Power semiconductor devices are based on high-purity, single-crystal silicon. Single crystals of several meters long and with the required diameter (up to 150 mm) are grown in the so-called float zone furnaces. Each huge crystal is sliced into thin wafers, which then go through numerous process steps to turn into power devices.
The most commonly used semiconductors are silicon and germanium (Group IV in the periodic table and gallium arsenide (Group V). Silicon materials cost less than germanium materials and allow diodes to operate at higher temperatures. For this reason, germanium diodes are rarely used.
Silicon is a member of Group IV of the periodic table of elements, that is, having four electrons per atom in its outer orbit. A pure silicon material is known as an intrinsic semiconductor with resistivity that is too low to be an insulator and too high to be a conductor. It has high resistivity and very high dielectric strength (over 200 kV/ cm). The resistivity of an intrinsic semiconductor and its charge carriers that are available for conduction can be changed, shaped in layers, and graded by implantation of specific impurities.
The process of adding impurities is called doping, which involves a single atom of the added impurity per over a million silicon atoms. With different impurities, levels and shapes of doping, high technology of photolithography, laser cutting, etching, insulation, and packaging, the finished power devices are produced from various structures of n-type and p-type semiconductor layers.
• n-type material:
If pure silicon is doped with a small amount of a Group V element, such as phosphorus, arsenic, or antimony, each atom of the dopant forms a covalent bond within the silicon lattice, leaving a loose electron. These loose. electrons greatly increase the conductivity of the material. When the silicon is lightly doped with an impurity such as phosphorus, the doping is denoted as n-doping and the resultant material is referred to as n-type semiconductor. When it is heavily doped, it is denoted as n+ doping and the material is referred to as n+-type semiconductor.
•p-type material:
If pure silicon is doped with a small amount of a Group III element, such as boron, gallium, or indium, a vacant location called a hole is introduced into the silicon lattice. Analogous to an electron, a hole can be considered a mobile charge carrier as it can be filled by an adjacent electron, which in this way leaves a hole behind. These holes greatly increase the conductivity of the material. When the silicon is lightly doped with an impurity such as boron, the doping is denoted as p-doping and the resultant material is referred to as p-type semiconductor.
When it is heavily doped, it is denoted as p+ doping and the material is referred to as p+-type semiconductor.
Therefore, there are free electrons available in an n-type material and free holes available in a p-type material. In a p-type material, the holes are called the majority carriers and electrons are called the minority carriers. In the n-type material, the electrons are called the majority carriers and holes are called the minority carriers. These carriers are continuously generated by thermal agitations, they combine and recombine in accordance to their lifetime, and they achieve an equilibrium density of carriers from about 10¹0 to 10¹3/cm³ over a range of about 0°C to 1000°C. Thus, an applied electric field can cause a current flow in an 1-type or p-type material.
Silicon carbide (SiC) (compound material in Group IV of the periodic table) is a promising new material for high-power/high-temperature applications. SiC has a high bandgap, which is the energy needed to excite electrons from the material’s valence band into the conduction band. Silicon carbide electrons need about three times as much energy to reach the conduction band as compared to silicon. As a result, SiC based devices withstand far higher voltages and temperatures than their silicon counter parts.
Silicon devices, for example, can’t withstand electric fields in excess of about 300 kV/cm. Because electrons in SiC require more energy to be pushed into the conduction band, the material can withstand much stronger electric fields, up to about 10 times the maximum for silicon. As a result, an SiC-based device can have the same dimensions as silicon device but can withstand 10 times the voltage. Also, an SiC device can be less than a tenth the thickness of a silicon device but carry the same voltage rating.
These thinner devices are faster and have less resistance, which means less energy is lost to heat when a silicon carbide diode or transistor is conducting electricity.
Key Points:
• Free electrons or holes are made available by adding impurities to the pure silicon or germanium through a doping process. The electrons are the majority carriers in the n-type material whereas the holes are the majority carriers in a p-type material. Thus, the application of electric field can cause a current flow in an n-type or a p-type material.
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