![]() ![]() The change in C gd with drain-to-gate voltage can be as much as a factor of 100 or more.įigure 10 shows the intrinsic capacitances from a circuit perspective. However, C gs has only a small voltage change across it compared to C gd and consequently a small capacitance change. These capacitances are independent of temperature, so MOSFET switching speed is also insensitive to temperature (except for a minor effect related to the threshold voltage changing with temperature).įigure 9: Power MOSFET Structure CapacitancesĬapacitances C gs and C gd, vary with the voltage across them because they are affected by depletion layers within the device. The values of the capacitances are determined by the structure of the MOSFET, the materials involved, and by the voltages across them. A clear advantage of such a device is that there is no need for channel doping and expensive technological steps like ion implantation and high temperature annealings can be avoided, keeping the thermal budget low.Figure 9 shows the locations of power MOSFET intrinsic capacitances. One of the main limitations of such a device is strongly related to the presence of this current that makes it difficult to properly switch it off. Setting the gate voltage to 0 V suppresses the tunneling current and enables only a lower current due to thermionic events. In the opposite case of a negative voltage applied to both junctions the band diagram is bent upwards and holes can be injected and flow from the drain to the source. For an isolated metal, the work function Φ M voltage is always implied) due to direct tunneling. To a first approximation, the barrier between a metal and a semiconductor is predicted by the Schottky–Mott rule to be proportional to the difference of the metal-vacuum work function and the semiconductor-vacuum electron affinity. There are no impurities at the interface between the two materials.No interdiffusion of the metal and the semiconductor is taken into account.The contact between the metal and the semiconductor must be intimate and without the presence of any other material layer (such as an oxide).Īt the basis of the description of the Schottky barrier formation through the band diagram formalism, there are three main assumptions: ![]() This happens both when the semiconductor is n-type and its work function is smaller than the work function of the metal, and when the semiconductor is p-type and the opposite relation between work functions holds. When a metal is put in direct contact with a semiconductor, a so called Schottky barrier can be formed, leading to a rectifying behavior of the electrical contact. Main article: Metal–semiconductor junction Not all metal–semiconductor junctions form a rectifying Schottky barrier a metal–semiconductor junction that conducts current in both directions without rectification, perhaps due to its Schottky barrier being too low, is called an ohmic contact. The value of Φ B depends on the combination of metal and semiconductor. One of the primary characteristics of a Schottky barrier is the Schottky barrier height, denoted by Φ B (see figure). Schottky barriers have rectifying characteristics, suitable for use as a diode. Schottky, is a potential energy barrier for electrons formed at a metal–semiconductor junction. Ī Schottky barrier, named after Walter H. Band diagram for n-type semiconductor Schottky barrier at zero bias (equilibrium) with graphical definition of the Schottky barrier height, Φ B, as the difference between the interfacial conduction band edge E C and Fermi level E F. The semiconducting silicon (center) makes a Schottky barrier against one of the metal electrodes, and an ohmic contact against the other electrode. ![]() Potential energy barrier in metal–semiconductor junctions 1N5822 Schottky diode with cut-open packaging. ![]()
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