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HSMS-270C Datasheet(PDF) 7 Page - AVAGO TECHNOLOGIES LIMITED |
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HSMS-270C Datasheet(HTML) 7 Page - AVAGO TECHNOLOGIES LIMITED |
7 / 9 page 7 Applications Information Schottky Diode Fundamentals The HSMS-270x series of clipping/clamping diodes are Schottky devices. A Schottky device is a rectifying, metal-semiconductor contact formed between a metal and an n-doped or a p-doped semiconductor. When a metal-semiconductor junction is formed, free electrons flow across the junction from the semiconductor and fill the free-energy states in the metal. This flow of electrons creates a depletion or potential across the junction. The difference in energy levels between semiconductor and metal is called a Schottky barrier. P-doped, Schottky-barrier diodes excel at applications requiring ultra low turn-on voltage (such as zero-biased RF detectors). But their very low, breakdown-voltage and high series-resistance make them unsuitable for the clipping and clamping applications involving high forward currents and high reverse voltages. Therefore, this discussion will focus entirely on n-doped Schottky diodes. Under a forward bias (metal connected to positive in an n-doped Schottky), or forward voltage, VF, there are many electrons with enough thermal energy to cross the barrier potential into the metal. Once the applied bias exceeds the built-in potential of the junction, the forward current, IF, will increase rapidly as VF increases. When the Schottky diode is reverse biased, the potential barrier for electrons becomes large; hence, there is a small probability that an electron will have sufficient thermal energy to cross the junction. The reverse leakage current will be in the nanoampere to microampere range, depending upon the diode type, the reverse voltage, and the temperature. In contrast to a conventional p-n junction, current in the Schottky diode is carried only by majority carriers (electrons). Because no minority-carrier (hole) charge storage effects are present, Schottky diodes have carrier lifetimes of less than 100 ps. This extremely fast switching time makes the Schottky diode an ideal rectifier at fre- quencies of 50 GHz and higher. Another significant difference between Schottky and p-n diodes is the forward voltage drop. Schottky diodes have a threshold of typically 0.3 V in comparison to that of 0.6 V in p-n junction diodes. See Figure 6. Through the careful manipulation of the diameter of the Schottky contact and the choice of metal deposited on the n-doped silicon, the important characteristics of the diode (junction capacitance, CJ; parasitic series resistance, RS; breakdown voltage, VBR; and forward voltage, VF,) can be optimized for specific applications. The HSMS- 270x series and HBAT-540x series of diodes are a case in point. Both diodes have similar barrier heights; and this is indicated by corresponding values of saturation current, IS. Yet, different contact diameters and epitaxial-layer thickness result in very different values of CJ and RS. This is seen by comparing their SPICE parameters in Table 1. Table 1. HSMS-270x and HBAT-540x SPICE Parameters. Parameter HSMS- 270x HBAT- 540x BV 25 V 40 V CJ0 6.7 pF 3.0 pF EG 0.55 eV 0.55 eV IBV 10E-4 A 10E-4 A IS 1.4E-7 A 1.0E-7 A N 1.04 1.0 RS 0.65 Ω 2.4 Ω PB 0.6 V 0.6 V PT 2 2 M 0.5 0.5 At low values of IF ≤ 1 mA, the forward voltages of the two diodes are nearly identical. However, as current rises above 10 mA, the lower series resistance of the HSMS- 270x allows for a much lower forward voltage. This gives the HSMS-270x a much higher current handling capabil- ity. The trade-off is a higher value of junction capacitance. The forward voltage and current plots illustrate the differences in these two Schottky diodes, as shown in Figure 7. PN CURRENT 0.6 V + – BIAS VOLTAGE PN JUNCTION CAPACITANCE METAL N CURRENT 0.3V + – BIAS VOLTAGE SCHOTTKY JUNCTION CAPACITANCE VF – FORWARD VOLTAGE (V) .01 10 1 .1 300 100 0 0.1 0.3 0.2 0.5 0.4 0.6 HSMS-270x HBAT-540x Figure 6. Figure 7. Forward Current vs. Forward Voltage at 25°C. |
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