GaN Switch #4 - Plextek RFI, design house, design … · Case Study DC to 20GHz High Linearity GaN SPDT Switch MMIC GaN technology is now widely adopted for high power microwave amplifiers

Case Study

DC to 20GHz HighLinearity GaN SPDT

Switch MMIC

GaN technology is now widelyadopted for high powermicrowave amplifiers. Thehigh breakdown voltage of thetransistors is key to theirability to provide high poweramplification and it also meansthey can be used to realise veryhigh linearity RF andmicrowave switches. This casestudy describes the design andimplementation of a DC to20GHz Single Pole DoubleThrow (SPDT) GaN MMICswitch. It offers an IP3 of62dBm with an insertion loss ofjust 0.75dB at 1GHz rising to1.3dB at 20GHz. Isolation isover 45dB and the die size isjust 1.6mm2. GaN transistors as switches

GaN transistors can easily beconfigured to perform an RF switchingfunction. Their suitability for switchrealisation stems from the fact that thedrain-source resistance behaves as avoltage variable resistor, the resistancebeing set by the gate-source voltage.When used as a switch, the transistoris operated with the drain and sourceat zero volts DC. The RF signal pathis drain to source and the gate is thecontrol terminal.

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In the region of Vds=0V, the Vds/Idscharacteristic approximates a resistance(Ids µVds). For Vgs=0V this is a lowdrain-source resistance (the transistor ison) and for Vgs below pinch-off(typically around -3 to -5V for GaNprocesses) it is a high drain-sourceresistance. This gives rise to the simpleapproximate equivalent circuit, shownbelow. A gate resistor (Rg) is included asa simple and effective means of providingisolation between the RF signal path andcontrol port, a value of several kΩ istypically used. The parasitic capacitance,in parallel with the resistor, limits theisolation that the transistor can provide,and tolerating this is one of the keychallenges in the design process.

Sheet Code R

Fi0607

DC-20GHz SPDT GaN Switch Layout



� Switch transistors do not include asource coupled field plate. Thisimproves the RF performance of theswitch (reducing parasiticcapacitances) but reduces thebreakdown voltage. Although thebreakdown voltage is reduced it isstill impressively high and controlvoltages in the range -25V to -40Vare typically used.

The basic approach for designing RFGaN switches is straightforward. Thesimplest SPDT switch uses a singleseries mounted transistor in each arm,as depicted below. Control voltages V1and V2 are complimentary (V1 being"low" when V2 is "high" and vice-versa). This topology can offer low lossbut limited isolation, which degradesfurther with increasing frequency. Thisdegradation in isolation is caused by theparasitic drain-source capacitanceshown in the simplified equivalentcircuit above. The choice of transistorsize (total gate width) largelydetermines the performance of thissimple SPDT. Increasing the total gatewidth of the transistor would reduce theon-case insertion loss but would alsodegrade the isolation. This topology isonly suitable for use at relatively lowRF frequencies and more complextopologies must be adopted to extendthe switch performance up into themicrowave frequency range.

Design approach for the DC to20GHz switchThe simplest way to extend the upperoperating frequency of the SPDTshown below is to add a shunt deviceto the output of each arm. These operateunder complimentary control to thecorresponding series device and extendthe practical upper operating frequency.However, whilst this approach doesextend the upper operating frequency itdoes not come close to allowing usefulperformance to 20GHz. For such widebandwidths as this a distributedarchitecture is most appropriate, whichwas the approach adopted in this design.The circuit schematic of the DC-20GHzSPDT is depicted on the following page.

The key to extending the upperoperating frequency of the switch is toembed the shunt transistors (Q2, Q3 andQ4) in a low pass filter structure. Whenthese transistors are pinched-off theirshunt capacitance combines with theseries inductors (L2, L3 and L4) to forma low pass filter with low insertion lossover the required operating band.

The layout of GaN transistors foroptimum performance as an RF switchis slightly different to the layout ofamplifying transistors. The GaN processitself is the same and switch transistorsand amplifying transistors can berealised on the same die. The maindifferences between switch transistorsand amplifying transistors are:

� The amplifying transistors includeintegral grounded sources(connected to the back side of thedie using through substrate vias)

� The gate finger pitch of theamplifying transistors is greater thanthat of the switch transistors. Thisimproves the thermal performance(ability to dissipate heat withoutgetting too hot) but does increase thedie area occupied by the transistor.As a switch transistor normallydissipates much less power than anamplifying transistor it can toleratea smaller gate pitch.

� Switch transistors feature a gateplaced centrally between the sourceand drain. This symmetric structureresults in maximum RF powerhandling, with Vds=0V and in theoff-state, for a gate bias voltagecentred between the breakdown andpinch-off voltages for the process.

