gallium arsenide products designers information...

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Gallium Arsenide Products

Designers’ Information


IMPORTANT NOTICE

TriQuint Semiconductor (TQS) reserves the right to make changes to or to discontinue anysemiconductor product or service identified in this publication without notice. TQS advisesits customers to obtain the latest version of the relevant information to verify, before placingorders, that the information being relied upon is current.

TQS warrants performance of its semiconductor products to current specifications in accor-dance with TQS’s standard warranty. Testing and other quality control techniques areutilized to the extent TQS deems necessary to support this warranty. Unless mandated bygovernment requirements, specific testing of all parameters of each device is not necessar-ily performed.

TQS assumes no liability for TQS applications assistance, customer product design, soft-ware performance, or infringement of patents or services described herein. Nor does TQSwarrant or represent that license, either express or implied, is granted under any patentright, copyright, mask work right, or other intellectual property right of TQS covering orrelating to any combination, machine, or process in which such semiconductor products orservices might be or are used.

TriQuint Semiconductor products are not intended for use in life-support appliances, de-vices, or systems. Use of a TQS product in such applications without the written consent ofthe appropriate TQS officer is prohibited.

Copyright 1992, TriQuint Semiconductor, Incorporated


ContentsTitle Page

GaAs Device Reliability and Reliability Test Results of TriQuint MMICs ...............

GaAs Device Thermal Characterization ....................................................................Introduction ........................................................................................................Definition of Channel Temperature ....................................................................Method of Calculating Channel Temperature .....................................................Analytical Solution Methods ...............................................................................Numerical Models ..............................................................................................Application of Thermal Model Results ................................................................Example of Thermal Model Results for the TGA8622 Distributed Amplifier .......

Radiation Hardness of TI GaAs MMICs .....................................................................

ESD and Guidelines for Handling Electrostatic-Discharge Sensitive (ESDS)Devices and Assemblies ............................................................................................

Cause .................................................................................................................Recommendation ...............................................................................................

MMIC Assembly Procedures: Manual and Automated ............................................Solder Attachment ..............................................................................................Component Placement and Adhesive ................................................................Interconnect .......................................................................................................

Testing Considerations and Typical Test Set Configurations ................................Amplifier Biasing ................................................................................................

Dual-Supply Single-Gate Amplifiers ........................................................Dual-Supply Dual-Gate Amplifiers ...........................................................Single-Supply Amplifiers .........................................................................

Switch Biasing ....................................................................................................FET Switches ..........................................................................................Typical Test Set Configurations ...............................................................

Operating Dual-Supply MMICs From a Single Supply .............................................

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GaAs Device Reliability and Reliability Test Results ofTriQuint Semiconductor MMICs

GaAs device reliability involves probability statistics, time, and a definition of failure. Given a failurecriteria, the most direct way to determine reliability is to submit a large number of samples to actual useconditions and monitor their performance against the failure criteria over time. Since most applicationsrequire device lifetimes of many years, this approach is not economical in most cases. To acquireMMIC reliability data in a reasonable amount of time, TriQuint Semiconductor has used acceleratedlife tests at high temperatures. The rationale behind high-temperature life tests is that mostphysiochemical processes are accelerated with temperature by a constant called the activation en-ergy. The premise is that most semiconductor failure mechanisms follow the Arrhenius equation.

To properly analyze life-test data requires the adoption of a mathematical failure distribution. Severalare commonly used including the normal, lognormal, exponential and Weibull distributions. TriQuintSemiconductor has adopted the lognormal distribution because it most closely fits the measured reli-ability data from life- tested GaAs semiconductor devices. TQS analyzes life-test data using the log-normal graph. The lognormal graph is a plot of normal cumulative-percent-failure versus log time. If thelife-test data fits a straight line on this graph, it indicates the data fits the lognormal distribution. Theintersection of this line with 50% cumulative failure indicates the median lifetime. Median life is the timeit takes for half of the devices to fail. An “S” curve on the lognormal graph indicates the measuredfailure data is bimodal. In the case of a bimodal distribution, the inflection point of the “S” indicates thepercentage of the weaker population. For additional information, reference MIL-STD-883, method 1016.To predict lifetimes at normal operating temperatures, at least three different high temperature lifetests must be performed. The median life from each of the three tests is transferred to an Arrheniusplot and fit with a line. The slope of the line is the activation energy. Median life at any temperature canthen be determined. For further information, reference JC50.1-91-165 committee letter ballot datedOctober 29, 1991.


