Stavan Patel( ) Vedant Patel( ) Vidhi Patel( ) 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.

I R-G negligible dominant p+p+ n BB I r-g negligible Mn-Si

where  B is Schottky barrier height, V A 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 series of general-purpose Schottky barrier diodes T = 100ºC T = 25ºC T = -50ºC I F Temperature Coeffiecient 10  A -2.3mV/ºC 100  A -1.8mV/ºC 1.0mA -1.3mV/ºC 10mA -0.7mV/ºC 100mA -0.2mV/ºC Forward voltage (mV) I-V Curve Showing Typical Temperature Variation for Series Schottky Diodes (a) Forward current (mA) 100

Capacitance (pF) V R -Reverse voltage (V) Series Typical Capacitance vs. Reverse Voltage at T A = 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.

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