Semiconductors and Electromagnetic Waves 23 5 Semiconductor Devices

Semiconductors and Electromagnetic Waves

23. 5 Semiconductor Devices Semiconductor devices such as diodes and transistors are widely used in modern electronics. “Technology has clearly revolutionized society, but solid-state electronics is revolutionizing technology itself”.

The Electron volt Small particles use small amounts of energy. The electron volt (e. V) is the magnitude of the amount of energy it take for one electron to move through a potential difference of one volt. 1 e. V = 1. 6 x 10 -19 Joules

Semiconductors • Silicon is the most common material used as a semiconductor (germanium is also used). • It has 4 valence electrons and forms a stable lattice structure. • All electrons are used in the bonding process. None are free to move through the lattice structure, therefore pure Si is a poor conductor.

Band Gap • The band gap (EG) is the minimum amount of energy required for an electron to break free of its bound state. • When the band gap energy is met, the electron is excited into a free state, and can therefore participate in conduction • The band gap determines how much energy is needed from the sun for conduction, as well as how much energy is generated. • A hole is created where the electron was formerly bound. This hole also participates in conduction. • The band gap energy of Si is 1. 1 e. V • http: //pveducation. org/pvcdrom/pn-junction/band-gap

23. 5 Semiconductor Devices SEMICONDUCTORS The semiconducting materials (silicon and germanium) used to make diodes and transistors are doped by adding small amounts of an impurity element.

n-TYPE SEMICONDUCTORS • Small amounts of a material with 5 valence electrons added to the silicon (e. g phosphorus). • Extra electron is a mobile negative charge carrier which increases overall conductivity. • The n-type semiconductor is electrically neutral. The doping process increased conductivity only.

n-type semiconductor doping http: //www. ece. utep. edu/courses/ee 3329/Studyguide/Shockwave/Fundamentals /Demos/Donor. html

p-TYPE SEMICONDUCTORS • Small amounts of a material with 3 valence electrons are added to the silicon (e. g boron). • Extra “electron hole” is a mobile positive charge carrier which increases overall conductivity. • Note that the p-type semiconductor is electrically neutral, just like the n-type material.

p-type semiconductor doping http: //www. ece. utep. edu/courses/ee 3329/Studyguide/Shockwave/Fundamentals/ Demos/acceptor. html http: //pveducation. org/pvcdrom/pnjunction/equilibrium-carrier-concentration

23. 5 Semiconductor Devices What do you get when you put an p-type and an n-type semiconductor together? overall neutral, but with moving, positive holes overall neutral, but with moving, negative electrons

You get a p-n junction, of course! • Mobile electrons from the blue, n-type material move left to fill the holes in the pink p-type material ( left, in Fig a). One may think of the square electron holes as moving right. • The layer at the end of p-type material becomes negative and vice versa. This results in an electric field, pointing from n-type material to p-type material (Fig b). • The resulting structure is called a diode. • No current flows because the diode is electrically neutral.

PN junction demo http: //www. ece. utep. edu/courses/ee 3329/e e 3329/Studyguide/Shockwave/PNjunctions /Demos/PNJunction. Diode. html

Connect a voltage source with a diode.

A solar cell is a diode. • Photons in sunlight hit the solar panel. • The energy ionizes atoms in the charge layers. • Electrons are ejected from their atoms, allowing them to flow through the material to produce electricity. • Due to the composition of solar cells, the electrons are only allowed to move in a single direction. As a result, the solar cell develops a positive and negative terminal, much like a battery.

A solar cell is a diode. When the energy of a photon is equal to or greater than the band gap of the material, the photon is absorbed by the material and excites an electron into the conduction band. Both a minority and majority carrier (i. e electron and hole) are generated when a photon is absorbed. The generation of charge carriers by photons is the basis of the photovoltaic production of energy. http: //pveducation. org/pvcdrom /pn-junction/absorption-of-light

Light-generated current Two key processes: 1. Absorption of a photon with energy greater than EG creates an electron -hole pair. However, If this pair recombines, then there will be no current.

Light-generated current (cont’d) 2. Separation of carriers at the p-n junction due to the electric field. If the light-generated minority reaches the p-n junction, it is swept across the junction by the electric field at the junction, where it is now a majority carrier. The majority carrier is prevented from crossing the pn junction so travels through the external circuit to recombine.

Light-generated current http: //pveducation. org/pvcdrom/solar-cell-operation/light-generated-current Note that in this animation, the blue is the p-type, and pink is the ntype Blue carriers are positive “holes”, and red are negative electrons.

