Front-end in GaAs. D.V. Camin, G. Pessina and E, Prevital_i. Table 1

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1 Nuclear Instruments and Methods in Physics Research A315 (1992) North-Holland NUCLEAR INSTRUMENTS &METHODS IN PHYSICS RESEARCH SectionA Front-end in GaAs Presented by D.V. Gamin D.V. Camin, G. Pessina and E, Prevital_i Dipartimento 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 cryogenic temperatures.the high electron mobility permits to obtain high gain-bandwidth products with low power dissipation. Voltage-sensitive preamplifiers for 4 K operation have been developed and are presently used with bolometric particle detectors. Charge-sensitive preamplifiers for liquid calorimetry have also been developed and are used for the readout of the Accordeon LAr calorimeter prototype. 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 metalsemiconductor field-effect transistors (MESFETs) have been introduced some time ago for the signal amplification of cryogenic particle detectors [1]. The adoption of GaAs was based on the rather narrow choice of low-noise electronic devices capable to operate at cryogenic temperatures. In effect, silicon JFETs start to exhibit carrier freeze-out at temperatures below 100 K. Presently available bipolar transistors rapidly decreases their current amplification factor when cooling [2]. On the other hand Si MOSFETs compensate carrier freeze-out by virtue of the strong electric field under the gate which makes it possible to operate these devices even at 4 K; nevertheless there are two characteristics that have to be considered : a) 11f noise in the MOSFET channel do not reduce and even increases at low temperatues [3] limiting its use for very low frequency applications like in thermal detectors; b) at present, speed limits applications at very fast shaping with large detector capacitances like in cryogenic liquid calorimeters. So far satisfactory results have been obtained for low detector capacitances or at long shaping times [4]. In any case, readily available monolithic integration of a large number of channels might make radiation-hard MOS a choice in cryogenic applications for future accelerators, when detectors with very low output capacitances are involved. The most relevant properties of GaAs MESFETs will be reviewed, and examples of GaAs based voltage-sensitive preamplifiers used for the signal amplification of thermal detectors and charge sensitive preamplifiers developed for liquid argon calorimetry will be described. 2. Properties of GaAs MESFETs Finally, the characteristics of a recently developed charge-sensitive preamplifier based on a monolithic array of MESFETs will also be reported. Nature has given GaAs fundamental physical parameters which make it an attractive option for the realization of low-noise cryogenic and fast amplifiers. In fact, the high electron mobility and low electric field for carrier peak velocity make it possible to obtain high transconductance to input capacitance ratios at low power dissipation. This is a parameter of prime importance in charge-sensitive preamplifiers as it is related to the product of the charge sensitivity times the speed for a given detector capacitance. In addition, the low ionization energy of dopant impurities keeps limited the freeze-out of carriers even at 4 K. Table 1 GaAs Electron 300K mobility 77K [cm V` s- 11W 3000 nd Electric field at peak velocity IV/p.mj Ionization energy of dopant impurities [mev] 6 50 Energy bandgap at 300 K [ev Si /92/$ Elsevier Science Publishers B.V. All rights reserved VI1. FRONT-END ELECTRONICS

2 386 D.V. Camin et al. / Front-end in GaAs Some fundamental parameters of GaAs are indicated in table 1 and, to make a comparison with the well established Si technology, the corresponding values for silicon are also given. A doping level of 1017 cm- 3 have been assumed in both cases [5,6]. So far, only MESFETs have been used for the realization of low noise preamplifiers for particle detectors, although other devices like AlGaAs/GaAs high electron mobility transistors (HEMTs) may be used to take advantage of their impressive electron mobilities which, from a value of 6000 cm 2/V s at room temperature can reach 3 x 105 cm2/v s at 77 K and 2 x 106 cm 2/V s at 4 K [7]. The selection of suitable MESFETs is done among those device types exhibiting low series noise at low frequencies which is normally the main limiting parameter. At room temperature series noise in MESFETs is mainly of generation-recombination type which dominates at low frequencies. The distribution of the spectral power density is proportional to 1/f due to the presence of multiple traps, each one contributing with a T/(1 + ui 2TJ, where T is the