Median life should not be confused with mean-time-to-failure (MTTF). MTTF is the reciprocal of theinstantaneous failure rate. MTTF is not constant with time due to the lognormal failure distribution. Onemust specify an operating time to calculate the exact MTTF. However, a close approximation to theaverage

Ideally, accelerated life tests should be conducted with very large sample sizes. However, this is notalways practical. The sample size determines the confidence in the lifetime predictions. The smallerthe sample size, the less confidence we have in the prediction. Confidence limits are defined in termsof percentage. For example, an upper and lower 90% confidence limit would indicate that repeatingthe life test 10 times, 9 out of the 10 tests would predict a median life between the two limits. Confi-dence limits can be calculated for median life with the following equation.

Since most GaAs device failures occur in the FET channel, all life-test data is referenced to the chan-nel temperature. The importance of accurately determining the channel temperature of each devicesubmitted to life test cannot be overstressed. Variables affecting the channel temperature include:ambient temperature, device thermal impedance, package and mounting materials, power dissipation,and RF levels.TriQuint Semiconductor life time predictions are all referenced to a 140°C channel tem-perature. See section on GaAs devices thermal characterization for further information. TriQuint Semi-conductor has performed extensive reliability life tests on numerous GaAs components since the early1980s through participation in several government-sponsored and internally-funded programs. By theend of 1991, over 6 million device hours have been accumulated in life tests of over two dozen compo-nents, including power amplifier MMICs, low-noise amplifier MMICs, power and low-noise FETs, andvarious passive structures. Measured activation energies range from 1.2 eV to 1.9 eV. Figure 1 showsthe results of life-test data taken on over 200 375-µm FETs at three temperatures. This data is repre-sentative of TriQuint Semiconductor 0.5-µm ion-implanted MESFETS, both low-noise and power pro-files, under typical power and low-noise bias conditions. The failure mechanism represented is cata-strophic failure. In the event that actual test data on a specific MMIC is unavailable, this FET data canbe used for a rough prediction of its reliability. The procedure is to use the channel temperature of thehottest FET on the MMIC to predict the MMIC life time. Again, accurate channel temperature determi-nation is critical.


The FET life-test data shown in Figure 1 obeys the Arrhenius relationship. The equation may be usedto predict median life (tm ) at any channel temperature given the following constants.

Good correlation has been observed between discrete FET life times and MMIC life times, since al-most all MMIC failures are due to FET failure. Typically, each MMIC has one FET that is hotter than therest, which dominates the lifetime for the entire MMIC.

TriQuint Semiconductor has proven to have and continues to monitor a very reliable GaAs process. Anaverage measured sigma of 0.7 from most life tests indicates a mature process. Most MMIC designshave a MTTF better than 10 million hours at 140°C channel temperature.


GaAs Device Thermal Characterization

Introduction

Thermal performance characteristics of GaAs devices directly affect their reliability. The purpose ofthis section is to discuss some of the factors affecting thermal performance and to discuss techniquesavailable for predicting temperature rises in a GaAs device. Predicting accurate channel temperaturesfor a GaAs device is difficult and requires detailed knowledge of the power dissipation, geometry of thegold-plating layers around the channel(s), method of attaching the die to a substrate, and the thermalboundary conditions of the substrate. Compared to silicon devices, the relative channel temperature ofa GaAs device may be much hotter than the surrounding GaAs due to the small feature size of thechannels and the fact that GaAs has approximately one-third the thermal conductivity of silicon.

Definition of Channel Temperature

Heat dissipation occurs in the GaAs directly below the FET gate finger(s), resulting in a concentratedzone of heat dissipation. The choice of the correct area for the heat generation zone must be consis-tent with the definition of channel temperature used in reliability calculations. TriQuint Semiconductorreliability calculations use an area of the gate length times the gate width with the heat generated at thetop surface of the GaAs.

Method of Calculating Channel Temperature

Due to the small size of GaAs devices, it is reasonable to assume that all of the heat generated willtravel by conduction through the GaAs to the die-attach material and then through the substrate. Twomethods of solving the heat equation are used for modeling these devices. The first is an analyticalapproach using simplified boundary conditions, and the second involves solving the equation numeri-cally through the use of finite difference or finite element computer models.

