Nuclear Instruments and Methods in Physics Research A315 (1992) 385-392North-Holland

Front-end in GaAsPresented by D.V . Gamin

D.V. Camin, G. Pessina and E, Prevital_iDipartimento di Fisica dell'Unicersita' and Istituto Nazionale di Fisica Nucleare, Sezione di Milano, (20133) Milano, Italy

The use of GaAs MESFETs in the realization of low-noise preamplifiers for particle detectors are analized in this paper.Fundamental properties of GaAs are reviewed . The low ionization energy of dopant impurities allows operation at cryogenictemperatures.The high electron mobility permits to obtain high gain-bandwidth products with low power dissipation . Voltage-sensi-tive preamplifiers for 4 K operation have been developed and are presently used with bolometric particle detectors . Charge-sensi-tive preamplifiers for liquid calorimetry have also been developed and are used for the readout of the Accordeon LAr calorimeterprototype . A version based on a monolithic array of MESFETs was tested as a first step towards a monolithic preamplifier version .So far, hybrid techniques have been used for the preamplifier manufacturing with a very high yield .

1. Introduction

Low noise preamplifiers based on GaAs metal-semiconductor field-effect transistors (MESFETs) havebeen introduced some time ago for the signal amplifi-cation of cryogenic particle detectors [1] . The adoptionof GaAs was based on the rather narrow choice oflow-noise electronic devices capable to operate at cryo-genic temperatures . In effect, silicon JFETs start toexhibit carrier freeze-out at temperatures below 100 K.Presently available bipolar transistors rapidly decreasestheir current amplification factor when cooling [2] . Onthe other hand Si MOSFETs compensate carrierfreeze-out by virtue of the strong electric field underthe gate which makes it possible to operate thesedevices even at 4 K; nevertheless there are two charac-teristics that have to be considered : a) 11f noise in theMOSFET channel do not reduce and even increases atlow temperatues [3] limiting its use for very low fre-quency applications like in thermal detectors; b) atpresent, speed limits applications at very fast shapingwith large detector capacitances like in cryogenic liquidcalorimeters . So far satisfactory results have been ob-tained for low detector capacitances or at long shapingtimes [4] . In any case, readily available monolithicintegration of a large number of channels might makeradiation-hard MOS a choice in cryogenic applicationsfor future accelerators, when detectors with very lowoutput capacitances are involved.

The most relevant properties of GaAs MESFETswill be reviewed, and examples of GaAs basedvoltage-sensitive preamplifiers used for the signal am-plification of thermal detectors and charge sensitive

0168-9002/92/$05 .00 © 1992 - Elsevier Science Publishers B.V . All rights reserved

2. Properties of GaAs MESFETs

NUCLEARINSTRUMENTS&METHODSIN

PHYSICS

RESEARCHSectionA

preamplifiers developed for liquid argon calorimetrywill be described . Finally, the characteristics of a re-cently developed charge-sensitive preamplifier basedon a monolithic array of MESFETs will also be re-ported.

Nature has given GaAs fundamental physical pa-rameters which make it an attractive option for therealization of low-noise cryogenic and fast amplifiers .In fact, the high electron mobility and low electric fieldfor carrier peak velocity make it possible to obtain hightransconductance to input capacitance ratios at lowpower dissipation. This is a parameter of prime impor-tance in charge-sensitive preamplifiers as it is relatedto the product of the charge sensitivity times the speedfor a given detector capacitance . In addition, the lowionization energy of dopant impurities keeps limitedthe freeze-out of carriers even at 4 K.

VI1. FRONT-END ELECTRONICS

Table 1

GaAs Si

Electron 300K 5000 800mobility 77K 10000 3000[cm V` s- 11W 3000 nd

Electric field at peakvelocity IV/p.mj 0.7 3

Ionization energy ofdopant impurities [meV] 6 50

Energy bandgap at 300K [eV1 1.4 1.1

386

h 1/2ENC= 8kTX -1 CD

WT TM

D.V. Camin et al.

