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Rev. V3 AG312 Visit www.macomtech.com for additional data sheets and product information. M/A-COM Technology Solutions Inc. and its affiliates reserve the right to make changes to the product(s) or information contained herein without notice. ADVANCED: Data Sheets contain information regarding a product M/A-COM Technology Solutions is considering for development. Performance is based on target specifications, simulated results, and/or prototype measurements. Commitment to develop is not guaranteed. PRELIMINARY: Data Sheets contain information regarding a product M/A-COM Technology Solutions has under development. Performance is based on engineering tests. Specifications are typical. Mechanical outline has been fixed. Engineering samples and/or test data may be available. Commitment to produce in volume is not guaranteed. Introduction The PIN diode finds wide usage in RF, UHF and mi- crowave circuits. It is fundamentally a device whose impedance, at these frequenc ies, is controlled by its DC excitation. A unique feature of the PIN diode is its ability to control large amounts of RF power with much lower levels of DC. PIN Diode Modeling The PIN diode is a current controlled resistor at radio and microwave frequencies. It is a silicon semicon- ductor diode in which a high resistivity intrinsic I- region is sandwiched between a P-type and N-type region. When the PIN diode is forward biased, holes and electrons are injected into the I-region. These charges do not immediately annihilate each other; Instead they stay alive for an average time called the carrier lifetime, τ. This results in an average stored charge, Q, which lowers the effective resistance of the I-region to a value R When the PIN diode is at zero or reverse bias there is no stored charge in the I-region and the diode ap- pears as a capacitor, C T, shunted by a parallel resis- tance RP. PIN diodes are specified for the following parameters: RS series resistance under forward bias C T total capacitance at zero or reverse bias Rp parallel resistance at zero or reverse bias VR maximum allowable DC reverse voltage τ carrier lifetime θAVE average thermal resistance or P D maximum average power dissipation θpulse pulse thermal impedance or PP maximum peak power dissipation By varying the I-region width and diode area it is pos- sible to construct PIN diodes of different geometrics to result in the same RS and CT characteristic. Figure 1 These devices may have similar small signal char- acteristics. However, the thicker I-region diode would have a higher bulk or RF breakdown voltage and better distortion properties. On the other hand the thinner device would have faster switching speed. These is a common misconception that carrier life time, τ , is the only parameter that determines the lowest frequency of operation and distortion pro- duced. This is indeed a factor, but equally impor- tant is the thickness of the I-region, W, which re- lates to the transit time frequency of the PIN diode. Low Frequency Model At low frequencies (below the transit time frequency of the I-region) and DC the PIN diode behaves like a silicon PN junction semico nductor diode. Its I-V characteristics determines the DC voltage at the forward bias current level. PIN diodes often are rated for the forward voltage, V F, at a fixed DC bias. The reverse voltage ratings on a PIN diode, V R, are a guarantee from the manufacturer that no more than a specified amount, generally 10µA, of reverse current will flow when V R is applied. It is not neces- sarily the avalanche or bulk breakdown voltage, VB, which is determined by the I-region width (approximately 10 V / µm.) PIN diodes of the same bulk breakdown voltage may have different voltage ratings. Generally, the lower the voltage rating the less expensive the PIN diode.