caracteristic time constant of the trap. At low temperatures this dominant 1/f noise decreases strongly due to the exponential dependance of T with 1/T. A reduction of two orders of magnitude in the spectral power density when temperature decreases from 300 K to 77 K was observed. An additional factor of five in noise reduction occurs when cooling to 4 K [8]. White noise is the limiting parameter in the signalto-noise ratio at very high frequencies or, equivalently, at short shaping times. The large transconductance/input capacitance ratio of GaAs MESFETs make it possible to obtain low dependance of the equivalent noise charge (ENC) on the detector capacitance in charge preamplifiers while keeping low the power dissipation. In effect : 1/2 h ENC = 8kTX -1 CD WT TM where X is about 1.2 for GaAs at 300 K, cot = gm/c; the gain-bandwith product of the input device, h 1 is a numerical parameter depending on the shaper considered and the rest are the usual terms [9]. MESFETs presently used in cryogenic preamplifiers have OT/2Tr values in excess of i GHz with less than 400 gw power dissipation at 77 K. Regarding radiation hardness, GaAs devices have been thoroughly investigated in particular in what concerns static and dynamic characteristics [10]. How noise depends on neutron fluence and on gamma, radiation dose is not yet fully understood. A preliminary study on commercial MESFETs, obviously not radiation hardened, have been performed and the results show that noise at 20 ns shaping time increased by 5% after E Vd [VI Fig. 1. Static output characteristics of a 3SK164 MESFET at 4 K. Dotted lines shows the simulation according to a UCB GaAs model whose parameters have beeen extracted from the measurement. being subject to a neutron fluence of 1014 n/cm2 and by a factor of two after irradiation to 106 rad (Si) with a 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 MESFETs at cryogenic temperatures, fig. 1 reports the static characteristic of a 3SK164 MESFET at 4 K. In the same figure the simulated I-V curves of the device are shown. They correspond to a UCB GaAs MESFET model whose parameters have been extracted using a computer program (IC-CAP) driving a Semiconductor Parameter Analyzer (HP 4142A). Fig. 2 shows the static characteristic of a NE25139-U71 at 77 K. The simulated characteristics using parameters extracted and optimized around [VDs = 2 V ; ID = 7.5 ma] are also plotted. In the realization of complex circuit configurations, extraction of semiconductor models parameters at any temperature between 4 K and 300 K, and a further SPICE simulation proved to be an advantageous tool. So far, GaAs MESFETs, specially designed for the utilization as amplifying element in front-end electronics for detectors used in particle physics, are still not available. For accelerator use, the aspect of radiation Fig. 2. Same as fig. 1 but for a NE25139-U71 at 77 K.

3 D.V. Camin et al. / Front-end in Ga4s 387 hardness has still to be inves*igatcd. For low frequency detectors at low temperatures, like thermal detectors, devices with very low 1 /f noise should be developed to improve the present results. The improvements on the low-frequency noise performances will most probably be obtained at the expense of increasing the input capacitance which, in any case, is not a noise limiting factor when working with voltage sensitive preamplifiers. Nevertheless, a systematic characterization of commercially avaiiable MESFETs led to the identification of many device types with very satisfactory static and noise performances. With these devices several versions of low-noise preamplifiers have been developed and later-on fabricated in series of more than 200 units with a yield larger than 95%. Nowadays, the low dispersion in component's characteristics makes it possible to construct much larger quantities keeping high the yield. A description of the most significant instruments developed will be described in the next paragraph. 3. Voltage-sensitive preamplifiers for 4 K operation The need to reduce the parasitic capacitance, the pick-up of electromagnetic interference and the thermal power injection through the connecting leads in bolometric particle detectors, led to the development of GaAs voltage-sensitive preamplifiers which had to operate between 1 K and 4 K inside a dilution refrigerator. The main feature looked for was to obtain a very low series noise keeping low the power dissipation ; preamplifier's parallel noise is not a matter of concern at cryogenic temperatures as the gate leakage current reduces exponentially with 1/T. Speed is also not critical for these applications. Succesive versions have been developed in the last years [12] and at present a design with