Some fundamental parameters of GaAs are indi-cated in table 1 and, to make a comparison with thewell established Si technology, the corresponding val-ues for silicon are also given. A doping level of 1017cm- 3 have been assumed in both cases [5,6] .

So far, only MESFETs have been used for therealization of low noise preamplifiers for particle de-tectors, although other devices like AlGaAs/GaAs highelectron mobility transistors (HEMTs) may be used totake advantage of their impressive electron mobilitieswhich, from a value of 6000 cm2/V s at room tempera-ture can reach 3 x 105 cm2/V s at 77 K and 2 x 10 6cm2/V s at 4 K [7] .

The selection of suitable MESFETs is done amongthose device types exhibiting low series noise at lowfrequencies which is normally the main limiting param-eter . At room temperature series noise in MESFETs ismainly of generation-recombination type which domi-nates at low frequencies . The distribution of the spec-tral power density is proportional to 1/f due to thepresence of multiple traps, each one contributing witha T/(1 + ui 2TJ, where T is the caracteristic time con-stant of the trap. At low temperatures this dominant1/f noise decreases strongly due to the exponentialdependance of T with 1/T. A reduction of two ordersof magnitude in the spectral power density when tem-perature decreases from 300 K to 77 K was observed.An additional factor of five in noise reduction occurswhen cooling to 4 K [8] .

White noise is the limiting parameter in the signal-to-noise ratio at very high frequencies or, equivalently,at short shaping times. The large transconductance/in-put capacitance ratio of GaAs MESFETs make it pos-sible to obtain low dependance of the equivalent noisecharge (ENC) on the detector capacitance in chargepreamplifiers while keeping low the power dissipation.In effect :

where X is about 1.2 for GaAs at 300 K, coT = gm/C;the gain-bandwith product of the input device, h1 is anumerical parameter depending on the shaper consid-ered and the rest are the usual terms [9] . MESFETspresently used in cryogenic preamplifiers have OT/2Trvalues in excess of i GHz with less than 400 gW powerdissipation at 77 K.

Regarding radiation hardness, GaAs devices havebeen thoroughly investigated in particular in what con-cerns static and dynamic characteristics [10] . How noisedepends on neutron fluence and on gamma, radiationdose is not yet fully understood. A preliminary studyon commercial MESFETs, obviously not radiationhardened, have been performed and the results showthat noise at 20 ns shaping time increased by 5% after

/ Front-end in GaAs

1.5

E

0.5

0.00.0 0.5 1 .0 1.5

Vd [VIFig . 1 . Static output characteristics of a 3SK164 MESFET at4 K. Dotted lines shows the simulation according to a UCBGaAs model whose parameters have beeen extracted from the

measurement.

being subject to a neutron fluence of 1014 n/cm2 andby a factor of two after irradiation to 106 rad (Si) witha 6°Co gamma source [11] . New studies on other MES-FETs types will be done in the near future .

To illustrate on the behaviour of GaAs MESFETsat cryogenic temperatures, fig. 1 reports the staticcharacteristic of a 3SK164 MESFET at 4 K. In thesame figure the simulated I-V curves of the device areshown . They correspond to a UCB GaAs MESFETmodel whose parameters have been extracted using acomputer program (IC-CAP) driving a SemiconductorParameter Analyzer (HP 4142A) . Fig . 2 shows thestatic characteristic of a NE25139-U71 at 77 K. Thesimulated characteristics using parameters extractedand optimized around [ VDs = 2 V; ID = 7.5 mA] arealso plotted . In the realization of complex circuit con-figurations, extraction of semiconductor models param-eters at any temperature between 4 K and 300 K, and afurther SPICE simulation proved to be an advanta-geous tool .