24. 1 The Nature of Electromagnetic Waves This picture shows an electromagnetic wave, such as a light wave, or radio wave. An EM wave is a transverse wave that does not need a medium, e. g. air, or water, to propagate. • In 1865, long before experimental evidence, the English physicist Maxwell correctly predicted that, in a vacuum: ε 0 = 8. 85 x 10 -12 C 2/(N m 2 μ 0 = 4π x 10 -7 T m/A.

24. 3 The Speed of Light • The American physicist Albert Michaelson improved on attempts to measure the speed of light. • By placing his mirrors on top of 2 Southern California mountains, he obtained a value of c that was less than 0. 0014% different that the currently accepted value. • He definitely got a A on that lab.

24. 2 The Electromagnetic Spectrum Like all waves, electromagnetic waves have a wavelength and frequency, related by:

Fig. 18 -2, p. 430

24. 2 The Electromagnetic Spectrum Example 1 The Wavelength of Visible Light Find the range in wavelengths for visible light in the frequency range between 4. 0 x 1014 Hz (red) and 7. 9 x 1014 Hz (violet). These wavelengths correspond to 0. 75 µm (microns) and 0. 38 µm, respectively.

The Crab Nebula is a remnant of a star that underwent a supernova. This event was recorded in the year 1054 A. D (see Anasazi pictograph, below). The Crab Nebula is located at a distance of 6. 0 x 1016 km away from the earth. How long ago did the supernova happen?

The Crab Nebula is a remnant of a star that underwent a supernova. This event was recorded in the year 1054 A. D (see Anasazi pictograph, below). The Crab Nebula is located at a distance of 6. 0 x 1016 km away from the earth. How long ago did the supernova happen? - 7300 years ago from 2010

24. 4 The Energy Carried by Electromagnetic Waves Electromagnetic waves, such as the microwaves shown below, carry energy, much like sound waves

EM/Solar Radiation • Radiation is the heattransfer mechanism by which solar energy reaches our planet. • Energy transferred by radiation is called electromagnetic radiation and can travel through a vacuum. This radiation is NOT radioactive! • All radiation travels at the speed of light in a vacuum.

When radiation strikes an object • Transmission (no change in direction or temperature) • Scattering and Reflection (transmission in another direction) • Absorption, which is accompanied by change of temperature for object absorbing the radiation.

Solar Radiation in the Atmosphere

Reflection and Albedo • Reflection–electromagnetic radiation bouncing of from a surface without absorption or emission, no change in material or energy wavelength • Albedo – proportional reflectance of a surface – – – a perfect mirror has an albedo of 100% Glaciers & snowfields approach 80 -90% Clouds – 50 -55% Pavement and some buildings – only 10 -15% Ocean only 5%! Water absorbs energy.

Typical Albedos of Materials on the Earth

Absorption and Emission • Absorption of radiation – electrons of absorbing material are “excited” by increase in energy – Increase in temperature; physical/chemical change – Examples: sunburn, cancer • Emission of radiation – excited electrons return to original state; radiation emitted as light or heat – Example: earth absorbs short wave radiation from sun (i. e. visible light) and emits longwave (infrared or heat) into the atmosphere.

Laws Governing Radiation 1. All objects at a temperature greater than 0 K emit radiant energy. This includes the Earth, and its polar ice caps. 2. For a given size, hot object emit more energy than cold objects (Stefan. Boltzmann Law)

Laws Governing Radiation 3. The hotter the radiating body, the shorter the maximum wavelength (Wien’s Law). The Sun is a very hot body. Although it radiates in all parts of the spectrum, much of its radiation is short-wave radiation. The much cooler Earth radiates in longer wavelengths called (!) long-wave radiation.

Electromagnetic Spectrum Note the distinction between short-wave and long-wave radiation.

EM radiation from Sun and Earth

Laws Governing Radiation 4. Objects that are good absorbers of radiation are good emitters as well (Kirchoff’s Law). The Earth and the Sun absorb and radiate with nearly 100% efficiency for their respective temperatures 5. The gases of the atmosphere not so much. They absorb some wavelengths and then re-emit them. They let other wavelengths pass through with no absorption.

The Greenhouse Effect • Sun emits EM radiation of all wavelengths, but primarily shortwave (i. e. visible). – Earth’s surface absorbs this energy – Most is re-emitted upward, as IR (longwave) – “greenhouse gases” (water vapor, carbon dioxide, methane, etc. ) let shortwave energy pass, but absorb longwave energy radiated upward by the Earth. – this longwave energy is re-radiated in all directions, some of it returning to the Earth’s surface. This is what keeps our atmosphere at a livable temperature of about 15 degrees C (59 degrees F).

the Radiation Balance • Sun emits EM radiation of all wavelengths, but primarily shortwave (i. e. light). – Earth’s surface absorbs this energy – Most is re-emitted, as heat (longwave) • Greenhouse Effect – “greenhouse gases” let shortwave energy (light) pass through, but absorb and emit longwave energy radiated by the Earth, keeping it the atmosphere

Fig. 18 -7, p. 433

• Most solar energy is in the form of shortwave radiation (e. g. light, uv rays) • Earth absorbs this energy and re-emits as longwave radiation (infrared, “heat”) • Greenhouse gases (CO 2, CH 4 H 2 O) in the atmosphere absorb infrared radiation • This natural process allows the Earth to maintain an average yearly temperature of about 150 C (600 F).