Rev. V3 AG312 Visit www.macomtech.com for additional data sheets and product information. M/A-COM Technology Solutions Inc. and its affiliates reserve the right to make changes to the product(s) or information contained herein without notice. ADVANCED: Data Sheets contain information regarding a product M/A-COM Technology Solutions is considering for development. Performance is based on target specifications, simulated results, and/or prototype measurements. Commitment to develop is not guaranteed. PRELIMINARY: Data Sheets contain information regarding a product M/A-COM Technology Solutions has under development. Performance is based on engineering tests. Specifications are typical. Mechanical outline has been fixed. Engineering samples and/or test data may be available. Commitment to produce in volume is not guaranteed. Large Signal Model When the PIN diode is forward biased the stored charge, Q, must be much greater than the incre- mental stored charge added or removed by the RF current, I RF. To insure this the following inequality must hold: Q>> I RF 2πƒ Under reverse bias the diode should not be biased beyond its DC voltage rating, V R. The avalanche or bulk breakdown voltage, V B, of a PIN diode is proportional to the I-region width, W, and is always higher than V R. In a typical application maximum negative voltage swing should never exceed V B. An instantaneous excursion of the RF signal into the positive bias direction generally does not cause the diode to go into conduction because of the slow reverse to forward switching speed, T RF, of the PIN diode. Refer to Figure 2. Figure 2 Forward Bias Model R S = W2 (ohms) (µ n +µp) Q Where Q = IF χ τ (coulombs) W = I-region width IF = forward bias current τ = carrier lifetime µn = electron mobility µp = hole mobility Notes: 1. In practical diode the parasitic resistance of the diode package and contact limit the lowest re- sistance value 2. The lowest impedance will be affected by the parasitic inductance, L, which is generally less than 1 nH. 3. The equation is valid at frequencies higher than the I-region transmit time frequency, i.e., ƒ > 1300 (where frequency is in MHz and W in µm). W 2 4. The equation assumes that the RF signal does Zero or Reverse Bias Model Cτ = W = dielectric constant of silicon A = area of diode junction Notes: 1. The above equation is valid at frequencies above the dielectric relaxation frequency of the I-region, i.e. ƒ = 1 (where p is the resistivity of the I-region) 2 πp At lower frequencies the PIN diode acts like a varactor. 2. The value of R P is proportional to voltage and inversely proportional to frequency. In most RF applications its value Is higher than the reac- tance of the capacitance, CT, and is less signifi- cant. Ae e Where
Rev. V3 AG312 Visit www.macomtech.com for additional data sheets and product information. M/A-COM Technology Solutions Inc. and its affiliates reserve the right to make changes to the product(s) or information contained herein without notice. ADVANCED: Data Sheets contain information regarding a product M/A-COM Technology Solutions is considering for development. Performance is based on target specifications, simulated results, and/or prototype measurements. Commitment to develop is not guaranteed. PRELIMINARY: Data Sheets contain information regarding a product M/A-COM Technology Solutions has under development. Performance is based on engineering tests. Specifications are typical. Mechanical outline has been fixed. Engineering samples and/or test data may be available. Commitment to produce in volume is not guaranteed. In a 50 ohm system where the condition of a totally mis- matched antenna must be considered this equation re- duced to: By using these equations it can be shown that using a MA4P709 (or equivalent) insulated stud and MA4P709- 150 stud mounted diode biased at 1 ampere where the RS value is < .2 Ω and is installed in a 50°C heat sink where the MA4P709-985 is rated at 20 watts that a power level of 2.5 kW may be safely controlled even for a totally mismatched antenna. For a perfectly matched antenna, 10 kW may be controlled. The MA47266 is an axial leaded PIN diode rated at 1.5 W dissipation at 1/2” (12. 