Analytical Solution Methods

Solving the heat equation for a simplified geometry results in a series solution requiring the summationof terms to predict temperature. The required geometry simplifications are that each stack-up materialmust be the same size and continuous. This approach does not allow for features such as gold-platinglayers on the top surface. The heat inputs are normally described as rectangles on the top surface withuniform flux. The National Bureau of Standards program TXYZ is one such example of this type ofprogram. TXYZ allows up to three different stack-up materials and does not permit any temperaturevariation of thermal conductivity. The analytical approach also requires the bottom surface of the bot-tom layer be isothermal. It should also be noted that when using this approach for predicting channeltemperatures with small gate lengths, summing a large number of series terms is required for conver-gence. Due to the required simplifications with an analytical model, a significant deviation in channeltemperature prediction compared to a numerical model including the effects that cannot be included inthe analytical model can occur. GaAs device thermal modeling at TriQuint Semiconductor has demon-strated up to a 25% difference in temperature rise predictions between the two methods. Model verification with IR scans and liquid crystal techniques have shown that the numerical models are moreaccurate, and as a result, TriQuint Semiconductor generates detailed thermal models for GaAs de-vices undergoing reliability life testing.


Numerical Models

Numerically modeling the GaAs device allows the inclusion of gold-plating layers, temperature-depen-dent thermal conductivities, and substrates of a size different than the chip. In order to resolve thesmall feature size, a large number of subdivisions or nodes are required. GaAs device thermal modelsfor life testing at TriQuint Semiconductor have been between 15 and 20 thousand nodes.

Application of Thermal Model Results

The results from a thermal model are usually reduced to a thermal resistance expressed in o C/W,which applies to the temperature rise from the base plate to the channel. Multiplying the dissipatedpower times the thermal resistance gives a temperature rise. The thermal resistance should also specifythe applicable mounting configuration. For a GaAs device with multiple FETs, the resistance shouldspecify the dissipated power distribution within the device and whether the resistance is based on anindividual FET power or total device power. The thermal conductivity of GaAs is a fairly strong functionof temperature, which means that the operating temperature of the device should be considered.GaAs thermal conductivity varies with absolute temperature to the minus 1.2 power. Using this relationship, an estimate of the thermal resistance at a higher base-plate operating temperature can bemade and is:

This equation assumes that the majority of the thermal resistance is in the GaAs as is the case withgold tin solder and a molybdenum carrier plate. The fact that some of the resistance is in the die attachand substrate and the three-dimensional heat spreading aspects of the problem make this equationonly an approximation.

Example of Thermal Model Results for the TGA8622 Distributed Amplifier

Thermal models were created for life testing the TGA8622 distributed amplifier and some of the resultsare summarized below as an example. The basic mounting configuration for the device was for theGaAs to be soldered using 2 mils of gold tin solder to a 20-mil-thick molybdenum base plate. Initially,an analytical model using TXYZ was created to determine which channel was the hottest and to give acheck on a finite difference model solution. The TXYZ model gave a temperature rise above base plateof 37°C, while the corresponding finite difference model gave a temperature rise of 38°C, which iswithin 3% agreement. Neither of these models accounted for the gold-plating layers on the top surface,and they both assumed a constant value of thermal conductivity for the GaAs (at 90°C). Adding thegold-plating layers to the finite difference thermal model brought the temperature rise prediction downto 30°C, which is a little over a 20% reduction in channel temperature rise. Finally, temperature-depen-dent properties were added to the thermal model, and the model was used to generate the informationillustrated in Figure 1. The channel temperature rises were then divided by the dissipated


power (0.9 W) to generate Figure 2. Both of these figures show the trend the base-plate temperatureincreases. These figures also give an example of the difference in channel temperatures depending onthe device mounting method. The two die-attach materials (AuSn and silver-filled epoxy) and the twobase-plate materials (Molybdenum and Kovar) are typical of device mounting configurations at TriQuintSemiconductor. Changing to different materials or cases with thicknesses other than those listed willchange the model predictions.

Thermal model results may be available for other GaAs devices. Contact TriQuint Semiconductor forfurther information.


Radiation Hardness of TI GaAs MMICs

Radiation hardness for TQS GaAs MMICs is summarized in Table 1. Initial studies of radiation effectswere carried out from 1984–1988 in cooperation with the Naval Research Laboratory. Subsequenttesting of TQS MMICs under MIMIC Phase I, 1989–1991, confirmed the levels given in Table 1. MMICusers wishing more details and specific test data are referred to the publications in Table 2.

Table 2. Published Data on Radiation Effects on TI GaAs MMICs

“Combined Pulsed Neutron and Flash X-ray Radiation Effects in GaAs MMICs,” W. T. Anderson,R. C. Harrison, J. Gerdes, J. M. Beall, and J.A. Roussos, 1988 GaAs IC Symposium.

