Physics

A procedure to model the difference between conductors, insulators and semiconductors

1. Open an empty egg carton (keep sides joined together) and place a ball bearing into each of the egg holders for ONE side. This represents the VALENCE band. The half of the carton WITHOUT the egg holders filled represents the CONDUCTION BAND.
   
   
2. Now slowly tilt the half of the carton with ball bearings towards the empty half.
   
   
3. Repeat the above steps EXCEPT separate the two halves of the carton and place them side-by-side and 15 cm apart. This will represent the forbidden energy gap of an insulator.
   
   
4. Repeat the above steps EXCEPT separate the two halves of the carton and place them side-by-side and 5 cm apart. This will represent the reduced forbidden energy gap of a semiconductor.

Background

When two atoms are close enough to interact with each other the allowed energy levels that the electrons can occupy splits into two distinct, but closely spaced, energy levels. In a three atom system there are three energy levels, and so on. In a crystalline solid there are so many atoms interacting that the energy levels are very close to each other. The electrons in the structure are restricted to one or other of these energy levels. Depending on the nature of the chemical bonding, electrons at particular energy levels can be grouped into bands. There are several types of bands, including:

• the conduction band where the electrons are free to move
• the valence band, which contains electrons that, given the right conditions, can be induced to move into the conduction band
• between these two bands is often a third bans or region which prevents electrons moving between the conduction and valence bands (forbidden energy band).

• In a conductor, the conduction and valence bands overlap. This allows the valence electrons to easily move along the conduction band giving the material low electrical resistance.
  
  
• In insulators, there is a large forbidden energy band, which makes it difficult for valence electrons to move into the conduction band giving the material a high electrical resistance.
  
  
• In semiconductors, the forbidden energy band is not too wide. Under certain conditions, electrons in the valence band can gain sufficient energy to cross the gap. This reduces the electrical resistance of the material.

perform an investigation to model the behaviour of semiconductors including the creation of a hole or positive charge on the atom that has lost the electron and the movement of electrons and holes in opposite directions when an electric field is applied across the semiconductor

• Perform the investigation for whatever aspect you decide to test (elecrton and or whole movement). You may need to make modifications to your procedures as you go. Write an account of your experiment to explain how the model demonstrates the behaviour of electrons and holes in a semiconductor.

identify absences of electrons in a nearly full band as holes, and recognise that both electrons and holes help to carry current

• When an electron in a semiconductor moves into the conduction band it leaves a “hole”, that is, an atom with one less valence electron than normal. An electron from a nearby atom in the valence band can move and fill the hole. This then creates another hole, and so on.
  
  
• The creation of holes and the movement of electrons to fill them is equivalent to an electric current in the semiconductor. Electrons flow in one direction. The apparent movement of holes in the opposite direction can be considered as a flow of positive charge.

compare qualitatively the relative number of free electrons that can drift from atom to atom in conductors, semiconductors and insulators

• Conductors contain high numbers of free electrons in the conduction band. Under normal conditions, insulators and semiconductors have far fewer free electrons than conductors.
  
  
• Raising the temperature, using certain lighting conditions or applying a potential difference, can induce electrons in some semiconductors to move into the conduction band.

gather, process and present secondary information to discuss how shortcomings in available communication technology lead to an increased knowledge of the properties of materials with particular reference to the invention of the transistor

• This investigation can be conducted by gathering a range of resources including scientific journals, CD-ROM encyclopaedias and Internet sites, like the one given below. Focus on collecting information about the shortcomings of vacuum tube technology and the development and strengths of transistor technology.
  
  Transistorized The American Institute of Physics, USA.
  
  
• To process the sources you find, assess their reliability by comparing the information provided. Look for consistency of information.
  
  
• Present the information to other students. You may use visual aids, such as overhead transparency graphics or power point presentation. Keep the information simple and produce just the summary asked for in the syllabus point.

identify that the use of germanium in early transistors is related to lack of ability to produce other materials of suitable purity

• At first, germanium was widely used as a semi-conductor because it was easier to purify than other known semi-conductors, such as silicon.
  
