HSMS-2852-BLKG

HSMS-285x Series Surface Mount Zero Bias Schottky Detector Diodes Data Sheet Description Features Avago’s HSMS-285x family of zero bias Schottky detector ­diodes has been designed and optim­ized for use in small signal (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. RS METAL PASSIVATION N-TYPE OR P-TYPE EPI PASSIVATION LAYER SCHOTTKY JUNCTION Cj Rj N-TYPE OR P-TYPE SILICON SUBSTRATE CROSS-SECTION OF SCHOTTKY BARRIER DIODE CHIP 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 HSMS-285A/6A fig 9 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. -5 R j = 8.33 X 10 n T = RV– Rs IS + Ib = 0.026 at 25°C I S + Ib where V -(see IRS table of SPICE parameters) n = ideality factor I = IS (exp T = temperature in °K - 1) 0.026 IS = saturation current (see table of SPICE parameters) Ib = externally applied bias current in amps RS = Rd – 0.026 IS is a function of diode If barrier height, and can range from picoamps for high barrier diodes to as much as 5 µA for very low barrier diodes. RV ≈ 26,000 IS + Ib  ( ) The current-voltage character­istic of a Schottky barrier 0.026 = at room temperature at 25°C diode is described by the following I +I equation: S b I = IS (exp V - IR ( 0.026 ) - 1) S On a semi-log plot (as shown in the Avago catalog) the RS = graph Rd – 0.026 current will be a straight line with inverse slope If 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 necessarRV ≈ 26,000 ily the same value IS + Ib 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). 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. CP LP RV RS Cj 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. -5 R j = 8.33 X 10 n T = RV– Rs IS + Ib 0.026 = at 25°C I S + Ib RS is perhaps the easiest to ­measure accurately. The V-I curve is measured for the diode under forward bias, and - IRScurve is taken at some relatively high the slope ofVthe I = IS (exp - 1)as 5 mA). This slope is converted value of current (such 0.026 into a resistance Rd. ( ) RS = Rd – 0.026 If RV and CJ are very difficult to measure. Consider the imped­ance26,000 of CJ = 0.16 pF when measured at 1 MHz — it RV ≈ is approximately IS + Ib 1 MΩ. For a well designed zero bias Schottky, RV is in the range of 5 to 25 KΩ, and it shorts out the junction capacitance. Moving up to a higher frequency enables the measurement of the capaci­tance, but it then shorts out the video ­resistance. The best measurement technique is to mount the diode in series in a 50 Ω microstrip test ­circuit and measure its insertion loss at low power levels (around -20 dBm) using an HP8753C ­network analyzer. The resulting display will appear as shown in Figure 7. Detector Circuits When DC bias is available, Schottky diode detector circuits can be used to create low cost RF and microwave receivers with a sensitivity of -55 dBm to -57 dBm.[1] These circuits can take a variety of forms, but in the most simple case they appear as shown in Figure 8. This is the basic ­detector circuit used with the HSMS‑285x family of diodes. In the design of such detector ­circuits, the starting point is the equivalent circuit of the diode, as shown in Figure 6. Of interest in the design of the video portion of the circuit is the diode’s video impedance — the other four elements of the equiv­alent circuit disappear at all ­reasonable video frequencies. In general, the lower the diode’s video impedance, the better the design. RF IN Z-MATCH NETWORK VIDEO OUT -10 50 Ω INSERTION LOSS (dB) -15 0.16 pF 50 Ω -20 IS + Ib -25 0.026 I S + Ib at 25°C Figure=8. Basic Detector ­ Circuits. 