So far, GaAs MESFETs, specially designed for theutilization as amplifying element in front-end electron-ics for detectors used in particle physics, are still notavailable . For accelerator use, the aspect of radiation

Fig. 2 . Same as fig . 1 but for a NE25139-U71 at 77 K.

hardness has still to be inves*igatcd . For low frequencydetectors at low temperatures, like thermal detectors,devices with very low 1 /f noise should be developed toimprove the present results. The improvements on thelow-frequency noise performances will most probablybe obtained at the expense of increasing the inputcapacitance which, in any case, is not a noise limitingfactor when working with voltage sensitive preampli-fiers.

Nevertheless, a systematic characterization of com-mercially avaiiable MESFETs led to the identificationof many device types with very satisfactory static andnoise performances. With these devices several ver-sions of low-noise preamplifiers have been developedand later-on fabricated in series of more than 200 unitswith a yield larger than 95%. Nowadays, the low dis-persion in component's characteristics makes it possi-ble to construct much larger quantities keeping highthe yield. A description of the most significant instru-ments developed will be described in the next para-graph.

3 . Voltage-sensitive preamplifiers for 4 K operation

The need to reduce the parasitic capacitance, thepick-up of electromagnetic interference and the ther-mal power injection through the connecting leads inbolometric particle detectors, led to the developmentof GaAs voltage-sensitive preamplifiers which had tooperate between 1 K and 4 K inside a dilution refriger-ator. The main feature looked for was to obtain a verylow series noise keeping low the power dissipation ;preamplifier's parallel noise is not a matter of concernat cryogenic temperatures as the gate leakage currentreduces exponentially with 1/T. Speed is also notcritical for these applications . Succesive versions havebeen developed in the last years [12] and at present adesign with outstanding performances is used in theexperiments with bolometric detectors performed bythe group of Milano [13].

Fig . 3 shows the schematic diagram of the circuitdeveloped . Series input noise is kept low by paralleling10 3SK164 MESFETs at the input, Q 1 , and operatingthem at low drain-source voltage and low drain current[0.6 V; 0.6 mA] where the 1 /f noise is low . A doublecascode Q2 , Q3 amplify the relatively low outputimpedance at the drain of Q1 and the output signal ofthis first amplifying stage is developed at the dynamicload 04, Q 5 . A second, inverting stage Q6 providesadditional gain and the feedback network R2, R 1 fixesthe closed loop gain to a nominal value of 52 . Transis-tors Q7-Qlo determine a network which provides lowimpedance, thermally compensated biasing voltagesources as required by the circuit . A detailed descrip-tion of this preamplifier is given in ref. [13] .

D. V. Camin et al. / Front-end in Ga4s

VCC

looR13

° 3KsVREGFig . 3. Circuit diagram of a 4 K GaAs voltage-sensitive pream-

plifier used in experiments with bolometric detectors.

The preamplifier components have been originallymounted onto a ceramic substrate which showed to bevery much sensitive to thermal shocks when cooled to 4K. At present, components are surface mounted onto aFR4 printed circuit board as shown in fig. 4. The use ofceramic substrates at 4 K has not been excluded aspositive results have been obtained when using highquality hybrid manufacturing techniques .

Fig. 5 shows the total series noise of the voltage-sensitive preamplifier at 4 K and 77 K. A noise level of9 :.V/ ~Hz at 100 Hz with 1/ VFf distribution and 0.5nV/ Vffz at 100 kHz was obtained with a power dissi-pation of 50 mW. The response to a step input voltagepulse has a rise time of 40 ns .

4 . Charge-sensitive preamplifiers

387

R_-1K3

Charge-sensitive preamplifiers in GaAs have beenoriginally developed for high impedance bolometric

Fig . 4. The voltage-sensitive preamplifier circuit mountedonto a FR4 printed circuit board .

VII . FRONT-END ELECTRONICS

388

detectors and other thermal detectors. Later-on theyhave been proposed as front-end for cryogenic liquidcalorimeters taking into account the favourable noisebehaviour at short shaping times and the fast speed ofresponse and good dynamic range with low powerdissipation.

In order to make an evaluation of the relativequality of different versions of charge-sensitive pream-plifiers, a factor of merit can be calculated as shownbelow.