7 mm) total length to a 50°C contact. The resistance of this diode is a 0.5 Ω (max) at 50 mA. A quarter-wave switch using 2 MA47266s may then be computed to handle 40 watts with a totally mismatched antenna. It should be pointed out that the shunt diode of the quarter-wave antenna switch dissipates about as much power as the series diode. This may not be apparent from Figure 17; however, it may be shown that the RF current in both the series and shunt diode is practically identical. Broadband antenna switches using PIN diodes may be designed using the series connected diode circuit shown in Figure 18. The frequency limit ation of this switch re- sults primarily from the capacitance of D2. In this case forward bias is applied either to D 1 during transmit or D 2 during receive. In high power application (<50 W) it is often necessary to apply reverse voltage on D2 during transmit. This may be accomplished either by a negative polarity power supply at Bias 2 or by having the forward bias current of D 1 flow through resistor R to apply the required negative voltage. The selection of diode D 1 is based primarily on its power handling capability. It nee not have a high voltage rating since it is always forward biased in its low resistance state when high RF power is applied. Diode D 2 does not pass high RF current but must be able to hold off the RF voltage generated by the transmi tter. It is primarily se- lected on the basis of its capacitance which determines the upper frequency limit and it s ability to operate at low distortion. Using an MA47266 as D 1, and a 1N5767 which is rated at 0.4 pF max, as D 2, greater than 25 dB receiver isola- tion may be achieved up to 400 MHz. The expected transmit and receive insertion loss with the PIN diodes biased at 50 mA are 0.1 dB and 0.3 dB respectively. This switch can handle RF power levels up to 40 watts. Figure 17 Quarter Wave Antenna Switches
Rev. V3 AG312 Visit www.macomtech.com for additional data sheets and product information. M/A-COM Technology Solutions Inc. and its affiliates reserve the right to make changes to the product(s) or information contained herein without notice. ADVANCED: Data Sheets contain information regarding a product M/A-COM Technology Solutions is considering for development. Performance is based on target specifications, simulated results, and/or prototype measurements. Commitment to develop is not guaranteed. PRELIMINARY: Data Sheets contain information regarding a product M/A-COM Technology Solutions has under development. Performance is based on engineering tests. Specifications are typical. Mechanical outline has been fixed. Engineering samples and/or test data may be available. Commitment to produce in volume is not guaranteed. Although there are other me thods for providing AGC functions such as varying the gain of the RF transistor amplifier, the PIN diode ap proach generally results in lower power drain, less frequency pulling, and lower RF signal distortion. The latter results are especially true, when diodes with thick I-region s and long carrier life- times are used in the attenuator circuits. Using these PIN diodes, one can achieve wide dynamic range at- tenuation with low signal distortion at frequencies ranging from below 1 MHz up to well over 1 GHz. Reflective Attenuators An attenuator may be designed using single series or shunt connected PIN diode sw itch configurations as shown in figure 21. These a ttenuator circuits utilize the current controlled resistance characteristic of the PIN diode not only in its low loss states (very high or low re- sistance) but also at in-between, finite resistance values. The attenuation value obtained using these circuits may be computed from the following equations: Attenuation of Series Connected PIN Diode Attenuator Attenuation of Shunt Connected PIN Diode Attenuator These equations assume the PIN diode to be purely re- sistive. The reactance of the PIN diode capacitance, however, must also be taken into account at frequencies where its value begins to approach the PIN diode resis- tance value. Matched Attenuators Attenuators built from switch design are basically reflec- tive devices which attenuate the signal by producing a mismatch between the source and the load. Matched PIN diode attenuator designs, which exhibit constant input impedance across the entire attenuation range, are also available which use either multiple PIN diodes bi- ased at different resistance points of band-width-limited circuits utilizing tuned elements. They are described as follows: Quadrature Hybrid Attenuators Although a matched PIN attenuat or may be achieved by combining a ferrite circulator with one of the previous simple reflective devices, the more common approach makes use of quadrature hybrid circuits. Quadrature hy- brids are commonly available at frequencies from below
10 MHz to above 1 GHz, with bandwidth coverage often
exceeding a decade. Figures 21 and 22 show typical quadrature hybrid circuits employing series and shunt connected PIN diodes. The following equations summa- rize this performance: Quadrature Hybrid (Series Connected PIN Diodes) Quadrature Hybrid (Shunt Connected PIN Diodes) Figure 21 SPST PIN Diode Switches
