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HSMS-286F Datasheet(PDF) 6 Page - Broadcom Corporation. |
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HSMS-286F Datasheet(HTML) 6 Page - Broadcom Corporation. |
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6 / 18 page  6 Applications Information Introduction Avago’s HSMS‑286x family of Schottky detector diodes has been developed specifically for low cost, high volume designs in two kinds of applications. In small signal detector applications (Pin < ‑20 dBm), this diode is used with DC bias at frequencies above 1.5 GHz. At lower frequencies, the zero bias HSMS‑285x family should be considered. In large signal power or gain control applications (Pin> ‑20 dBm), this family is used without bias at frequencies above 4 GHz. At lower frequencies, the HSMS‑282x family is preferred. 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 7, along with its equivalent circuit. The Height of the Schottky Barrier The current‑voltage characteristic of a Schottky barrier diode at room temperature is described by the following equation: HSMS-285A/6A fig 9 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 Figure 7. 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 capacitance of the diode, controlled by the thickness of the epitaxial layer and the diameter of the Schottky contact. Rj is the junction resistance of the diode, a function of the total current flowing through it. RS RV Cj Figure 8. Equivalent Circuit of a Schottky Diode Chip. RS is perhaps the easiest to measure accurately. The V‑I curve is measured for the diode under forward bias, and the slope of the curve is taken at some relatively high value of current (such as 5 mA). This slope is converted into a resistance Rd. 8.33 X 10 -5 n T R j = = R V - R s I S + I b 0.026 = at 25 °C I S + I b V - IR S I = I S (exp ( ) -1) 0.026 8.33 X 10 -5 n T R j = = R V - R s I S + I b 0.026 = at 25 °C I S + I b V - IR S I = I S (exp ( ) -1) 0.026 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. On a semi‑log plot (as shown in the Avago 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 necessar‑ ily 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 characteristics 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) and DC biased detectors. Measuring Diode Linear Parameters The measurement of the many elements which make up the equivalent circuit for a packaged Schottky diode is a complex task. Various techniques are used for each element. The task begins with the elements of the diode chip itself. (See Figure 8). R V = R j + R S 0.026 R S = R d - If For n‑type diodes with relatively low values of saturation current, Cj is obtained by measuring the total capaci‑ tance (see AN1124). Rj, the junction resistance, is calcu‑ lated using the equation given above. |
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