4.1. A factor of merit for charge-sensitive preamplifiers

In first approximation, the rise time of the responseto a 8 input current is given by the following expres-sion, which assumes a dominant pole in the open loopgain:t r = 2.2Ri(Cd +Ci + Cf + C,),

(2)

tr is the rise time of the output pulse, Ri is thepreamplifier input resistance, Cd, C i , Cf and C, are thedetector, input, feedback and test capacitances respec-tively . Taking into account that R i is given by:

Ri = gm 1C0/Cfwhere gm is the transconductance of the input transis-tor, and Co the capacitance determining the dominantpole, a preamplifier's transition frequency f, =gm/2zrCO can be calculated from eqs . (2) and (3) bymeasuring the rise time for a given detector capaci-tance .In effect,

ft =0.35tr'Cf'(Cd +Ci +Cf +Ct )

(4)

now, considering that t r ' expresses the speed and Cf'the charge sensitivity of the preamplifier, and that theirproduct increases only if the open-loop gain is im-

40n

rmsV/rHz

In

400p

D. V. Camin et aL / Front-end in Ga4s

proved, which means increasing the power dissipationPd , it is useful to calculate for each design the factor ofmerit :

F=f~lPd =0.35tr'Cf 1 (Cd +Ci +Cf +Ct )Pd 1

which relates the speed-sensitivity product to the powerdissipation for a given detector capacitance . Also, ahigh f means low white noise which depends on1/

g �, .So far it was found that the factor of merit of GaAs

charge preamplifiers is higher than that of siliconpreamplifiers of similar characteristics, and this is justa consequence of the higher mobility of electrons inGaAs which allows to obtain higher trans-conductances/input capacitance ratio at a lower powerdissipation .

4.2. Cryogenic charge-sensitive preamplifiers

IOOHz 1kHz

Fig . 5 . Series noise at 4 K and 77 K of the circuit of fig . 3 .

The first cryogenic charge-sensitive preamplifier de-veloped using exclusively GaAs devices was reported inref. [8] . It matches detector capacitances of about 10pF, at 4 K has a minimum ENC of 20 electrons rms at0 pF detector capacitance, and dissipates 9 mW, theresponse to a S current has a rise time of 20 ns with 1pF feedback capacitance and the operating tempera-ture extends from 1 K to 120 K. The upper tempera-ture was limited by the voltage drop in the 109 SZfeedback resistor caused by the gate leakage current .Fig . 6 shows the preamplifier's ENC as a function ofthe shaping time at 4 K. The factor of merit of thispreamplifier is F = 78 MHz/mW but it has a lowdynamic range . At present it is used to amplify thesignal of a photodiode coupled to a CaF, scintillatingcrystal which will be used in an experiment on double-beta decay of 48Ca.

IOk xlz

1OOkH2

IN

240[

210 -

180~

150~

120

90

60

Fig . 6 . Equivalent noise charge at 4 K of the first GaAs charge sensitive preamplifier developed.

Further versions have been developed to matchdetector capacitances of 80 pF [14] and 400 pF [11] andto reach pulse amplitudes of 1 V at the receiving-endof a 50 SZ coaxial cable terminated at both ends. Aslew rate of 100 V/Ws has been measured . In bothcases transistors in parallel at the input have been usedto increase the matching detector capacitance at theexpense of increasing the power dissipation . The inputresistance of the first 400 pF version was 22 fl and thefactor of merit was about 7 MHz/mW. The ENC at 77K was 5500 electrons rms at 100 ns unipolar Gaussianshaping and 104 electrons rms at a 100 ns peaking timebipolar shaping . 64 channels of the first prototype ofthe LAr Accordeon calorimeter have been equippedwith this instrument .

The circuit configuration of a very recently devel-oped version of charge-sensitive preamplifier is indi-cated in fig . 7. It uses five transistors in parallel at the

Rb

D. V. Cumin et al. / Front-end in GaAs

T = 4K

Fig . 7. Circuit diagram of a GaAs charge-sensitive preampli-fier optimized for the LAr Accordeon calorimeter prototype .