50 Ω 9 KΩ -30 50 Ω -35 -40 Z-MATCH RF 8.33 X 10-5 n T = RVIDEO R j = NETWORK – Rs OUT IN V 3 10 100 1000 3000 FREQUENCY (MHz) Figure 7. Measuring C J and RV. At frequencies below 10 MHz, the video resistance domHSMS-285A/6A fig 10 inates the loss and can easily be calcu­lated from it. At frequencies above 300 MHz, the junction capacitance sets the loss, which plots out as a straight line when frequency is plotted on a log scale. Again, ­calculation is straightforward. LP and CP are best measured on the HP8753C, with the diode ­terminating a 50 Ω line on the ­input port. The resulting tabulation of S11 can be put into a ­ microwave linear analysis ­ program having the five element equivalent circuit with RV, CJ and RS fixed. The optimizer can then adjust the values of LP and CP ­until the ­calculated S11 matches the measured values. Note that extreme care must be taken to ­ de‑embed the parasitics of the 50 Ω test fixture. The situation is somewhat more complicated in the design of the V -RF IRS impedance matching net­work, which I = IS (exp the pack­age ­ - 1) inductance and capacitance includes 0.026 (which can be tuned out), the ­series resistance, the junction ­capacitance and the video ­resistance. Of these five elements of the diode’s equiv­alent circuit, the four paraR = Rd – 0.026 sitics Sare constants If and the video resistance is a ­function of the current flowing through the diode. ( ) RV ≈ 26,000 IS + Ib where   IS = diode saturation current in µA   Ib = bias current in µA Saturation current is a function of the diode’s design,[2] and it is a constant at a given tempera­ture. For the HSMS-285x series, it is typically 3 to 5 µA at 25°C. Saturation current sets the detection sensitivity, video resistance and input RF impedance of the zero bias Schottky detector diode. Since no external bias is used with the HSMS-285x series, a single transfer curve at any given frequency is obtained, as shown in Figure 2. [1] Avago Application Note 923, Schottky Barrier Diode Video Detectors.  The most difficult part of the ­design of a detector circuit is the input impedance matching ­ network. For very broadband ­detectors, a shunt 60 Ω resistor will give good input match, but at the expense of detection ­sensitivity. When maximum sensitivity is ­ required over a narrow band of frequencies, a reactive matching network is optimum. Such net­works can be realized in either lumped or distributed elements, depending upon frequency, size constraints and cost limitations, but certain general design ­ principals exist for all types.[3] ­ Design work begins with the RF impedance of the HSMS-285x ­series, which is given in Figure 9. FREQUENCY (GHz): 0.9-0.93 Figure 11. Input Impedance. The input match, expressed in terms of return loss, is HSMS-285A/6A fig 15 given in Figure 12. 2 0.2 0.6 5 0 1 3 4 5 6 Figure 9. RF Impedance of the HSMS‑285x Series at-40 dBm. 915 MHz Detector Circuit Figure 10 illustrates a simple ­ impedance matching network for a 915 MHz detector. 65nH VIDEO OUT WIDTH = 0.050" LENGTH = 0.065" -5 -10 -15 -20 0.9 HSMS-285A/6A fig 13 RF INPUT RETURN LOSS (dB) 1 GHz 2 100 pF WIDTH = 0.015" LENGTH = 0.600" TRANSMISSION LINE DIMENSIONS ARE FOR MICROSTRIP ON 0.032" THICK FR-4. Figure 10. 915 MHz Matching Network for the HSMS-285x Series at Zero Bias. 0.915 0.93 FREQUENCY (GHz) Figure 12. Input Return Loss. As can be seen, the band HSMS-285A/6A fig 16over which a good match is achieved is more than adequate for 915 MHz RFID applications. Voltage Doublers To this point, we have restricted our discus­sion to single diode ­ detectors. A glance at Figure 8, however, will lead to the suggestion that the two types of single diode detectors be combined into a two diode voltage doubler[4] (known also as a full wave rectifier). Such a detector is shown in Figure 13. A 65 nH inductor rotates the ­impedance of the diode to Z-MATCH a point on the Smith Chart HSMS-285A/6A fig 14 where a shunt inductor can RF IN NETWORK pull it up to the ­center. The short length of 0.065" wide microstrip line is used to mount the lead of the diode’s SOT‑323 package. A shorted shunt stub of length