Rev. V3 AG312 Visit www.macomtech.com for additional data sheets and product information. M/A-COM Technology Solutions Inc. and its affiliates reserve the right to make changes to the product(s) or information contained herein without notice. ADVANCED: Data Sheets contain information regarding a product M/A-COM Technology Solutions is considering for development. Performance is based on target specifications, simulated results, and/or prototype measurements. Commitment to develop is not guaranteed. PRELIMINARY: Data Sheets contain information regarding a product M/A-COM Technology Solutions has under development. Performance is based on engineering tests. Specifications are typical. Mechanical outline has been fixed. Engineering samples and/or test data may be available. Commitment to produce in volume is not guaranteed. the nominal impedance in the plane of the PIN diode, the current and voltage stress can usually be adjusted to be within the device ratings. In general, this implies lower- ing the nominal impedance to reduce the voltage stress in favor of higher RF currents. For PIN diodes, the maxi- mum current rating should be specified or is dependent upon the power dissipation rating while the maximum voltage stress at RF frequencies is dependent on I- region thickness. PIN Diode Distortion Model The beginning sections of this article concerned with large signal operation and thermal considerations allows the circuit designer to avoid c onditions that would lead to significant changes in PIN diode performance or exces- sive power dissipation. A subtle but often significant operating characteristic is t he distortion or change in signal shape which is always produced by a PIN diode in the signal it controls. The primary cause of distortion is any variation or nonlin- earity of the PIN diode impedance during the period of the applied RF signal. These variations could be in the diode’s forward bias resistance, R S, parallel resistance, RP, capacitance, CT, or the effect of the low frequency I- V characteristic. The level of distortion can range from better than 100 dB below, to levels approaching the de- sired signal. The distortion could be analyzed in a fourier series and takes the traditional form of harmonic distor- tion of all orders, when applied to a single input signal, and harmonic intermodulation distortion when applied to multiple input signals. Non-linear, distortion generating behavior is often de- sired in PIN and other RF oriented semiconductor di- odes. Self-biasing limiter diodes are often designed as thin I-region PIN diodes oper ating near or below their transmit time frequency. In a detector or mixer diode the distortion that results from the ability of the diode to fol- low its I-V characteristic at high frequencies is exploited. In this regard the term “square law detector” applied to a detector diode implies a second order distortion genera- tor. In the PIN switch circ uits discussed at the beginning of this article, and the atte nuator and other applications discussed here, methods of selecting and operating PIN diodes to obtain low distortion are described. There is a common misconception that minority carrier lifetime is the only significant PIN diode parameter that affects distortion. This is indeed a major factor, but an- other important parameter is the width of the I-region, which determines the transit ti me of the PIN diode. A diode with a long transmit time will have more of a ten- dency to retain its quiescent level of stored charge. The longer transmit time of a thick PIN diode reflects its ability to follow stored charge model for PIN diode resistance according to: Where: IF = forward bias current τ = carrier lifetime W = I region width µn = electron mobility µp = hole mobility Rather than the non-linear I-V characteristic. The effect of a carrier lifetime on distortion related to the quiescent level of stored charge induced by the DC forward bias current and the ratio of this stored charge to the incremental stored charge added or removed by the RF signal. Distortion in PIN Diode Switches The distortion generated by a forward biased PIN diode switch has been analyzed* and has been shown to be related to the ratio of stored charge to diode resistance and the operating frequency. Prediction equations for the second order intermodutation intercept point (IP2) and the third order intermodut ation intercept point (IP3) have been developed from PIN semiconductor analysis are presented as follows: Where: F = frequency RS = PIN diode resistance ohms Q = Stored charge in nC In most applications, the distortion generated by a re- versed biased diode is smaller than forward biased gen- erated distortion for small or moderate signal size. This is particularly the case when the reverse bias applied to the PIN diode is larger than the peak RF voltage prevent- ing any instantaneous swing into the forward bias direc- tion. Distortion produced in a PIN diode circuit may be re- duced by connecting an additional diode in a back to back orientation, (cathode to cathode or anode to an- ode). This results in a cancellation of distortion currents.
