Conductors, Semiconductors and Insulators Conductors require free negative charge carriers : electrons (shown in red) These conductor electrons are the outermost or “ valence” electrons in the furthest orbit of the atom and are free to move about. In conductors, they are not tightly bound in by the positive nucleus that is why are free to move around. The black dot here represents the nucleus plus all the electron below the outer orbit: so it is positive. Conductors In terms of energy bands, the valance band overlaps with the conduction band. This results in the electrons being free to flow under and electric field applied by the battery. In the diagram below the electrons flow from right to left. + - Insulators In an insulator all the outermost electrons within the structure are “bound in” and unable to move freely. Above is shown the structure of a plastic, where all of the outermost electrons are covalently bonded. That is each outermost electron is attracted to both its own nucleus and simultaneously its nearest neighbouring nucleus. It is thus unable to move freely as a mobile charge carrier. Insulators In terms of a band diagram and insulator looks like this:- The valance band is the shell or orbit of a materials outermost electrons. There are no electrons in the conduction band and a very large band gap between the valance band and the conduction band. The valence band is completely filled. Only if a very high voltage is placed across an insulator (e.g. a plastic) will electrons jump up into the conduction band from the valance band. This would then rupture the plastic with a small lightening spark. Semiconductors Intrinsic Silicon In an intrinsic semiconductor all the outermost electrons within the structure appear to be “bound in” and unable to move freely -just like an insulator. And at absolute zero no conduction takes place. However only a small amount of thermal energy is needed to free an electron from its location and become a free negative charge carrier . At the same time a hole is left behind: i.e. a free positive charge carrier is created. So at room temperature there are an equal number of both free positive and negative charge carriers i.e. holes and electrons. How “holes”-positive charge carriers move Cathode -ve Anode +ve As electrons jump into the adjacent hole in the structure, the hole appears to move across the structure, much the same way as a positive charge would. We call these positive charge carriers “holes”. If a voltage is applied to an intrinsic semiconductor, then both the electron and the holes can contribute to a small current flow. Cathode -ve Anode +ve The holes flow towards the cathode away from the anode, and the electrons towards the anode and away from the cathode. Intrinsic semiconductor –band diagram At absolute zero there is no thermal energy and so all the electrons are bound into the valance band. Above zero however , since the band gap is very small, thermal excitation can cause electrons to jump from the valance band into the conduction band leaving a corresponding hole behind in the valance band. Temperature at absolute zero Temperature above absolute zero Intrinsic semiconductor band diagram –current flow. If a voltage is applied to an intrinsic semiconductor, then both the electron and the holes can contribute to a small current flow. n type doping A neutral valence 5 atom is added, resulting a free electron being produced. At absolute zero -with no thermal energy available, the electron lies just below the conduction band. However with only a little thermal energy it moves up to the conduction band and the electrons are then free to move. The resistance decreases with n type doping, more negative free charge carriers become available, but the semiconductor remains NEUTRAL. p type doping p-type band structure at absolute zero In p type doping, neutral valence 3 atoms are added, resulting in free “holes” being produced (at room temperature). p-type band structure above absolute zero At absolute zero -with no thermal energy available, the electron (that will move to produce a hole) lies at the top of the valance band. No free “holes” are present. But above 0k ,with only a little thermal energy the electron moves up to the acceptor level and the holes are then free to move at the top of the valance band. The resistance decreases with p type doping, more positive free charge carriers become available, but the semiconductor remains NEUTRAL. p-n junction With a diode or a p-n junction, p type and n type semiconductors are joined. At the junction a depletion layer forms: electrons from the n type jump into the holes at the p type. This leaves the n type side of the junction positive and makes the p type side negative. Reverse bias Vs If the p-n junction is connected to a dc supply with the positive side connected to the n-type and the negative side to the p-type the depletion layer is widened. A tiny leakage negligible current is the result. Vs “Neglegible” leakage current (micro amperes) I (μA) p-n junction in forward bias Vs I (mA) A p-n junction is a diode – current flows in forward bias but not in reverse bias Vo Vs The voltage required to overcome the depletion layer is known as the “striking voltage” Vo. If the p-n junction is connected to a dc supply with the negative side connected to the n-type and the positive side to the p-type the depletion layer is reduced to zero, as the supply voltage is increased. A p-n junction (diode) graph I (mA) Forward bias: After overcoming the depletion layer the current then increases with the voltage Vs (volts) Reverse bias: “Neglegible” leakage current (micro amperes) When a light emitting diode is operating in forward bias and a current is flowing, the electrons and holes meet at the junction and recombine, resulting in a photon being emitted. The energy of the photon is equal to the energy gap. Since the energy of the photon has the equation βπ πΈπβ = βπ = λ It means the bigger the frequency given off by the LED the bigger the band gap. Or the longer the wavelength given off by the LED the smaller the band gap. LED in forward bias-colour and striking voltage I (mA) Vs (volts) Vo Vo Vo Vo Vo Vo As the frequency of the light of the LED increases so the striking voltage Vo increases It turn out that E=eVo gives an approximation to the band gap energy the electron jumped to produce the photon. (https://www.ecse.rpi.edu/~schubert/Light-Emitting-Diodes-dot-org/LED-slide-show.pdf page 35 “Forward voltage is approximately equal to Eg / e” ( i.e. Egap≈eVo) Work done W=QV The work done moving a charge across a voltage is W=QxV Hence the energy moving an electron charge e through a voltage Vo is W=QV=eVo The charge on an electron is e=1.6x10^-19 coulombs. So the work done moving an electron across 1V is W=QV=eV=1.6x10^-19 x1=1.6x10^-19 J i.e. 1eV=1.6x10^-19 J Eph=hf=hc/λ When a photon of light is emitted, an electron moves from a high energy level to a lower energy level. At a p-n junction of an LED in forward bias, electrons from the conduction band descend to the valence band and, as electron hole recombination takes place, photons are emitted. The bigger the band gap the higher the frequency and the longer the wavelength. Irradiance=Power/Area (Intensity) Units watts per metre squared, Wm-2 Irradiance is the light energy per second per square metre So I=Nhf Since Eph=hf (=hc/λ) and N=number of photons per second per square metre The irradiance is a measure of the number of photons being produced per second (per m2) so will increase as the number of electron hole recombinations occur. As the current increases, electron hole recombination will increase so the irradiance should increase. Intensity ?? –investigate!! –by experiment current Back up data from internet –by google image https://www.google.co.uk/search?q=Luminance+versus+current+for+a+LED&safe=strict&biw=1280&bih=878&tbm=isch&tbo=u&source=univ&sa=X&ved=0CBwQsARqFQoTCPX0vN3NmckCFYhXGgodRVALnA https://engineering.purdue.edu/~ece495/Power_Electronics_Lab/LED_Basics.pdf http://www.societyofrobots.com/electronics_led_tutorial.shtml http://www.screens.ru/en/2003/7.html http://www.screens.ru/en/2003/7.html How threshold voltage relates to LED colour http://www.chemistry.wustl.edu/~edudev/LabTutorials/PeriodicProperties/MetalBonding/MetalBonding.html Bonds, Bands, and Doping: How Do LEDs Work?