“Circuit Response of GaAs MMICs and Bias Networks to Ionizing Radiation Transients,”J. M. Beall and W. T. Anderson, 1986 Military Communications Conference.

“Neutron Radiation Effects in GaAs MMICs and FETs,” W. T. Anderson, J. K. Callahan, andJ. Beall, 1986 Conference for Hardened Electronics and Radiation Technology.

“GaAs MMIC Technology Radiation Effects,” W. T. Anderson, M. Simmons, A. Christou, andJ. Beall, IEEE Transactions on Nuclear Science, December 1985.

Table 1. Radiation Hardness Summary

TOTAL DOSE: Up to 10 7 – 10 8 rads(GaAs) without significant parametricdegradation

HIGH DOSE RATE: Operates through events up to 10 8 rads(GaAs)/sec withoutsignificant upset in operation. Survives >10 11 rads(GaAs)/sec.Recovery in 5 ms with some parametric shifts lasting for several 10sor 100s of milliseconds. Recovery rate and survivability dependpartly on design of off-chip bias circuitry.

NEUTRON: Up to 10 14 /cm 2 1 MeV equivalent without significant parametricdegradation

ELECTROMAGNETICPULSE

No data yet; however some limited experience indicates that MMICscan survive no more than 5 V to 10 V overvoltage at bias terminals.Off-chip circuitry must limit transients to below this level.


ESD and Guidelines for Handling Electrostatic-Discharge Sensitive (ESDS)Devices and Assemblies

Cause

Several elements that are used in MMICs can be damaged by electrostatic discharge (ESD) if nothandled properly. Damage may occur at tune-and-test, assembly, inspection, and other places, ifproper precautions are not taken.

The FETs that make up the MMICs are “Class 1”, as defined by DOD-STD-1686. That is, they can bedamaged by ESD voltages in the 20- to 2000-V range. Tests have shown that noncatastrophic damage can occur in the 50- to 75-V range for some devices. This damage is characterized by a slightincrease in gate leakage current. As an example, typical leakage currents of 8 mA have been seen toincrease to 30 to 40 mA after being subjected to 60- to 75-V ESD per MIL-STD-883 test methods. TheMMIC used in the test still operated properly and met all RF specifications. Catastrophic damage hasbeen observed in the 150- to 200-V range.

Thin-film capacitors and resistors can be damaged by static charges of less than 2000 V and aretherefore also “Class 1” devices. The voltages needed to damage these components are, however,much higher than those needed to damage FETs. Several hundred volts would damage these circuitelements; FETs are more susceptible to damage than capacitors and resistors. Input- and output-blocking capacitors will not protect internal FETs from damage in most cases. The ESD is usuallypresent in the form of voltage transients and as such will be coupled through most capacitors. There-fore, it is recommended that all operators be careful when connecting these devices to RF test set ups.A good practice is for test technicians to ground themselves prior to connecting the bias or RF leads.It is not known what impact noncatastrophic damage will have on device lifetimes. Tests on intention-ally damaged devices have shown that they continue to operate for over 500 hours at 85°C withoutdegrading further. It is anticipated, however, that lifetimes will be shortened when compared to undam-aged devices.

Recommendation

At TriQuint Semiconductor, all workstations within the GaAs Operations Group have been rigorouslyevaluated for static charges, and weekly checks have been implemented to ensure the stations arefree of damaging static charges. Workstations conform to DOD-HDBK-263 and DOD-STD-1686, andmany use the recommended air ionizers as well. In addition, all personnel have undergone ESD training and are required to repeat the training on an annual basis.

Inspection and packaging is carried out in a static-free environment to ensure delivered product is freeof damage. Devices are packaged in conductive carriers and delivered in static-free bags.

It is recommended that all handling and inspection be performed in a similar environment. Figure1shows a workstation capable of meeting “Class 1” handling requirements. It is also good practice toensure that terminals leading to RF and dc connections within the device are not touched, even thoughwrist straps are being worn.

When handled properly, TQS MMICs have demonstrated MTTFs of 10 million hours (or better) at140°C channel temperature. TQS utilizes various designs in military equipment such as the HARMmissile, ATF-phased array radar, LANTIRN, ELS, GEN-X, and others. All of these applications haveexhibited excellent reliability.