Rev. V3 AG312 Visit www.macomtech.com for additional data sheets and product information. M/A-COM Technology Solutions Inc. and its affiliates reserve the right to make changes to the product(s) or information contained herein without notice. ADVANCED: Data Sheets contain information regarding a product M/A-COM Technology Solutions is considering for development. Performance is based on target specifications, simulated results, and/or prototype measurements. Commitment to develop is not guaranteed. PRELIMINARY: Data Sheets contain information regarding a product M/A-COM Technology Solutions has under development. Performance is based on engineering tests. Specifications are typical. Mechanical outline has been fixed. Engineering samples and/or test data may be available. Commitment to produce in volume is not guaranteed. The cancellation should be total, but distortion produced by each PIN diode is not exactly equal in magnitude and opposite in phase. Approximately 20 dB distortion im- provement may be expected by this back to back con- figuration. Distortion in Attenuator Circuits In attenuator applications, distortion is directly relatable to the ratio of RF to DC stored charge. In such applica- tions, PIN diodes operated only in the forward bias state and often at high resistance values where the stored charge may be very low. Under these operating condi- tions, distortion will vary with charges in the attenuation level. Thus, PIN diodes se lected for use in attenuation circuits need only be chosen for their thick I-region width, since the stored charge at any fixed diode resistance, Rs 1, is only dependent on this dimension. Consider an MA4PH451 PIN diode used in an applica- tion where a resistance of 50 Ω is desired. The MA4PH451 datasheet indicates the 1 mA is the typical diode current at which this occurs. Since the typical car- rier lifetime for this diode is ≈ 5 µS, the stored charge for the MA4PH451 diode at 50 Ω is 5 nC. If two MA4PH451 PIN diodes, however are inserted in series, to achieve the same 50 Ω resistance level, each diode must be bi- ased at 2 mA. This results in a stored charge of 10 nC per diode or a net stored c harge of 20 nC. Thus, adding a second diode in series multiplies the effective stored charge by a factor of 4. This would have a significant positive impact on reducing the distortion produced by attenuator circuits. Measuring Distortion Because distortion levels are often 50 dB or more below the desired signal, special pr ecautions are required in order to make accurate second and third order distortion measurements. One must fi rst ensure that the signal sources used are free of distortion and that the dynamic range of the spectrum analyz er employed is adequate to measure the specified level of distortion. These require- ments often lead to the us e of fundamental frequency band stop frequencies at the device output as well as pre-selectors to clean up the signal sources employed. In order to establish the a dequacy of the test equipment and signal sources for making the desired distortion measurements, the test circui t should be initially evalu- ated by removing the diodes and replacing them with passive elements. This approach permits one to opti- mize the test setup and es tablish basic measurement limitations. Since harmonic distortion appears only at multiples of the signal frequency, these signals may be filtered out in narrow band systems. Second order distortion, caused by the mixing of two input signals, will appear at the sum and difference of these frequencies and may also be filtered. As an aid to identifying the various distortion signals seen on a spec- trum analyzer, it should be no ted that the level of a sec- ond distortion signal will vary directly at the same rate as any change of input signal level. Thus, a 10 dB signal increase will cause a corresponding 10 dB increase in a second order distortion. Third order intermodulation distortion of two input signals at frequencies FA and FB often produce in-band, nonfilter- able distortion component s at frequencies of 2F A - F B and 2FB - F A. This type of distortion is particularly trou- blesome in receivers located nearby transmitters operat- ing on equally spaced channels. In identifying and measuring such signals, it should be noted that third or- der distortion signal levels vary at twice the rate of change of the fundamental signa l frequency. Thus a 10 dB change in input signal will result in a 20 dB change of third order signal distortion power observed on a spec- trum analyzer. *G. Hiller, R.Caverly, “Predict Distortions Intercept Points in PIN Diode Switches,” Microwaves and RF, Dec. 1985 and Jan. 1986. References Garver, Robert V., “Microwave Control Devices,” Artech House, Inc., Dedham, MA., 1976. Mortenson, K.E., and Borrego, J.M., “Design Per- formance and Application of Microwave Semiconduc- tor Control Components,” Artech House, Inc, Dedham, MA, 1972. Watson, H.A., “Microwave Semiconductor Devices and their Circuit Applications.” McGraw Hill Book Co., New York, NY., 1969. White, Joseph F., “Semiconductor Control,” Artech House, Inc., Dedham, MA., 1977. Caverly, R.H., Hiller, G., “Distortion in PIN Diode Control Circuits” IEEE Trans MIT, May 1987. Hiller, G., Caverly, R.H., “Establishing Reverse Bias for PIN Diodes in High Power Switches,” IEEE Trans MIT, Dec. 1990.