A Junção PN Site original: http://physics.slss.ie/forum Animações: © Declan O’Keeffe Adaptação e tradução para o português: © Ewaldo L. M. Mehl

Isolantes Isolantes possuem elétrons densamente ligados em sua camada de valência, geralmente em ligações covalentes Estes elétrons necessitam nível de energia muito elevado para torna-los elétrons de condução Vamos aplicar uma diferença de potencial sobre o material isolante acima… A energia fornecida a cada elétron não é suficiente para romper as ligações covalentes Isolantes possuem portanto alta resistividade Slide 2

Condutores Condutores tem elétrons fracamente ligados aos átomos em sua camada de valência Estes elétrons requerem pouca quantidade de energia para torna-los elétrons de condução Valos aplicar uma DDP através do condutor… A força sobre cada elétron é suficiente para liberta-lo da sua órbita e assim os elétrons podem “pular” para outros átomos: existe portanto condução da corrente elétrica Condutores possuem portanto baixa resistividade Slide 3

Semicondutores Semicondutores apresentam resistividade intermediária entre a dos condutores e isolantes Os elétrons de um semicondutor participam de ligações covalentes e por isso, à princípio, não estariam disponíveis para a condução da corrente elétrica; no entanto, um nível de energia relativamente BAIXO já é capaz de colocar alguns elétrons em estado de condução Este fenômeno era já conhecido para o silício e para o germânio Slide 4

Silício (Si) O Siício tem valência 4, i.e. 4 elétrons na camada mais externa Cada átomo de silício compartilha seus 4 elétrons de valência com 4 átomos vizinhos Nesta representação os elétrons compartilhados são mostrados como ligações covalentes na forma de linhas horizontais e linhas verticais ligando os átomos Esta figura mostra apenas os elétrons compartilhados Slide 5

Silício: rede cristalina (simplificada) Agora estendemos o arranjo através de um pedaço de cristal de silício… Obtemos uma REPRESENTAÇÃO SIMPLIFICADA de um cristal de silício Esta seria a situação do cristal na temperatura de 0 K Não há elétrons livres : o cristal não conduz corrente elétrica e portanto se comporta como se fosse um MATERIAL SOLANTE Slide 6

Electron Movement in Silicon However, if we apply a little heat to the silicon…. An electron may gain enough energy to break free of its bond… It is then available for conduction and is free to travel throughout the material Slide 7

Hole Movement in Silicon Let’s take a closer look at what the electron has left behind There is a gap in the bond – what we call a hole Let’s give it a little more character… Slide 8

Hole Movement in Silicon This hole can also move… An electron – in a nearby bond – may jump into this hole… Effectively causing the hole to move… Like this… Slide 9

Heating Silicon We have seen that, in silicon, heat releases electrons from their bonds… This creates electron-hole pairs which are then available for conduction Slide 10

Intrinsic Conduction Take a piece of silicon… And apply a potential difference across it… This sets up an electric field throughout the silicon – seen here as dashed lines When heat is applied an electron is released and… Slide 11

Intrinsic Conduction The electron feels a force and moves in the electric field It is attracted to the positive electrode and re-emitted by the negative electrode Slide 12

Intrinsic Conduction Now, let’s apply some more heat… Another electron breaks free… And moves in the electric field. We now have a greater current than before… And the silicon has less resistance… Slide 13

Intrinsic Conduction If more heat is applies the process continues… More current… Less resistance… The silicon is acting as a thermistor Its resistance decreases with temperature Slide 14

The Thermistor The thermistor is a heat sensitive resistor When cold it behaves as an insulator i.e. it has a very high resistance When heated, electron hole pairs are released and are then available for conduction as has been described – thus its resistance is reduced Thermistor Symbol Slide 15

The Thermistor Thermistors are used to measure temperature They are used to turn devices on, or off, as temperature changes They are also used in fire-warning or frost-warning circuits Thermistor Symbol Slide 16

The Light Dependent Resistor (LDR) The LDR is very similar to the thermistor – but uses light energy instead of heat energy When dark its resistance is high As light falls on it, the energy releases electron-hole pairs They are then free for conduction Thus, its resistance is reduced LDR Symbol Slide 17

The Light Dependent Resistor (LDR) LDR’s are used as light meters LDR’s are also used to control automatic lighting LDR’s are used where light is needed to control a circuit – e.g. Light operated burgler alarm LDR Symbol Slide 18

The Phosphorus Atom Phosphorus is number 15 in the periodic table It has 15 protons and 15 electrons – 5 of these electrons are in its outer shell Slide 19

Doping – Making n-type Silicon Relying on heat or light for conduction does not make for reliable electronics Suppose we remove a silicon atom from the crystal lattice… and replace it with a phosphorus atom We now have an electron that is not bonded – it is thus free for conduction Slide 20

Doping – Making n-type Silicon Let’s remove another silicon atom… and replace it with a phosphorus atom As more electrons are available for conduction we have increased the conductivity of the material Phosphorus is called the dopant If we now apply a potential difference across the silicon… Slide 21

