Physics Notes- Transistors

8/10/2019 Physics Notes- Transistors

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9.4.3 Limitations of past technologies and increased research into the structureof the atom resulted in the invention of transistors

Previous knowledge: discuss how the length, cross-sectional area, temperature and type of material affect themovement of electricity through a conductor, describe the relationship between potential difference, current and

power dissipated, recall the argu ments relating to the ‘wave - particle’ debate of matter and energy

Identify that some electrons in solids are shared between atoms and move freely

Pauli Exclusion Principle

- Proposed by Wolfgang Pauli in 1925- Stated that no two electrons can occupy the same energy level

o This meant that electrons could not all collapse into the lowest energy shell In a gas, two electrons that were well separated could exist in the same energy state, but

when the atoms came closer as a solid , the electrons could not stay in that energy level andspread into an ‘energy band’ (levels that have slight differences forms a band)

Metals- In conducting solids, there is a ‘sea’ of delocalised electrons (free to move around)

o These electrons are the valence electrons which are loosely bound to the atoms and so it’s sharedaround the lattice of positive ions

o Under an electric field (when there is a potential difference), the random motion of the electronsdecreases and begins to have a net motion against the electric field direction

InsulatorsEl b d d i h l i i b d d h bl fi i



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Compare qualitatively the relative number of free electrons that can drift from atom to atom inconductors, semiconductors and insulators

Material Number of free electronsConductors LotsSemi-conductors There are a few once the semi-conductor receives enough energyInsulator None

Identify that the use of germanium in early transistors is related to lack of ability to produce othermaterials of suitable purity

- Semiconductor materials are usually made from group 4 lattices which formcovalent bonds with each other

- Silicon and germanium were predicted to be ideal from the group 4elements due to their semiconducting properties

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Identify differences in p and n-type semiconductors in terms of the relative number of negativecharge carriers and positive holes

Conductor P-Type N-typeNumber of negative chargecarriers

None More

Positive holes More None

Describe differences between solid state and thermionic devices and discuss why solid state devicesreplaced thermionic devices

Thermionic Devices- Used to:

o control the direction of current flowo convert AC to DC (rectification)o

Switch current flow on or offo Amplify a current

- Prior to the invention solid state diodes or the transistor, we used thermionic/valve devices- Uses thermionic emission

o When the filament is heated with an electric current, electrons are liberated and the filament acts asa cathode

DiodeH h d d l ( d )



in the grid current will amplify signals in the anode

Solid State Devices- Uses transistors and integrated circuits to act as a diode

P-N Junction- When a P-type and N-type semiconductor are placed adjacently, the electrons flow from the N-type to P-

typeo This means electrical current only flows in 1 directiono Therefore, it rectifies current in positive half-cycles

- However, as electrons move across to the p-type, we are left with anions at the p-type and cations at the n-type which creates an electric field



Forward Bias (and reverse bias)- By providing a voltage through the P-N Junction with the positive terminal at N and the negative at P, the

depletion zone will narrow as the holes will drift towards the zone and the electrons also drift towards thezone so that electrons can again, ‘cross’ the depletion zone

o 0.6V is required for silicon to conducto 0.3V is required for germanium to conduct

- BUT, with a reverse bias, there is very little ‘leakage’ cu rrent or no current flow and so the diode acts as alarge resistor

o This causes the holes and electrons to flow to their respective terminals and thus, widens thedepletion zone

Device Thermionic Devices Solid State Devices

Examples Radio valves/amplifiers MicroprocessorsSize Large and bulky due to large power requirements

and so batteries were largeSmaller led to miniaturisation ofelectronic devices

EnergyEfficiency- heat

Lots of heat generated by valves sometimes heatwould damage surrounding electronics

Less. More efficient

Energyefficiency-voltage

High voltages required to correctly bias triodes foramplification

Silicon transistor required 0.6V to dothe same job

Durability Fragile- Made of glass- Filament can burn out

More durable, flexible, shock andvibration-resistant

Lifetime Shorter- Valves used in 1950 telephones became

intolerable

Longer

Dangers Metal on the cathode slowly boiled off and reacted

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o One person can direct themselveso People who run on tight schedules (business people) will be able to take the most efficient path

without too much prior planning- Easier to communicate to other people very far away due to computers and mobile phones which require

microchips- Society can access and share knowledge easily through their mobile phones and computers which access theinternet

Advantages- environment

- Better analytical technology for the environment- we can more accurately measure the concentration ofpollutants using more sophisticated apparatus such as AAS

- More energy efficient technology SSD uses less energy than thermionic devices and therefore newmodels of old technology is generally more energy efficient and environmentally friendly

- Since SSDs are smaller than thermionic devices, they require less material to make

Disadvantages-society

- Overly dependent on technologyo If technology fails, much of our society will fail too

Much of the technology is integral to our society e.g. global communications networko Since so little of our lives are left ‘technology -free’, our technological footprin t is hard to cover and

thus, there is less privacy in our lives- Increase in unemployment due to technological changes

o Industrialisation of society- introduction of machines to replace low-level labour- Leisure activities have become more complex

o Younger people generally spend more time indoors using electronic devices rather than doingphysical activities. Made life more sedentary.



an otherwise pure crystal latticeDonor Atoms Impurity atoms that are in excess and therefore

promoted to the conduction bandDiode A thermionic device that contains only 2

electrodes enclosed in a glass vacuum tube.Like a valve

Valve Thermionic device that contains 2 or moreelectrodes enclosed in a glass vacuum tube

Rectification Converting AC current into DC currentTransistor Tiny switch that changes the size/direction of

current.Forbidden Energy Gap Distance between the conduction and valence

energy bands

Bibliography- Charles Sturt University, Physics 9.4: From Ideas to Implementation: 3. Transistors

http://hsc.csu.edu.au/physics/core/implementation/9_4_3/943net.html- S. Bosi, J. O’Bryne, P. Fletcher, J. Khachan, J. Stanger, S. Woodwar d, In2Physics @ HSC

- R. Abeysuriya, Student’s guide to HSC Physics (2009) - M. Andriessen, P. Pentland, R. Gaut, B. McKay, J. Tacon, Jacaranda Physics HSC 2 (2008)



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