To summarize, the following recommendations are made:

l ensure that all workstations are static freel handle devices at static-free workstations onlyl implement ESD training for all operatorsl control relative humidity within 40-60%l transport all work in static-free containersl ground yourself before handling device leads, even though a wrist strap is being worn

Following these recommendations will ensure devices are free of damage and will result in many yearsof trouble-free service.


MMIC Assembly Procedures: Manual and Automated

Solder AttachmentSolder attachment is performed utilizing a reflow process during which the solder preform or paste isbrought to its melting temperature and attaches the GaAs die to the substrate or base-plate material.Because of its metallic properties, the solder attachment alloy provides optimum electrical and thermalcontinuity between the two surfaces as well as mechanical integrity. The attachment is formed whenthe alloy and the two surfaces are heated, causing the alloy to melt at a specific temperature, depending on its elemental composition.

The heat can be produced by various techniques, and the simplest of these is a hot plate. Typicalreflow of materials on a hot plate in air usually requires flux. This is a rosin-based material that pro-motes wetting by cleaning the two surfaces to be mated, thus removing oxides and contaminants andinhibiting oxidation of the mating surfaces during the attachment operation. Fluxes cannot be utilized inthe assembly of GaAs devices due to electrical and mechanical degradation upon exposure. Fluxesmay be eliminated through the use of a forming gas blanket over a hot plate. The forming gas atmo-sphere provides an adequate reducing environment to inhibit oxide formation during the reflow opera-tion. It is for that reason that these types of alloy stations are commonly employed for rework when fluxcannot be utilized.

Most microwave assemblies are reflowed in a conveyor belt furnace. Because of the reducing atmosphere within the furnace, there is no need to employ flux during the attachment operation. An atmo-sphere of forming gas or hydrogen adequately reduces the oxides from the surfaces to be attached.Solvent cleaning of the preforms and substrates are advised. The GaAs MMICs can be cleaned usingan oxygen or plasma cleaning process prior to reflow; however, if the GaAs is stored properly in drynitrogen, no cleaning should be necessary.

AuSn (80/20) is the alloy most commonly used in the industry for GaAs assemblies due to its compat-ibility with gold-based components and its long-term reliability. Because it is a hard solder and GaAs isa very brittle material, special care must be taken to ensure that the coefficient of thermal expansion(CTE) of the substrate or base-plate material matches the CTE of GaAs (5.73 ppm/°C). A typical CTErange that is acceptable places the GaAs in slight compression (6 to 10 ppm/°C). Some materialswithin this CTE range include Al2O3, CuW, CuMo, and Alloy 46, which is a common FeNi alloy. Mate-rials that have a CTE lower than that of GaAs, such as Kovar or AlN, should not be utilized due tostress fractures that may develop in the device during assembly or during environmental exposure orconditioning. Extended time at or above 300°C is harmful to GaAs devices. Each three minutes ofexposure at 300°C is equivalent to approximately 7000 hours of 100°C operational life. Reflow pro-cess assembly notes:

l limited exposure to temperatures at or above 300°Cl alloy station or conveyor furnace with reducing atmospherel no fluxes should be utilizedl coefficient of thermal expansion matching is critical for long-term reliabilityl storage in dry nitrogen atmosphere


Component Placement and Adhesive

Component placement involves the pick up and placing of components in the assembly of circuits andmicrowave modules. Vacuum pencils and/or vacuum collets are the preferred method of pick up.Although tweezers may be used, chipouts of the edge can take place. These chipouts may propagate,causing reliability concerns.

In any automated placement, avoidance of air-bridge locations should be practiced to prevent deformation and other air-bridge damage. Force impact is critical during pick up and placement in an auto-mated mode of operation because of the brittle nature of the GaAs device. An evaluation should beconducted prior to implementation of auto pick-and-place on GaAs products to define the acceptableforce profile on the specific piece of equipment.

Attachment of GaAs devices on substrates can be performed using an organic adhesive such asepoxy or polyimide. These organics typically have properties of thermal conductivity an order of mag-nitude below that of solder and should only be used in low-power applications where good heat trans-fer from the device is not an issue. Epoxies cure at temperatures of 100°C to 200°C. Curing can beeffected in a convection oven with exhaust for the expulsion of diluents and solvents during the cureprocess. Microwave or radiant curing techniques may result in differential heating of the assembly.This can potentially cause failure to the GaAs device due to excessive black body heating, where thedevice heats up faster than the surrounding components and absorbs all of the heat energy beinggenerated.

