It focuses on basic semiconductor physics and the physics of three important devices: 1 the PN junction, 2 the bipolar junction transistor BJT , and 3 the metal-oxide-semiconductor field-effect transistor MOSFET. This course is about basic semiconductor physics and the physics of three important devices: 1 the PN junction, 2 the bipolar junction transistor BJT , and 3 the metal-oxide-semiconductor field-effect transistor MOSFET.
The course is divided into three parts. The first part is an introduction to quantum mechanics and solid-state physics energy bands, electrons and holes, the Fermi function , doping and carrier densities, carrier transport and generation-recombination, and the so-called semiconductor equations, which provide a complete, semi-classical, mathematical description of electrons and holes in semiconductors, subject to some important simplifying assumptions.
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Controlling the amount and type of impurity, however, can fine-tune the properties of the semiconductor. For example, adding a phosphorus impurity with five valence electrons to silicon with four valence electrons effectively adds one extra electron for each phosphorus atom added to the crystal.
Thus the phosphorus is a donor to silicon. The dopant phosphorus atoms will displace silicon atoms in the crystal, but the overall crystal structure does not change, nor does the band structure. The extra electron must go into the conduction band, however, since the valence band was already full. This yields an n -type semiconductor. Conversely, doping silicon with aluminum provides one too few electrons, leaving one hole in the valence band for each aluminum atom added. Thus aluminum is an acceptor from silicon.
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Having lost electrons, the holes bear a positive charge, forming a p -type semiconductor. Because the gap in energy between the donor levels and the conduction band E n or between the acceptor levels and the valence band E p is very small, these doped semi-conductors will exhibit a greater conductivity and much less temperature dependence than is observed for an intrinsic semiconductor.
The P — N junction. The Fermi level of a p -type semiconductor is lower in energy than that of an n -type semiconductor. When p - and n -type semi-conductors are joined, a common Fermi level is created by the combination of holes and electrons at the interface. No carriers are present in this depletion zone at the junction.
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When a positive voltage is applied to the p -type side of the p-n junction and a negative voltage to the n -type side, current can flow since the positive voltage pushes the holes toward the. By contrast, hooking the battery up in reverse will increase the size of the depletion zone, because the negative voltage attached to the p -side of the junction will pull more holes toward that electrode and the positive voltage attached to the n -type semiconductor will pull electrons toward the electrode and away from the junction, leaving a larger depletion zone.
As a result there are no free carriers and no current can flow. Thus this p-n junction forms the simplest solid-state device, known as a diode. Diodes are essentially one-way valves for electronic conductivity. Such a device is very important in a power supply that converts AC to DC current, necessary for many electronic devices. Connecting a single diode to an AC electrical circuit will block either the positive or negative voltage swings, described as half rectification.
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The connection of four diodes as shown in Figure 6 will give full wave rectification, converting AC to DC current. The addition of capacitors to this circuit will smooth the voltage oscillations for actual device operation. When electrical current is passed through any p-n junction, electrons traveling through the conduction band or holes traveling through the valence band can recombine with holes or electrons, respectively, across the band gap. This recombination primarily occurs to electrons that have been pushed into the p -type region of the diode and to holes that have been pushed into the n -type region of the diode by the external voltage.
As electrons and holes are recombined across the band gap, energy is released, often as a photon with an energy equal to that of the band gap. Semiconductors with band gaps between 1. These are known as LEDs.
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In fact, all diodes emit light, but most have a band gap that is smaller than the energy of a visible photon. As a result, most diodes emit infrared photons. Such infrared emission is used for signaling in many remote control devices. Transistors are solid-state switching and amplifying devices, and it was their invention in that truly began the electronics revolution of the late twentieth century.
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An n-p-n bipolar transistor is formed by connecting n -type semiconductors on either side of a thin p -type semiconductor. As shown in Figure 7, the transistor acts as a switch for the circuit connected to the 10 V power supply. If a small negative voltage is applied to the base contact B , the holes are removed from the p -type semiconductor, creating an insulating barrier. This is the same process as trying to force current the wrong way through two p-n junctions.
As a result, no current will flow between any of the contacts. If a small positive voltage is applied to the base contact, however, some current will flow through the control loop electrons going from the emitter to the base. Because the voltage supplied between the emitter contact E and the collector contact C is much larger than the base voltage, a much greater fraction of the electrons will cross the thin p -type layer, giving the much larger collector current.
Because the base current is small and the collector current is large, this same solid-state device can also be used as a signal amplifier. The ratio of the collector current to the base current is essentially constant for a given device.