For Silicon Wafers or Silicon-On-Insulator (SOI) substrates, resistivity is typically adopted on datasheets to characterize their fundamental electrical parameters. Silicon wafers are generally classified into high-resistivity silicon and low-resistivity silicon. High-resistivity silicon contains extremely low impurity concentrations, usually below \(10^{12}\ \mathrm{cm^{-3}}\), resulting in ultra-high resistivity ranging from 1,000 to 10,000 Ω·cm. Compared with low-resistivity silicon, the fabrication of high-resistivity silicon is extremely challenging, as higher resistivity demands superior silicon purity. It should be noted that the resistivity of many high-resistivity silicon wafers as-marked will degrade to a certain extent after annealing treatment.

The concept of sheet resistance was developed primarily for thin films and greatly simplifies the electrical design of MEMS devices. It can be analogized to square building blocks. Once the sheet resistance value is known, the total resistance can be easily calculated by counting the number of squares (the ratio of resistor length to width). Therefore, the introduction of the square unit and sheet resistance drastically streamlines communication between design and manufacturing engineers. For polysilicon films or ion-implanted monocrystalline silicon layers, circuit design can be readily completed solely by measuring sheet resistance, without precise information about implantation depth (junction depth).