Coefficient of thermal expansion matching of the substrate to the GaAs device remains a concer neven with organic attachment. In an unmatched situation, initial failure might not be seen because ofthe ability of the organic to absorb the stress. Instead, failure may occur after environmental conditioning when changes to the organic, such as hardening due to additional cross linking of the polymerchains, promote stress transfer from the unmatched substrate. The preferred CTE range is 6–10 ppm°C for substrate to GaAs attachment.

Component placement and adhesive attachment assembly notes:

l vacuum pencils and/or vacuum collets preferred method of pick upl avoidance of air bridges during placementl force impact critical during auto placementl organic attachment can be used in low-power applicationsl curing should be done in a convection oven; proper exhaust is a safety concernl microwave or radiant curing should not be used because of differential heatingl coefficient of thermal expansion matching is critical

Interconnect

Assembly interconnection involves the electrical connection of components within an assembly throughthe use of wires and/or ribbons. For GaAs assemblies, the wires and ribbons are typically 99.99%gold. The most commonly-employed technology is the thermosonic wire-bonding technique. Inthermosonic interconnect, the wire or ribbon bond is formed with pressure, heat, and ultrasonic en-ergy. The heat is provided through the bonding stage, the bond head movement exerts pressure onthe wire or ribbon, and ultrasonic energy travels from the transducer through the bond tool to the bond.This compares to the historically older thermocompression technique where the bond is formed prima-rily through heat and pressure.


While the level of heat in thermosonic bonding is usually kept at approximately 150°C, in thethermocompression technique, heat is applied through the stage at approximately 200°C as well asthrough the bonding tool at over 300°C. A maximum stage temperature of 200°C is recommended tominimize device degradation. Pressure or force, bond time, ultrasonic power, loop configuration, andbond locations are programmed parameters in automated bonding. Force, time, and ultrasonic powerare critical to reliability of the bonded device. The majority of interconnections are formed through theuse of ball-bonded wire. The circuit connection on one end of the wire is made by a ball with a wedgeor stitch at the opposite end of the wire.

Since GaAs devices currently have gold-plated bonding pads, only gold wire or ribbon should be usedfor interconnection. Aluminum wire causes reliability problems, due to intermetallic growth and theresultant “Kirkendall voiding” within the bonded interface. Minimum bond-pad size for ball bonding with0.001-inch gold wire is 0.004 inch × 0.004 inch. Discrete FET devices are generally wedge bondedwith 0.0007-inch wire because of pad size constraints.

Interconnect process assembly notes:

l thermosonic ball bonding is the preferred interconnect techniquel force, time, and ultrasonics are critical parametersl aluminum wire should not be usedl discrete FET devices with small pad sizes should be bonded with 0.0007-inch wirel maximum stage temperature: 200°C


Testing Considerations and Typical Test-Set Configurations

Generic test-set configurations for testing common RF and microwave parameters of the TriQuintSemiconductor (TQS) MMICs and simple biasing techniques to safely bias up the TQS MMIC amplifi-ers and FET switches are discussed below in detail.

Amplifier Biasing

For reliable operation of TQS GaAs FET amplifiers, VDS should not exceed 9 V, and the positivecurrent should be typically 0.5-IDSS for class A operation. Refer to data sheets for operational param-eters. Do not exceed the absolute maximum ratings stated in the device data sheet because exposureto absolute-maximum-rated conditions for extended periods of time may affect device reliability.

For devices that require a negative supply voltage, a sequencing power supply is recommended toensure positive supply voltage is applied only when negative voltage is present. The following proce-dures should be observed when testing these devices:

Dual-Supply Single-Gate AmplifiersTurn-on procedure:

1. All test equipment, tools, and personnel should be properly grounded.2. A regulated power supply is recommended.3. Adequate voltage spike protection on both bias lines is recommended.4. Apply the negative voltage at about –4 V for power amplifiers or –2 V to –2.5 V for general-purpose amplifiers.5. Slowly apply the positive voltage and observe the positive supply current.6. Readjust the negative voltage to obtain the required positive supply current.7. Apply RF drive.

Turn-off procedure:1. Turn off the RF drive.2. Set the positive supply voltage to 0.3. Set the negative supply voltage to 0.

Dual-Supply Dual-Gate AmplifiersTurn-on procedure:

1. All test equipment, tools, and personnel should be properly grounded.2. A regulated power supply is recommended.3. Adequate voltage spike protection on all bias lines is recommended.4. Apply the negative voltage at about –4 V for power amplifiers or –2 V to –2.5 V for general-purpose amplifiers. Make sure the positive supply voltage and gate 2 supply voltage are set to 0.5. Slowly apply the positive voltage and observe the positive supply current.6. Set gate 2 supply voltage to nominal.7. Readjust the negative voltage to obtain the required positive supply current.8. Apply RF drive.