Correlation of the rise in atmospheric carbon dioxide concentration (blue line) with the rise in average temperature (red line)

How CO 2 in atmosphere relates to temperature

EM Wave Intensity • Intensity – defined previously for sound waves as power to area ratio: Intensity = P/A. • Intensity is inversely proportional to the square of the distance from the source of the wave. • Recall power is the amount of energy transported per second.

24. 5 The Doppler Effect and Electromagnetic Waves Electromagnetic waves also can exhibit a Dopper effect, but it differs for two reasons: a) Sound waves require a medium, whereas electromagnetic waves do not. b) For sound, it is the motion relative to the medium that is important. For electromagnetic waves, only the relative motion of the source and observer is important. c) use plus if observer and source are moving together, minus if they are moving apart. d) vrel is a magnitude and therefore always positive.

EOC #37 A distant galaxy is simultaneously rotating and receding from the earth. As the drawing shows, the galactic center is receding from the earth at a relative speed of u. G = 2. 00 x 106 m/s. Relative to the center, the tangential speed is v. T = 5. 00 x 105 m/s for locations A and B, which are equidistant from the center.

EOC #37. When the frequencies of the light coming from regions A and B are measured on earth, they are not the same and each is different than the emitted frequency of 6. 20 x 1014 Hz. Find the measured frequency for the light from region A and region B.

EOC #37. Find the measured frequency for the light from region A and region B. A: 6. 17 x 1014 Hz B. 6. 15 x 1014 Hz

24. 5 The Doppler Effect and Electromagnetic Waves Example 6 Radar Guns and Speed Traps The radar gun of a police car emits an electromagnetic wave with a frequency of 8. 0 x 109 Hz. The approach is essentially head on. The wave from the gun reflects from the speeding car and returns to the police car, where on-board equipment measures its frequency to be greater than the emitted wave by 2100 Hz. Find the speed of the car with respect to the highway. The police car is stationary.

24. 5 The Doppler Effect and Electromagnetic Waves source frequency fs = 8 x 109 Hz frequency “observed” by speeding car reflected frequency observed by police car Replace f’ 0 with term for f 0 on the right side of equation: Replace f 0 with fs on the right side of the equation and expand the square:

24. 5 The Doppler Effect and Electromagnetic Waves Continuing: but we can make the assumption that vrel<< c, so the last term becomes 2:

Doppler weather radar uses the Doppler shift of reflected radar signals to measure wind speeds and gauge the severity of a storm. This picture is off the coast of Florida.

Red shifts and blue shifts: The Big Bang For light coming from astronomical objects, this Doppler equation is no longer correct, but it is still true that the light coming from an object moving closer has a higher frequency, while the light coming from a receding object has a lower frequency. We say light has been “blueshifted” for an object moving closer, and “red-shifted” for an object moving away.

Red shifts and blue shifts: The Big Bang The light coming from the stars and galaxies around us is red-shifted, leading to our present belief that the galaxy is expanding. Extrapolating back in time brings us to a point when the universe was contained in a volumeless point that “exploded”, aka The Big Bang.

23. 5 Semiconductor Devices There is an appreciable current through the diode when the diode is forward biased. Under a reverse bias, there is almost no current through the diode.

23. 5 Semiconductor Devices • The graph shows dependence of current on magnitude and polarity of voltage applied across an ideal p-n junction. • the arrow/bar is the symbol for diode (arrow shows the direction the diode allows conventional current to flow). • Reverse bias- regardless of how much voltage applied, no current flows. • Forward bias – after some “threshold” voltage applied (here slightly more than 0. 5 volts), current rises at an exponential rate.

23. 5 Semiconductor Devices • A more realistic graph for a silicon diode. • When reverse-biased, a real diode lets in a very small amount of current. • If you apply enough reverse voltage (V), the junction breaks down and lets current through (shown at far-left unlikely in normal circumstances). • When forward-biased, the threshold voltage for silicon is about 0. 7 volts. • A diode is a non-ohmic device; it does not obey Ohm’s Law. • If you apply more voltage (bigger battery), the current through the diode will increase, but the voltage drop will always remain at the threshold value.