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HSMS-2855 Datasheet(PDF) 5 Page - Agilent(Hewlett-Packard) |
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HSMS-2855 Datasheet(HTML) 5 Page - Agilent(Hewlett-Packard) |
5 / 13 page 5 Applications Information Introduction Agilent’s HSMS-285x family of Schottky detector diodes has been developed specifically for low cost, high volume designs in small signal (Pin < -20 dBm) applica- tions at frequencies below 1.5 GHz. At higher frequencies, the DC biased HSMS-286x family should be considered. In large signal power or gain con- trol applications (Pin > -20 dBm), the HSMS-282x and HSMS-286x products should be used. The HSMS-285x zero bias diode is not designed for large signal designs. Schottky Barrier Diode Characteristics Stripped of its package, a Schottky barrier diode chip consists of a metal-semiconductor barrier formed by deposition of a metal layer on a semiconductor. The most common of several different types, the passivated diode, is shown in Figure 5, along with its equivalent circuit. Figure 5. Schottky Diode Chip. RS is the parasitic series resistance of the diode, the sum of the bondwire and leadframe resistance, the resistance of the bulk layer of silicon, etc. RF energy coupled into RS is lost as heat — it does not contribute to the rectified output of the diode. CJ is parasitic junction capaci- tance of the diode, controlled by the thickness of the epitaxial layer and the diameter of the Schottky contact. R j is the junction resistance of the diode, a function of the total current flowing through it. 8.33 X 10-5 nT Rj = –––––––––––– = RV – R s IS + Ib 0.026 = ––––– at 25 °C IS + Ib where n = ideality factor (see table of SPICE parameters) T = temperature in °K IS = saturation current (see table of SPICE parameters) Ib = externally applied bias current in amps IS is a function of diode barrier height, and can range from picoamps for high barrier diodes to as much as 5 µA for very low barrier diodes. The Height of the Schottky Barrier The current-voltage characteristic of a Schottky barrier diode at room temperature is described by the following equation: V - IRS I = IS (exp (––––––) - 1) 0.026 On a semi-log plot (as shown in the Agilent catalog) the current graph will be a straight line with inverse slope 2.3 X 0.026 = 0.060 volts per cycle (until the effect of RS is seen in a curve that droops at high current). All Schottky diode curves have the same slope, but not necessarily the same value of current for a given voltage. This is determined by the saturation current, IS, and is related to the barrier height of the diode. Through the choice of p-type or n-type silicon, and the selection of metal, one can tailor the charac- teristics of a Schottky diode. Barrier height will be altered, and at the same time CJ and RS will be changed. In general, very low barrier height diodes (with high values of IS, suitable for zero bias applications) are realized on p-type silicon. Such diodes suffer from higher values of RS than do the n-type. Thus, p-type diodes are generally reserved for small signal detector applications (where very high values of RV swamp out high RS) and n-type diodes are used for mixer applications (where high L.O. drive levels keep RV low). Measuring Diode Parameters The measurement of the five elements which make up the low frequency equivalent circuit for a packaged Schottky diode (see Figure 6) is a complex task. Various techniques are used for each element. The task begins with the elements of the diode chip itself. LP RS RV Cj CP FOR THE HSMS-285x SERIES CP = 0.08 pF LP = 2 nH Cj = 0.18 pF RS = 25 Ω RV = 9 KΩ Figure 6. Equivalent Circuit of a Schottky Diode. RS Rj Cj ;; METAL SCHOTTKY JUNCTION PASSIVATION PASSIVATION N-TYPE OR P-TYPE EPI LAYER N-TYPE OR P-TYPE SILICON SUBSTRATE CROSS-SECTION OF SCHOTTKY BARRIER DIODE CHIP EQUIVALENT CIRCUIT |
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