Semiconductors
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Semiconductors have a very important place in electronics. They are used in diodes, transistors and chips. Semiconductor devices can be created with different materials, but the majority is created with silicon. Hence, the name silicon valley. Silicon is a very common substance. For example, it is the main element in sand.
Silicon has good insulating properties. The reason is that the silicon atoms use all their electrons to bond to the neighboring atoms in their lattice. This is different from metals, that have free electrons to pass on electric charge. The properties of the pure silicon are therefore changed by a process called doping. This means that a small amount of other substance (impurities) are added to the silicon crystal lattice.
Two different methods of doping exist: N-type and P-type. In N-type doping, a very small amount of arsenic or phosphorus is added to the silicon. These atoms have 1 electron more than the silicon. The result is that when they are integrated in the crystal, they have 1 free electron to flow through the lattice. This means that the silicon is now a slightly better conductor, or "semiconductor". The other doping, P-type, uses gallium or boron. In contradiction to arsenic, they have one electron less than silicon. This causes so called holes in the lattice that attract electrons. Because they attract charge, this doping is called P for positive doping.
A diode is the most simple example of a semiconductor device. Here a piece of N-type and P-type silicon are put together. If a voltage difference is applied to the device, with the positive lead at the N-type part and the negative lead at the P-type part, no current will flow. Although both N-type and P-type are conductors when they are used independently, in the combination they have very different properties. The reason that the diode doesn't conduct is that the positive holes in the P-type get pulled to the negative lead of the voltage source. The negative electrons in the N-type get pulled to the positive lead of the voltage source. At the junction there is no flow of current, because the holes and electrons are both pulled away from the junction. When the voltage source would be connected the other way around, the opposite would happen and a flow of current would happen at the junction in the diode. The diode has the very special property to conduct only in one direction. This property gives diodes some special applications. A simple example are devices with batteries. A diode makes sure that no current will flow when the batteries are inserted the wrong way.
Although the ideal diode has no resistance in one direction and infinite resistance in the other direction, in practice this doesn't exist. Practical diodes show a behavior as in the picture below.
Silicon has good insulating properties. The reason is that the silicon atoms use all their electrons to bond to the neighboring atoms in their lattice. This is different from metals, that have free electrons to pass on electric charge. The properties of the pure silicon are therefore changed by a process called doping. This means that a small amount of other substance (impurities) are added to the silicon crystal lattice.
Two different methods of doping exist: N-type and P-type. In N-type doping, a very small amount of arsenic or phosphorus is added to the silicon. These atoms have 1 electron more than the silicon. The result is that when they are integrated in the crystal, they have 1 free electron to flow through the lattice. This means that the silicon is now a slightly better conductor, or "semiconductor". The other doping, P-type, uses gallium or boron. In contradiction to arsenic, they have one electron less than silicon. This causes so called holes in the lattice that attract electrons. Because they attract charge, this doping is called P for positive doping.
A diode is the most simple example of a semiconductor device. Here a piece of N-type and P-type silicon are put together. If a voltage difference is applied to the device, with the positive lead at the N-type part and the negative lead at the P-type part, no current will flow. Although both N-type and P-type are conductors when they are used independently, in the combination they have very different properties. The reason that the diode doesn't conduct is that the positive holes in the P-type get pulled to the negative lead of the voltage source. The negative electrons in the N-type get pulled to the positive lead of the voltage source. At the junction there is no flow of current, because the holes and electrons are both pulled away from the junction. When the voltage source would be connected the other way around, the opposite would happen and a flow of current would happen at the junction in the diode. The diode has the very special property to conduct only in one direction. This property gives diodes some special applications. A simple example are devices with batteries. A diode makes sure that no current will flow when the batteries are inserted the wrong way.
Although the ideal diode has no resistance in one direction and infinite resistance in the other direction, in practice this doesn't exist. Practical diodes show a behavior as in the picture below.
Diodes can be forward or reverse biased. Forward biased means that a small voltage is needed to make the diode smart conducting. This voltage is around 2/3 volts with silicon. Reverse biased diodes block ideally all current. In practice they let through around 0.01 mA. With enough reverse voltage, the junction breaks and passes current. Because the breakdown voltage is usually very high compared to the circuit voltage, this value is often not important.
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http://matse1.matse.illinois.edu/sc/ware.html