Extrinsic Conduction – n-type Silicon A current will flow Note: The negative electrons move towards the positive terminal Slide 22

N-type Silicon This type of silicon is called n-type From now on n-type will be shown like this. This type of silicon is called n-type This is because the majority charge carriers are negative electrons A small number of minority charge carriers – holes – will exist due to electrons-hole pairs being created in the silicon atoms due to heat The silicon is still electrically neutral as the number of protons is equal to the number of electrons Slide 23

The Boron Atom Boron is number 5 in the periodic table It has 5 protons and 5 electrons – 3 of these electrons are in its outer shell Slide 24

Doping – Making p-type Silicon As before, we remove a silicon atom from the crystal lattice… This time we replace it with a boron atom Notice we have a hole in a bond – this hole is thus free for conduction Slide 25

Doping – Making p-type Silicon Let’s remove another silicon atom… and replace it with another boron atom As more holes are available for conduction we have increased the conductivity of the material Boron is the dopant in this case If we now apply a potential difference across the silicon… Slide 26

Extrinsic Conduction – p-type silicon A current will flow – this time carried by positive holes Note: The positive holes move towards the negative terminal Slide 27

P-type Silicon This type of silicon is called p-type From now on p-type will be shown like this. This type of silicon is called p-type This is because the majority charge carriers are positive holes A small number of minority charge carriers – electrons – will exist due to electrons-hole pairs being created in the silicon atoms due to heat The silicon is still electrically neutral as the number of protons is equal to the number of electrons Slide 28

The p-n Junction Suppose we join a piece of p-type silicon to a piece of n-type silicon We get what is called a p-n junction Remember – both pieces are electrically neutral Slide 29

The p-n Junction When initially joined electrons from the n-type migrate into the p-type – less electron density there When an electron fills a hole – both the electron and hole disappear as the gap in the bond is filled This leaves a region with no free charge carriers – the depletion layer – this layer acts as an insulator Slide 30

The p-n Junction 0.6 V As the p-type has gained electrons – it is left with an overall negative charge… As the n-type has lost electrons – it is left with an overall positive charge… Therefore there is a voltage across the junction – the junction voltage – for silicon this is approximately 0.6 V Slide 31

The Reverse Biased P-N Junction Take a p-n junction Apply a voltage across it with the p-type negative n-type positive Close the switch The voltage sets up an electric field throughout the junction The junction is said to be reverse – biased Slide 32

The Reverse Biased P-N Junction Thus, the depletion layer ( INSULATOR ) is widened and no current flows through the p-n junction Negative electrons in the n-type feel an attractive force which pulls them away from the depletion layer Positive holes in the p-type also experience an attractive force which pulls them away from the depletion layer Slide 33

The Forward Biased P-N Junction Take a p-n junction Apply a voltage across it with the p-type postitive n-type negative Close the switch The voltage sets up an electric field throughout the junction The junction is said to be forward – biased Slide 34

The Forward Biased P-N Junction Negative electrons in the n-type feel a repulsive force which pushes them into the depletion layer Positive holes in the p-type also experience a repulsive force which pushes them into the depletion layer Therefore, the depletion layer is eliminated and a current flows through the p-n junction Slide 35

The Forward Biased P-N Junction At the junction electrons fill holes Both disappear as they are no longer free for conduction They are replenished by the external cell and current flows This continues as long as the external voltage is greater than the junction voltage i.e. 0.6 V Slide 36

The Forward Biased P-N Junction If we apply a higher voltage… The electrons feel a greater force and move faster The current will be greater and will look like this…. The arrow shows the direction in which it conducts current The p-n junction is called a DIODE and is represented by the symbol… Slide 37

The Semiconductor Diode The semiconductor diode is a p-n junction In reverse bias it does not conduct In forward bias it conducts as long as the external voltage is greater than the junction voltage A diode should always have a protective resistor in series as it can be damaged by a large current Slide 38

The Semiconductor Diode The silver line drawn on one side of the diode represents the line in its symbol This side should be connected to the negative terminal for the diode to be forward biased Diodes are used to change alternating current to direct current Diodes are also used to prevent damage in a circuit by connecting a battery or power supply the wrong way around Slide 39

The Light Emitting Diode (LED) Some diodes emit light as they conduct These are called LED’s and come in various colours LED’s have one leg longer than the other The longer leg should be connected to the positive terminal for the LED to be forward biased LED’s are often used as power indicators on radios, TV’s and other electronic devices Symbol Slide 40

The Characteristic Curve of a Diode Diodes do not obey Ohm’s Law A graph of CURRENT vs VOLTAGE for a diode will not be a straight line through the origin The curve will look like this one Note how the current increases dramatically once the voltage reaches a value of 0.6 V approx. i.e. the junction voltage This curve is known as the characteristic curve of the diode Slide 41

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