  
• Silicon eventually replaced the germanium as semi conducting material of choice in transistors because :
  • it is the second most abundant element on earth by weight, which means it is relatively cheap
  • it retains its semi conducting properties at relatively high temperatures (when compared to germanium)
  • it can handle higher electric currents before overheating (which destroys its semi conductor properties)
  • it forms an oxide that can be doped and made into thin, flat layers
  • processing techniques were developed to produce very pure, single crystal forms
  • in single-crystal form (very pure silicon), the molecular structure of the material is uniform, thus ensuring consistency of properties.

describe how ‘doping’ a semiconductor can change its electrical properties

• Doping is the addition of an impurity (such as gallium or arsenic) to a semiconductor in the ratio of about one part per million.
  
  
• The atoms of the doping element need to fit reasonably well into the semi-conductor lattice structure so as not to distort it and impede electron flow.
  
  
• The doping element needs to have either one more valence electron than the semi-conductor itself or one less valence electron than the semi-conductor material.
  
  
• Doping increases the potential conductivity of the semiconductor (extra electrons or holes to act as charge carriers).

identify differences in p and n-type semiconductors in terms of the relative number of negative charge carriers and positive holes

• In p-type semiconductors there are more positive holes than negative charge carriers. Elements such as aluminium and gallium (3 valence electrons) are used as doping agents with silicon to produce p-type semiconductors.
• In n-type semiconductors there are more negative charge carriers than positive holes. Elements, such as arsenic and phosphorus (5 valence electrons), are used as doping agents with silicon to produce n-type semiconductors.

  

describe differences between solid state and thermionic devices and discuss why solid state devices replaced thermionic devices

• A thermionic device contains a cathode that emits electrons only when heated to a high temperature. It requires a separate heating circuit to heat the cathode, which takes time to heat up. A solid state device uses semiconductors to generate a flow of electrons and does not require a heating circuit.
  
  
• Solid state devices work immediately, require less power and then produce less heat than equivalent thermionic devices.
  
  
• A thermionic device requires a near vacuum to allow electrons to flow between the electrodes, thus they are commonly packaged in an evacuated glass tube. Solid state devices operate at normal air pressures and are commonly packaged in thermosetting plastic.
  
  
• Modern solid-state devices are very much smaller than thermionic devices, allowing electronic equipment to be reduced in size.
  
  
• The combined advantages of smaller size, simpler and cheaper construction, lower power requirements and speed of operation make solid state devices more attractive to electronics manufacturers than equivalent thermionic devices.

identify data sources, gather, process, analyse information and use available evidence to assess the impact of the invention of transistors on society with particular reference to their use in microchips and microprocessors

• In deciding the type of data necessary for this investigation, you need to:
  1. select appropriate sources
  2. consider the type of information about the invention of the transistor that must be gathered.
  
  
• Scientific journals and the Internet should be good data sources to gather information about the impact of the discovery of the transistor. Use a search engine and type in some words or phrases like History of the Transistor, How Microchips Work, The Basics of Microprocessors.
  
  
• You should process this information by only selecting material that is relevant to the discovery of the transistor and to its impact on
  
  
• Analyse the information to make a generalisation regarding the impact of the devices using transistors on society.
  
  
• You can justify the generalisation by seeking evidence for how aspects of society have changed due to the use of transistors in the devices you have chosen

identify data sources, gather, process and present information to summarise the effect of light on semiconductors in solar cells

• To find up to date information go to the Internet. Use a search engine and type in words such as 'light', 'semiconductor', 'solar cells' and 'effect'. Skim the information and choose that which is relevant to your task.You might choose two or three different sources. You may also look in CD ROM encyclopedias.

• Process the information by assessing the reliability of it from all the sources. You may choose to incorporate information from several sources.

• Present your data using an appropriate medium. What you choose will depend partly on the audience. You may choose a power point presentation, an overhead transparency or just give an oral or written presentation with a diagram.

• Some information that could get you started is: When light strikes a semiconductor material a certain portion of the light (depends upon the covering of the semiconductor) is absorbed into the semiconductor material. The energy of the absorbed light, in the form of photons, is transferred to the semiconductor resulting in electrons and positively charged holes moving across the PN-junction in opposite directions. An electric field within the photovoltaic cell acts to force the electrons in a certain direction. A metal grid on either side of the solar cell allows the electrons to collect and, if connected to an external circuit, a current will flow.