Turn-off procedure:1. Turn off the RF drive.2. Set the gate 2 supply voltage to 0.3. Set the positive supply voltage to 0.4 Set the negative supply voltage to 0.


Single-Supply Amplifiers

Turn-on procedure:1. All test equipment, tools, and personnel should be properly grounded.2. A regulated power supply is recommended.3. Adequate voltage spike protection on the bias line is recommended.4. Slowly apply the positive voltage and observe the positive supply current.5. Apply RF drive.

Turn-off procedure:1. Turn off the RF drive.2. Set the positive supply voltage to 0.

Switch Biasing

For reliable operation of TQS GaAs FET switches, the control voltage should not exceed –10 V. Do notexceed the absolute maximum ratings since exposure to absolute-maximum-rated conditions for ex-tended periods of time may affect device reliability. Selection of a switch output arm is dependent uponthe biasing configuration. Refer to the data sheet for operational parameters. The following proceduresshould be observed when testing these devices.

FET Switches

Turn-on procedure:1. All test equipment, tools, and personnel should be properly grounded.2. A regulated power supply is recommended.3. Adequate voltage spike protection on bias lines is recommended.4. Slowly apply bias voltage(s).5. Apply RF drive.

Turn-off procedure:1. Turn off the RF drive.2. Set bias voltage(s) to 0.


Figure 2 is a typical test-set block diagram for measuring noise figure. The frequency range is limitedby the mixer.

Figure 3 is a typical test-set block diagram for measuring gain compression. RF power levels, frequen-cies, and attenuation levels are computer controlled.


Figure 4 is a typical test-set block diagram for second- or third-order intercept (SOI or TOI) measure-ments. The RF power meter is used to calculate the calibration factors of the test set.


Operating Dual-Supply MMICs From a Single Supply

Monolithic Microwave Integrated Circuits (MMICs) often use via grounds for source terminals tomaximize high-frequency performance. Biasing these devices requires both a negative and a positivesupply and sequential biasing of the gate and drain terminals. For the hybrid designer, this type ofMMIC may be difficult to use as a direct replacement for MIC modules due to single supply restrictions.This application note presents a technique for which amplifiers, designed for dual bias supplies, cannow be operated from a single-supply. This single-supply operation is achieved by mounting the MMICon a silicon metal oxide (MOS) capacitor, thereby isolating the ground from dc.

One possible self-bias configuration is shown in Figure 1. The FET source is floated from ground at dcthrough the MOS capacitor. Current flows through the resistor RS causing a gate-to-source voltagedrop, VGS = –IDSRS. Operating current is obtained from the square-law transfer equation, IDS =IDSS[1–(VGS/Vp)]2 . By combining these equations, one can solve for RS:

The supply voltage is calculated from: VDD = VDS+IDSRS.

To ensure safe operating junction temperatures,the MMIC should be mounted on a MOS capacitorhaving a low thermal impedance; this requires a thin, high thermal conductivity substrate material. Alarge capacitor should be selected so that it is distributed under the entire MMIC to ensure an effectiveRF ground plane (see Figure 2). The MMIC may be mounted to the MOS capacitor using conductiveepoxy or gold-tin solder. One recommended source for MOS capacitors is M/A-COM (SDI). The mini-mum recommended value is 300–400 pF.


Transmission-line and resonator structures were used to determine the effects of MOS-capacitor groundplanes on microwave propagation characteristics. A 50-Ω microstrip line was constructed on 4-mil-thick GaAs. The thru structures were mounted to both MOS capacitors and directly to the metal carrierplate. Insertion loss and the reflection coefficient were measured over the 250-MHz to 20-GHz range.In Figure 3, the MOS cap ground is slightly lossier than the metal ground by approximately 0.15 dBmaximum. The MOS cap ground also showed a higher return loss from 10 to 15.5 GHz with a 1.6-dBdelta maximum (see Figure 4).


Two types of resonator or T structures were measured. Both were on thick alumina and consist of a50-Ω through line with either a 30-Ω or 50-Ω open circuit stub at its center. Measurements show onlya slight shift in the resonant frequencies. Figure 5 further indicates that the MOS capacitor’s effects onRF performance should be minimal.