22 .5

10.5

020 r.m.s . e-

5

6

7

8

9 A 10

11 r

389

input and a single cascode stage . 144 channels of theAccordeon LAr calorimeter prototype have beenequipped with these preamplifiers which have beenmounted onto double sided hybrids of 15 x 17 mm2.Fast signals have been read-out using 20 ns peakingtime bipolar shaping [15]. Only one MESFET type,NE25139-U71 have been used in the realization of thisproject. One inconvenience of this choice was thatsome channels have shown slow response and thisoccurred because the selected device changes its dooutput characteristics when large drain-source voltageexcursions are applied at cryogenic temperatures . Thiseffect was later-on investigated and attributed to thehot-electron trapping in the region between drain andgate, possibly in the interface between the channel andthe passivation layer [16].

Fig . 8. 8 current response of the bread-board prototype circuitof fig . 7. The manufactured hybrid preamplifiers have shown

less than 10 ns rise time .

VII . FRONT-END ELECTRONICS

390

Fig. 9. S current response of the preamplifier based around amonolithic array of MESFETs. The rise time is 9 ns .

The present version uses two types of MESFETs:NE25139-U71 at the input and in other positions wherenoise is the prime factor, and CF 739 where largevoltage excursion are expected. The latter transistortype behaves correctly at cryogenic temperatures evenfor voltage excursions in excess of 12 V. The responseto a 8 current is shown in fig . 8. The rise time is 5 ns inthe prototype and about 10 ns in the hybrids manufac-tured. As the feedback capacitance is CF = 33 pF, theinput resistance results lower than 10 fl . The power

veaw

a+

h

V

UZ

-200 -100 0

D. V. Camin et ai / Fïùnt-end in GaAs

Pre tipo G - Shaper Ortec 450 Tau=100ns

100 200

dissipation is 54 mW. The factor of merit F is 20MHz/mW for the prototype and 10 MHz/mW for thehybrids . For large signals, a slew rate of 120 V/ps wasmeasured .

4.3. Use of monolithic array of MESFETs

The high resistivity of the semi-insulating substrate,109 fl cm, makes it possible to have very high isolationbetween devices in GaAs monolithic structures if careis taken to limit the backgating effect .

As a first approach towards a monolithic integrationof GaAs low-noise preamplifiers, a charge-sensitiveversion based on a monolithic array of MESFETs havebeen developed .

The schematic diagram is similar to that of fig . 7,but the devices are this time the array of MESFETs16GO20 made by Gigabit Logic . The 1.26 x 1.24 mm2die contains 11 MESFETs of 22 GHz transition fre-quency . It is mounted in a rather large case (10 x 10mm2 36 pin leadless chip carrier) which is required asevery MESFET electrode is connected to a pin . Proto-types of this preamplifier have been realized by mount-ing the chip onto a 4 layer, 15 x 11 mm2 hybrid circuit.

Using a feedback capacitance of 33 pF, a rise timeof 9 ns was obtained at 77 K when detector capacitancewas 400 pF, fig . 9 . The input resistance is therefore 8f.

300 400 500

NS (e/pF)

15.5

7 .6

Fig . 10 . ENC of the charge-sensitive preamplifier based on the monolithic array of MESFETs . Shaping is unipolar Gaussian with7 = 100 ns.

40000

30000-

2000020000

10000-

-200 -100 0

The noise as a function of detector capacitance for300K and 77 K are shown in figs . 10 and 11. The largerdecrease in noise at longer shaping time correspondsto the rapid decrease of the 1/f noisewhen MESFETsare cooled down . In the same figure it can be observedthat the preamplifier matching capacitance is 120 pF.The noise slopes are also indicated in the figures.

The preamplifier power dissipation is 23 mW.Therefore, a factor of merit of 25 MHz/mW can becalculated .

The next step foreseen is the customization of theMESFET array by bonding the individual componentsinside the chip carrier itself. In this way it will bepossible to reduce size and parasitic capacitances .

