Modern electronic devices are constructed with resistors, diodes, transistors, integrated circuits which are made by semiconductor materials. Nowadays, silicon is the most used semiconductor in power electronic components: diodes, thyristors, IGBT, MOSFET transistors, etc. The reason is that the silicon is resistant to very high temperature and current. The maximum operation temperature of silicon transistors is 150oC while for example germanium transistor has up to 70oC. The silicon is not a conductor in the true sense of the word. It conducts electricity under certain conditions. The silicon is semiconductor material which is insulator at the absolute zero temperature (0K). With increasing temperature, thermal energy will cause a covalent electrons fraction which becomes free.
When an electrical field is applied they will move and become conduction electrons. That means that the silicon has a negative resistance temperature coefficient. Pure silicon has covalent bonds energy of 1.1 eV. That means how much energy it takes to free the valence electrons in the crystal structure.
Pure monocrystalline silicon is used as a wafer and mechanical support for integral circuits. The pure silicon poorly conducts the electrical energy. The silicon is doped with different impurities to increase the conductivity level of the material. The impurities have added the extra energy levels, and an energy band gap becomes extended. Semiconductors with wide band gap imply materials with the band gap energy above 2 eV. Those semiconductors are suitable for high power electronics, high temperature, and high operation frequency conditions. The Silicon Carbide (SiC) gives the best results in commercial electronic components production. It has band gap energy of 3.03 eV.
The silicon with added impurities can become N-type semiconductor or P-type semiconductor. If the impurity with five valence electrons donor (Nitrogen-N, Phosphorus-P, Arsenic-As, Antimony-Sb, Bismuth- Bi) is added to the pure tetravalent silicon, the four impurity electrons will be covalently tied up with four neighbourly Si atoms and forming covalent bonds. The fifth electron remains free and thanks to the thermal energy it chaotically moves in the crystal lattice. Free electrons conduct electricity if an external electric field exists. The P-type semiconductor is formed by adding trivalent impurity- acceptor (indium-In, boron-B, aluminum-Al, and gallium-Ga) to the pure tetravalent silicon the covalent bonds will be formed with three Si atoms. An empty space is known as a hole. The formed hole is free to move in the crystal lattice. In this case, the positively charged holes will conduct electricity.
Applications of Silicon Semiconductor
The main advantages of semiconductors based on the Si are long life cycle, small volume, lightweight, simple production, great mechanical strength, low supplying power, economical production. The Si is essential material in photovoltaic cells construction (98%). Semiconductor crystal diodes (rectifier) are made binding the P-type and N-type of semiconductor, known as PN junction. Depending on the supplied voltage polarity, the energy band gap will increase or decrease thus the diode resistance is changing and can be very small (Ohms) or very high (MOhms). Based on that, the diode will conduct electricity or not (rectifier diode effect). The nonlinear resistors (voltage-dependent resistors) as varistors are usually made of SiC (silicon carbide). Also, transistors, microchips are made by the silicon-based conductor.