Simplified model of a GaN transistor as an RF switch

Simple series only SPDT



When the shunt transistors are on (Vgs= 0V) their low resistance transformsthe low pass filter into a high valueattenuator providing isolation for theoff-state arm. Complimentary controlvoltages V1 and V2 (in this design0V/-40V) set each arm of the switch intoopposite states (if the top arm is theon-path the bottom arm will be theoff-path).

The series transistors Q1 are pinched-offwhen the shunt transistors in thecorresponding arm are set to their lowresistance state (i.e. high RF path loss).The off-state series transistors transformthe low impedance at Q2 to a highimpedance at the input. At the same timethe other pair of series transistors arebiased at Vgs = 0V and provide a lowimpedance through path. The followingshunt transistors are pinched-off and theswitch arm behaves as a low pass filter.Thus the overall switch has one low losson-path and one high isolation off-path.

The use of two series transistors Q1,rather than a single transistor, is relatedto providing improved linearity asdiscussed below.

Small Signal PerformanceSmall signal performance is shown inthe two plots below. Insertion loss ofthe on-state path is just 0.75dB at 1GHzrising gently with frequency but stillless than 1.3dB at 20GHz. Input andoutput return losses are better than18dB to 20GHz. The isolation of theoff-state path is higher than 45dB to20GHz.

Power Handling and LinearityTransistor large signal models arenormally optimised to simulate theperformance of the transistor as anamplifier and are often not well suitedto accurately modelling switchcompression and linearity.

Fortunately it is possible to estimate thepower handling performance with areasonable degree of accuracy. The peakpower handling of the switch MMIC isdetermined by the ability of off-statetransistors to handle the associated RFvoltages and the ability of the on-statetransistors to handle the associated RFcurrents. Power handling of both theon-state and off-state devices must beconsidered in order to determine thepower handling capabilities of the entireswitch.

For the on-state devices the peak RFcurrent flowing through the seriestransistors dominates. The larger theseries transistor the higher its currenthandling capability and the higher theRF power level it can pass beforecompression.

Schematic of the SPDT GaN switch



SummaryGaN transistors can be used to realiseRF and microwave switches with veryhigh power handling capabilities. Thedesign presented here is a Single PoleDouble Throw (SPDT) switch realisedon a 0.25µm gate length GaN on SiCprocess. The switch covers DC to20GHz with an insertion loss of just0.75dB at 1GHz rising to 1.3dB at20GHz. Isolation is over 45dB and IP362dBm. The overall die size is 1.6mm2.

Generally speaking the higher the upperoperating frequency of a switch thesmaller the series devices that need tobe used. Viewed simplistically the useof two series devices in cascade allowsthe size of the transistor to be doubledwhilst maintaining the same insertionloss and isolation. This means anincrease in power handling of 6dBcompared to using a single seriestransistor. In reality the additionalparasitics associated with a largertransistor mean that the performance atmicrowave frequencies actuallydegrades slightly for a cascade of twotransistors of double the size and thereis a trade-off between small signal andlarge signal performance. For thedesign presented here the estimated1dB compression point of the seriestransistors is 42dBm and the IP3 pointaround 62dBm.

For the off-state devices compressionoccurs when the RF voltage swingcauses the gate-drain breakdownvoltage to be exceeded on the positivehalf cycle or when it moves thetransistor out of pinch-off on thenegative half cycle.

The high breakdown voltage of the GaNtransistors allows a control voltage of-40V to be used. The effective P-1dBpoint of the off-state transistors isestimated at around 48dBm, whichmeans that the compression performanceof the on-state devices dominates.

The compression and linearity figuresabove are valid for frequencies abovearound 100MHz. The compressionperformance of HEMT switchtransistors degrades at very lowfrequencies. The is a well knownphenomenon, which is described in moredetail in a paper entitled "The Design ofIntegrated Switches and Phase Shifters"available for download from the PlextekRF Integration website:

http://www.plextekrfi.com/publications/publications-from-plextek-rfi

Consideration must also be given to themaximum power dissipation that theswitch can tolerate. It is possible todesign GaN switches with very hightheoretical P-1dB points that woulddissipate a damaging amount of powerbefore the P-1dB point was actuallyreached.

Insertion loss and return lossesof the GaN SPDT

Insertion loss and isolationof the GaN SPDT

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GaN Switch #4 - Plextek RFI, design house, design … · Case Study DC to 20GHz High Linearity GaN SPDT Switch MMIC GaN technology is now widely adopted for high power microwave amplifiers