Self-biasing experiments were conducted using the TGA8014-SCC, TGA8021-SCC, and the TGA8320-SCC MMICs. The TGA8014-SCC is a two-stage medium power amplifier that covers the 6-GHz to 18-GHz range. The device uses reactively matched, 900-µm and 1200-µm FETs that provide 0.5-W out-put power at 1-dB gain compression, and 11-dB nominal gain. The TGA8021-SCC, an X-band lownoise amplifier, uses a single-ended feedback design to produce 2.5-dB noise figure, with 28-dB nomi-nal gain. Our dc to 8-GHz general-purpose gain block is the TGA8320-SCC. Four 200-µm FETs pro-vide 9.5-dB nominal gain with 17-dBm output power at 1-dB gain compression.

All of the MMICs were mounted on 6-mil thick, Si MOS capacitors using conductive epoxy. Theseassemblies were mounted to molybdenum carrier plates. A self-biased TGA8320-SCC assembly isshown in Figure 6. Total saturated drain current, IDSS , is typically 144 mA for this device with a pinchoff voltage, Vp, of –1.3 V. For the desired operating current of 70 mA, VGS = –0.4 V (see eqn. 1). Thusthe nominal value for RS = 6 W (see eqn. 2). It is recommended that a multitap resistor be used fortuning purposes. Nominal RS values used for the TGA8014-SCC and TGA8021-SCC assemblies were5 Ω and 27 Ω, respectively.

Small-signal gain and power measurements were made on the TGA8014-SCC and TGA8320-SCCdevices at 25°C, –55°C, and 85°C. Noise figure and gain were measured on the TGA8021-SCC as-semblies over these temperatures. In order to get a direct comparison for dual versus self biasing, thesame circuits were modified for dual-bias testing by shorting the MOS capacitor to ground via bondwires or conductive epoxy. The same measurements were repeated under duplicate current condi-tions. Performance data is shown in Figures 7–13.


Due to the limited values of source resistors available, the devices were operated at 10% to 20% lowerthan the recommended operating currents (40% to 50% IDSS ). This accounts for the overall, slightlylower gain and power performance from the typical data sheet performance. Figures 7, 9, and 11 showthe small-signal gain performance of the TGA8021, TGA8320, and the TGA8014, respectively. Perfor-mance differences are negligible for the TGA8014-SCC and TGA8021-SCC devices. The low fre-quency roll off on the TGA8320 device is due to insufficient bypass capacitance. The MOS cap valueused was 210 pF. At low frequencies, this no longer looks like an RF short, instead it becomes a highimpedance.

Figure 8 shows noise figure performance over temperature for the TGA8021-SCC. Again, there isclose agreement between the dual and self-biased performance, only a 0.2-dB maximum delta atroom temperature.

Power at 1-dB gain compression is shown in Figures 10, 12, and 13. When the same TGA8320-SCCdevice is tested in both self- and dual-bias configurations (Figure 10) with the same dc bias applied,the self-biased version exhibits approximately 2 dBm less power. This is caused by the reduced volt-age across the FET due to the voltage drop across RS . Figure 13 compares power performance onself- and dual-bias TGA8014-SCC assemblies (with and without MOS capacitor grounds) on devicesfrom the same wafer. MMICs on the dual-bias assemblies were mounted directly to a molybdenumshim and biased at 50% IDSS . There is approximately a 1.5-dB degradation in power performance onthe TGA8014 self-biased devices.

A thermal analysis was performed comparing the self-bias and dual-bias TGA8014 assemblies. Aninfrared scan was performed on the 1200-µm FETs. Results show that for a case temperature of 25°Cand devices dissipating 1.75 W, the self-biased FET operated at 40.4°C/W and the dual-biased FET at39°C/W. Electrical thermal impedance measurements produced similar results: 43.8°C/W–MOS and38.9°C/W–molybdenum.

The self-bias technique offers several advantages; only one power supply is required, the need forbias sequencing (gate/drain) circuitry is removed, and the resistor in series with the source provides


voltage suppression. Possible disadvantages are that gate and drain voltages cannot be adjusted andthere is a slight performance degradation using MOS capacitors. Using the self-bias makes it possibleto replace most current MIC amplifier modules with dual-supply MMICs and without costly redesign.

Reference: D. Sabo, W. Ou, B. Hundley, S. Nelson, and J. Nuttall, “Silicon MOS Caps Carry GaAsMMIC to Singular Supply”. Microwaves and RF. December 1988, pp.103–108.

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