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•Stavan Patel(140130117088) •Vedant Patel(140130117089) •Vidhi Patel(140130117090) 1 • The Schottky diode (named after German physicist Walter H. Schottky; also known as hot carrier diode) is a semiconductor diode with a low forward voltage drop and a very fast switching action. • When current flows through a diode there is a small voltage drop across the diode terminals. A normal silicon diode has a voltage drop between 0.6–1.7 volts, while a Schottky diode voltage drop is between approximately 0.15–0.45 volts. This lower voltage drop can provide higher switching speed and better system efficiency. • • • A metal–semiconductor junction is formed between a metal and a semiconductor, creating a Schottky barrier (instead of a semiconductor– semiconductor junction as in conventional diodes). Typical metals used are molybdenum, platinum, chromium or tungsten; and the semiconductor would typically be N-type silicon. The metal side acts as the anode and N-type semiconductor acts as the cathode of the diode. This Schottky barrier results in both very fast switching and low forward voltage drop. The most important difference between the p-n and Schottky diode is reverse recovery time, when the diode switches from conducting to non-conducting state. Where in a p-n diode the reverse recovery time can be in the order of hundreds of nanoseconds and less than 100 ns for fast diodes, Schottky diodes do not have a recovery time, as there is nothing to recover from (i.e. no charge carrier depletion region at the junction). • The switching time is ~100 ps for the small signal diodes, and up to tens of nanoseconds for special high-capacity power diodes. With p-n junction switching, there is also a reverse recovery current, which in high-power semiconductors brings increased EMI noise. With Schottky diodes switching essentially instantly with only slight capacitive loading, this is much less of a concern. • Areas of Application Very high frequency range Lower noise figure Low-voltage or high-current power supplies AC-to-DC converters Radar systems Schottky TTL logic Fig. Approximate equivalent circuit for the Schottky diode Schottky diode is a metal-semiconductor (MS) diode Historically, Schottky diodes are the oldest diodes MS diode electrostatics and the general shape of the MS diode I-V characteristics are similar to p+n diodes, but the details of current flow are different. Dominant currents in a p+n diode – arise from recombination in the depletion layer under small forward bias. – arise from hole injection from p+ side under larger forward bias. Dominant currents in a MS Schottky diodes – Electron injection from the semiconductor to the metal. p+ M n n-Si dominant negligible B IR-G Ir-g negligible dominant qVkTA I I s e 1 where I s AA*T 2 e B kT where B is Schottky barrier height, VA is applied voltage, A is area, and A* is Richardson’s constant. The reverse leakage current for a Schottky diode is generally much larger than that for a p+n diode. Since MS Schottky diode is a majority carrier devices, the frequency response of the device is much higher than that of equivalent p+ n diode. Fig . Characteristics curves for Hewlett-Packard 5082-2300 series of general-purpose Schottky barrier diodes. 100 10 Forward current (mA) IF 1 .1 0.1 0 Temperature Coeffiecient 10A -2.3mV/ºC 100A -1.8mV/ºC 1.0mA -1.3mV/ºC 10mA -0.7mV/ºC 100mA -0.2mV/ºC T = 100ºC T = 25ºC 100 200 300 400 Forward voltage (mV) 500 600 700 T = -50ºC I-V Curve Showing Typical Temperature Variation for 5082-2300 Series Schottky Diodes (a) 1.2 Capacitance (pF) 1.0 0.8 0.6 2900 2303 0.3 0.2 2301 2302 2305 0 4 8 12 VR-Reverse voltage (V) 16 20 5082-2300 Series Typical Capacitance vs. Reverse Voltage at TA = 25ºC (c) The most evident limitations of Schottky diodes are the relatively low reverse voltage ratings for silicon-metal Schottky diodes, typically 50 V and below, and a relatively high reverse leakage current. Some higher-voltage designs are available; 200V is considered a high reverse voltage. • Reverse leakage current, because it increases with temperature, leads to a thermal instability issue. This often limits the useful reverse voltage to well below the actual rating. • While higher reverse voltages are achievable, they would be accompanied by higher forward voltage drops, comparable to other types; such a Schottky diode would have no advantage. • Thank You 13