What is covalent bonding? 9. What are the effects of covalent bonding on intrinsic semiconductor materials? 10. What is an electron-hole pair? 11. What is recombination? 12. How long is the typical lifetime of an electron-hole pair? 13. What is the relationship between temperature and conductivity? 14. Why aren’t electron-hole pairs generated in a semiconductor when its temperature drops to absolute zero? 

2. Doping Doping – process of adding impurity atoms to

intrinsic silicon or germanium to improve the conductivity of the semiconductor. Extrinsic – impure Two element types used for doping  Trivalent (acceptor atoms)  Pentavalent (donor atoms)

P-type material N-type material

Doping elements Trivalent impurities Aluminum (Al) Gallium (Ga) Boron (B) Indium (In)

Pentavalent impurities Phosphorus (P) Arsenic (As) Antimony (Sb) Bismuth (Bi)

N-type vs P-type materials N-type materials

 Pentavalent + intrinsic = n-type  N-type materials are still electrically neutral!  Majority carriers: conduction band/free electrons  Minority carriers: valence band holes

P-type materials oTrivalent + intrinsic = p-type oMajority carriers : hole oMinority carriers: free electrons oP-type materials are still electrically neutral!

Section review 1.What is doping? Why is it necessary? 2.What is an impurity element? 3.What are trivalent and pentavalent elements? 4.Despite their respective characteristics, n-type and p-type materials are still electrically neutral. Why? 5.In what ways are n-type and p-type materials similar? In what ways are they different?

1-3 The PN Junction n-type + p-type = useful pn junction Diffusion results:  One net + charge in the n-type material  One net – charge in the p-type material 

Depletion layer  At a large-scale picture:  Each electron that diffuses across the

junction leaves one positively charged bond in the n-type material and produces one negatively charged bond in the p-type material.  Both conduction-band electrons (free electrons) and valence-band holes are needed for conduction through the materials. When an electron diffuses across a junction, the n-type material lost a conduction-band electron. When the electrons fall into a hole in the p-type material, the material has lost a valence band- hole. Both bonds have been depleted of charge carriers.

Section review 1.What is the overall charge on an n-type covalent bond that has just given up a conduction-band electron? 2.What is the overall charge on an p-type covalent bond that has just accepted an extra valence-band electron? 3.Describe the forming of the depletion layer. 4.What is barrier potential? What causes it?

1-4 Bias A PN junction is useful because we can control

the width of the depletion layer. By controlling the depletion layer width, we control the resistance, thus we can control the amount of current that can pass through a device.  dep. layer width, resistance, junction current   dep. layer width, resistance, junction current

Bias – potential applied to a pn junction to

obtain a desirable mode of operation.

Bias Two types:  Forward bias  Reverse bias

Forward bias – potential used to reduce the

resistance of the pn junction Reverse bias – potential used to increase the resistance of the pn junction

Forward Bias  When the applied voltage causes the n-type material to

be more negative than the p-type material.  Allows current to pass  Majority carriers in the n-type and p-type materials are pushed toward the junction.  Minority carriers in the n-type and p-type materials are drawn away from the junction.

Forward bias  Bulk resistance  combined resistance of the n-type and p-type

materials in a FB pn junction  Typically in the range of 5 ohms or less  Forward voltage (VF)  Voltage across a FB pn junction  0.7 v for silicon  0.3 v for germanium

 Two ways of forward biasing a pn junction:  By applying a potential to the n-material that

drives it more negative than the p-material  By applying a potential to the p-material that drives it more positive than the n-material

Reverse bias  When the applied potential causes the n-type material to be

more positive than the p-type material.  Depletion layer becomes wider, junction current reduced to near zero  Majority carriers in the n-type and p-type materials are drawn away from the junction  Minority carriers in the n-type and p-type materials are pushed toward the junction  Diffusion current  Majority carrier current during the time the depletion layer is growing 



Reverse bias Two ways to reverse bias a junction:  By applying a potential to the n-type material

that drives it more positive than the p-type material  By applying a potential to the p-type material that drives it more negative than the n-type material

Bottom line Bias type

Junction polarities

Junction resistance

Forward

N-type is more Extremely low (-) than p-type

reverse

P-type is more Extremely high (-) than n-type

Final Note Silicon is more tolerant of heat and more

abundant than germanium. It is also a better insulator than germanium (at room temperature) Germanium oxide is water soluble, making germanium more difficult to process than silicon. When heated, silicon readily produces silicon dioxide, which is critical to the production of integrated circuits. Germanium lacks any comparable property.

Section review 1.What are the resistance and current characteristics of a pn junction when its depletion layer is at its maximum width? Its minimum width? 2.What purpose is served by the use of bias? 3.What effect does forward bias have on depletion layer width of a pn junction? 4.What is the junction resistance of a FB pn junction? 5.What are the approximate values of VF for FB Si and Ge pn junctions?

Section review 6.List the commonly used methods for forward biasing a pn junction? 7.What effect does reverse bias have on the depletion layer width of a pn junction? 8.What is the junction resistance of a reversebiased pn junction? 9.List the commonly used methods for reverser biasing a pn junction. 10.Why is silicon more commonly used than the germanium in the production of solid-state components? 11. 12.