D. V. Camin et al. / Front-end in GaAs

Pre tipo G - tp=20ns bipolar shaping

5. Conclusions

300 400 500100 200CD (pF)

Fig. 11 . Idem as fig. 10 but using bipolar shaping with a peaking time of 20 ns.

NS(el/pF)

77

50

391

Fig. 12 shows the different versions of hybridcharge-sensitive preamplifiers which have been de-scribed above.

Different verions of low-noise preamplifiers basedon GaAs MESFETs have been developed and testedwith success either at 4 K with bolometric detectorsand photodiodes, and at 87 K with a prototype of theLAr Accordeon calorimeter. For the latter, series ofabout 200 units have been manufactured with a yieldhigher than 95% and with less than 0.5% failures upon

Fig. 12 . Charge-sensitive preamplifiers described in the text have been manufactured using hybrid techniques as shown in thefigure.

VII. FRONT-END ELECTRONICS

392

D.V. Camin et al.

cooling down. The power dissipation can be kept lowwithout sacrificing speed or charge sensitivity, thanksto the high electron mobility of GaAs devices. A firststep towards monolithic integration have been taken bydeveloping a version based on a monolithic array ofMESFETs which will be customized in the near future .

Results obtained so far are quite satisfactory . Forbolometric detectors, effort will be put in reducing the1/f noise still further as it will be of benefit for largemass, very high resolution detectors. For acceleratorapplications, work still have to be done to test hownoise is affected in particular by gamma radiation asthe resistance to neutron fluence have been proved.

References

[1] A. Alessandrello, D.V. Camin, A. Giuliani and G.Pessina, Proc. Workshop on Low Temperature Devicesfor the Detection of Low Energy Neutrinos and DarkMatter, ed . G. Pretzl (Springer, Berlin-Heidelberg 1987)p. 122.

[2] B. Lengeler, Cryogenics 14 (1974) 439.[3] R.K . Kirschman (ed.), Low Temperature Electronics,

(IEEE Press, 1986) part VI .[4] V. Schoenberg et al ., Nucl . Instr. and Meth. A288 (1990)

191.

/ Front-end in GaAs

[5] S.M. Sze, Physics of Semiconductor Devices (Wiley, 1981)pp . 29-30.

[6] D.C . Look, Electrical Characterization of GaAs Materialand Devices, (Wiley, 1989) p. 92 .

[7] R. Kirschman, private communication.[8] A. Alessandrello, C. Brofferio, D.V. Camin, A. Giuliani,

G. Pessina and E. Previtali, Nucl . Instr. and Meth .A289(3) (1990) 426.

[9] E. Gatti and P.F. Manfredi, Riv. Nuovo. Cimento 9(1986) 93 .

[10] R. Zuleeg, Proc . of IEEE 77(3) (1989) pp. 389-407.[11] D.V . Camin, G. Pessina and E. Previtali, IEEE Trans.

Nucl . Sci. NS-38 (1991) 53 .[12] D.V. Camin, G. Pessina, E. Previtale and G. Ranucci,

Cryogenics 29 (1989) 857.[13] A. Alessandrello, C. Brofferio, D.V. Camin, A. Giuliani,

G. Pessina and E. Previtali, Nucl . Instr. and Meth . A295(1990)405.

[14] A. Alessandrello, C. Brofferio, D.V. Camin, A. Giuliani,G. Pessina and E. Previtali, IEEE Trans. Nucl . Sci .NS-37 (1990) 1242 .

[15] F. Gianotti, these Proceedings, Proc . 5th Pisa Meeting onAdvanced Detectors: Frontier Detectors for FrontierPhysics, La Biodola, Isola d'Elba, Italy, May 26-31, 1991,eds. A. Baldini, A. Scribano and G. Tonelli, Nucl . Instr.and Meth . A315 (1992) 285.

[16] D.V . Camin, G. Pessina and E. Previtali, Electron . Lett .27(24) (1991) 2297 .