GALLIUM ARSENIDE FIELD EFFECT TRANSISTORS - THEIR PERFORMANCE AND

APPLICATION UP TO X-BAND FREQUENCIES

H.E.G. LUXTON

ABSTRACT

The microwave performance of GaAs FETts is reviewed and examples oftheir application in systems operating up to X-band frequencies discussed.Examples of amplifiers (one giving 42 dB gain at 11.2 GHz) and oscillatorsoperating at frequencies hitherto unattainable by 2-port solid statedevices are used to illustrate the performance and potential of GaAs FET'sin microwave systems.

DEVICE PERFORMANCE

Gallium arsenide field effect transistors with gate lengths of 1 p.mor less are currently being produced in various laboratories throughoutthe World [iJ- [31 * These devices exhibit cut-off frequencies inexcess of 30 GHz and power gains of 10 dB at X-band frequencies. Suchdevices are now commercially available together with 2 jim and 4 pm gatelength types suitable for operation up to C and L-band frequenciesrespectively.

These devices have been developed principally for small signal, lownoise amplifier applications. At 10 GHz the noise figure of 1 Pm gatelength device is typically 6 dB. At S-band 2 1im gate length devices give10 d.B of gain with a 3 dB noise figure. Similar gains and noise figuresare obtained from 4 pm devices at L-band.

MICROWAVE AMPLIFIER APPLICATIONS

At frequencies below 6 GHz the GaAs FET has to compete with bipolarmicrowave transistors and, although it requires more careful matching,the YET offers a number of advantages at frequencies down to L-band. Theseare:

1. Low noise figure and high gain

2. Low intermodulation and cross-modulation distortion

3. Large dynami c range

4. Ease of d.c. biasing, which simplifies the microwave circuit design

5. High radiation resistance and thermal stability since the d.evice isunipolar.

The low noise and distortion of the GaAs YET has led to its use inamplifiers for S-band radar and L-band receiver systems. It is, however,at X-band frequencies that the GaAs YET is unique, being capable ofyielding gains of 10 d.B per device at 8 GHz with output power levels ofapproximately +10 dBm. The small variation of the device parameters withtemperature results in slight changes in the gain of amplifiers as the

Allen Clark Research Centre, The Plessey Company Limited, Caswell,Towcester, Northants. IN12 8EQ

92

ambient temperature varies. If this gain variation cannot be tolerated bythe rest of the system it may be corrected by compensation of the d.c.bias conditions.

TABLE I

GaAs PET MICROWAVE AMPLIFIERS

Number and NoiseDescription Gatelength Gain -1 dB Bandwidth Figure

of devices

L-band lumped 1 4 ,um 15dB 700MHz - 900MHz 3dBcomponent

l-band distributed 1 4 ,m 12dB 1.3GHz - 1.6GHz 3.5dBelement

S-band distributed 4 2 ,um 30dB 2.7GHz - 3.1GHz 3.7dBelement

C-band distributed, 2 1 im| 15dB 5.8GHz - 6.4GHz 7.5dBtuned

X-band distributed, 3 1 gin 20dB 11.0GHz - 11.5GHz 12dBtuned

The performance of a range of GaAs RET amplifiers is listed in TableI. All the devices were operated with zero gate and +5 volts drain-sourcebias. The amplifiers were optimised for gain and frequency response andthe noise figures are those of the completed amplifier including inputisolator, when used.

The L- and S-band amplifiers were designed using the measured s-parameters of the FET's to determine the characteristics of the matchingnetworks required. At X-band frequencies there are problems in accuratelymeasuring s-parameters and also in realising specific matching networks onalumina microstrip circuits. To avoid these difficulties X-band amplifiermodules have been produced by using a tuning technique. This requiresvirtually no circuit design and no precise s-parameter data for the devicesbeing used.

Figure 1

X-band amplifierconstruction

93

An 11.2 GHz amplifier module constructed using this technique isshown in Figure 1. Three 1 pVm gate length devices mounted on discsattached to the ground plane of the alumina substrate are interconnectedby lengths of 50 ohm microstrip lines. GaAs chip capacitors at the endsof these lines isolate the d.c. bias which is applied via bond wires fromthe microstrip bias "tees". The resistive film attached to the gate biasnetworks introduces loss at out of band frequencies hence stabilising thedevices and preventing low frequency oscillation. Metal discs arepositioned on the 50 ohm interconnecting lines to obtain the required gainresponse. When the optimum response has been achieved the discs arepermanently fixed to the substrate.

This versatile method of construction is ideally suited to producingprototype amplifiers amplifiers and for evaluating devices. It is quick,cheap, and tolerant of device parameter variations, yet capable of realis-ing the full potential of the FET's being used.

Figure 2

42 dB Gain

11 .2 GHz GaAs YET amplifier

Fi&ure 3

11.2 GHz amplifier gainresponse

10 7 11-2

FREQUENCY (GHz)11*7

A prototype 42 dB gain 11.2 GHz amplifier has been developed for asatellite repeater application which uses two tuned modules, Figure 2.

The gain response of this amplifier is shown in Figure 3 and otherparameters are listed in Table II. This amplifier has no provision forcompensating the small gain variation with temperature.

94

TABLE II

1 1 . 2 GHz AMPLIFIER PERFORMANCE

GainNoise figure

) at 20°C

I

Centre frequency

Bandwidth at -1 dB points

11.18 GH2

260 MH2

42 dB13.5 dB

1.5+ 1.2 dB

- 40 dB

8.5dBn

Input and Output VSWR's

Gain variation 70C - 35 0CThird order intermodulationlevel at output for two equalcarriers at input of -50 dB.neach

Output power level

V

Firire 4

Temperature compensated.11.2 GHz amplifier

A three module amplifier of similar performance but incorporating athermistor circuit to control the negative gate bias applied to the firstmodule has also been produced, Figure 4. This reduces the gain variationwith temperature to less than 1 dB total with the minimum gain occurringat 20 C

OYHER APPLICATIONS

The GaAs FET can be uped in microwave oscillator circuits. Pre-liminary designs at L- and X-band have given output powers of 10 mW and5 mW with efficiencies of 20% and 10%o respectively. Because the FET is ahigh impedance device, it does not lower the Q of the oscillator resonantcircuit hence signals of good spectral purity are obtained.

At S-band the performance of 2 um gate length devices as mixers hasalso been examined. [4]

95

CONCLUSION

It has been shown that GaAs PET's may be used to produce high gain,

low noise microwave amplifiers, suitable as solid state replacements for

travelling wave tube and other conventional amplifiers in microwavesystems.

A novel method of constructing X-band amplifier modules and compen-

sating the gain variation with temperature has been presented.

ACKNOWLEDGEMENThS

The author gratefully acknowledges the contributions of hiscolleagues at the Allen Clark Research Centre. This paper is publishedby permission of the Directors of The Plessey Company Limited. This work

has been carried out with the support of Procurement Executive, Ministryof Defence and the European Space Research Organisation.

REFERENCES

1 J.A. Thrner, 'Schottky-Barrier FET'S, Next low noise designst.

Microwaves, Vol. 11, No. 4, April 1972

;2 NaG. Bechtel, et al, 'X-band GaAs PET'.

Microwave Journal, Vol. 15, No. 11, Nov. 1972.

3 C.A. Liechti, et al, 'GaAs Microwave Schottky-Gate PET'.ISSCC Digest 72, P. 158.

4 J.E. Sitch and P.N. Robson, 'The Performance of GaAs FET's as

Microwave Mixerst, Proc. IEEE (USA), Vol. 61, No. 3,

March 1973.

96