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The simplest MOSFET structure is comprised of a substrate, which can be P-type or N-type, and two areas of silicon of the opposite polarity to the bulk, which make up the drain and the source (see Figure 3). Figure 2 shows the difference between P-type and N-type semiconductor dopants, and the effect they have on the silicon structure.įigure 2: Dopants – Donor vs. These impurities are also called acceptor impurities. On the other hand, elements with three electrons in the valence band will be lacking an electron, which is equivalent to contributing a hole, meaning the overall charge will be positive (P-type). Since they contribute an electron, these impurities in the silicon are called donor impurities. If an area of silicon has been doped with ions that have five valence electrons (group 5 in the periodic table) then there is an extra electron that is set free into the semiconductor, so the charge will be overall negative (N-type). The difference between these two silicon types is the charge stored by the dopant ions, which are charged particles that are injected into the silicon to create charge instability, making the element useful for electronic purposes. MOSFETs are semiconductor-based devices, and are mostly built using either P-type or N-type silicon. In most cases, the MOSFET’s bulk is connected to the source, which is why MOSFETs are commonly referred to as 3-pin devices.įigure 1: MOSFET P-Channel MOSFETs vs. When the MOSFET is turned on, current flows from the drain to the source of the MOSFET, through a channel created in the bulk (also called the body). The MOSFET is controlled by applying certain voltage conditions to the gate. These devices act as voltage-controlled current sources, and are mainly used as switches or for the amplification of electrical signals.