outstanding performances is used in the experiments with bolometric detectors performed by the group of Milano [13]. Fig. 3 shows the schematic diagram of the circuit developed. Series input noise is kept low by paralleling 10 3SK164 MESFETs at the input, Q 1, and operating them at low drain-source voltage and low drain current [0.6 V; 0.6 ma] where the 1 /f noise is low. A double cascode Q 2, Q 3 amplify the relatively low output impedance at the drain of Q1 and the output signal of this first amplifying stage is developed at the dynamic load 04, Q 5. A second, inverting stage Q6 provides additional gain and the feedback network R 2, R 1 fixes the closed loop gain to a nominal value of 52. Transistors Q7-Qlo determine a network which provides low impedance, thermally compensated biasing voltage sources as required by the circuit. A detailed description of this preamplifier is given in ref. [13]. loo R13 3Ks VREG Fig. VCC 3. Circuit diagram of a 4 K GaAs voltage-sensitive preamplifier used in experiments with bolometric detectors. The preamplifier components have been originally mounted onto a ceramic substrate which showed to be very much sensitive to thermal shocks when cooled to 4 K. At present, components are surface mounted onto a FR4 printed circuit board as shown in fig. 4. The use of ceramic substrates at 4 K has not been excluded as positive results have been obtained when using high quality hybrid manufacturing techniques. Fig. 5 shows the total series noise of the voltagesensitive preamplifier at 4 K and 77 K. A noise level of 9 :.V/ ~Hz at 100 Hz with 1/ VFf distribution and 0.5 nv/ Vffz at 100 khz was obtained with a power dissipation of 50 mw. The response to a step input voltage pulse has a rise time of 40 ns. 4. Charge-sensitive preamplifiers R_-1K3 Charge-sensitive preamplifiers in GaAs have been originally developed for high impedance bolometric Fig. 4. The voltage-sensitive preamplifier circuit mounted onto a FR4 printed circuit board. VII. FRONT-END ELECTRONICS

4 388 D.V. Camin et al / Front-end in Ga4s detectors and other thermal detectors. Later-on they have been proposed as front-end for cryogenic liquid calorimeters taking into account the favourable noise behaviour at short shaping times and the fast speed of response and good dynamic range with low power dissipation. In order to make an evaluation of the relative quality of different versions of charge-sensitive preamplifiers, a factor of merit can be calculated as shown below A factor of merit for charge-sensitive preamplifiers In first approximation, the rise time of the response to a 8 input current is given by the following expression, which assumes a dominant pole in the open loop gain: t r = 2.2Ri(Cd + C i + Cf + C,), (2) tr is the rise time of the output pulse, R i is the preamplifier input resistance, Cd, C i, C f and C, are the detector, input, feedback and test capacitances respectively. Taking into account that R i is given by: Ri = gm 1C0/Cf where g m is the transconductance of the input transistor, and Co the capacitance determining the dominant pole, a preamplifier's transition frequency f, = gm/2zrco can be calculated from eqs. (2) and (3) by measuring the rise time for a given detector capacitance. 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 their product increases only if the open-loop gain is im- proved, which means increasing the power dissipation Pd, it is useful to calculate for each design the factor of merit : F=f~lPd =0.35tr'Cf 1 (Cd +C i +C f +C t )Pd 1 which relates the speed-sensitivity product to the power dissipation for a given detector capacitance. Also, a high f means low white noise which depends on 1/ g,. So far it was found that the factor of merit of GaAs charge preamplifiers is higher than that of silicon preamplifiers of similar characteristics, and this is just a consequence of the higher mobility of electrons in GaAs which allows to obtain higher transconductances/input capacitance ratio at a lower power dissipation Cryogenic charge-sensitive preamplifiers The first cryogenic charge-sensitive preamplifier developed using exclusively GaAs devices was reported in ref. [8]. It matches detector capacitances of about 10 pf, at 4 K has a minimum ENC of 20 electrons rms at 0 pf detector capacitance, and dissipates 9 mw, the response to a S current has a rise time of 20 ns with 1 pf feedback capacitance and the operating temperature extends from 1 K to 120 K. The upper temperature was limited by the voltage drop in the 109 SZ feedback resistor caused by the gate leakage current. Fig. 6 shows the preamplifier's ENC as a function of the shaping time at 4 K. The factor of merit of this preamplifier is F = 78 MHz/mW but it has a low dynamic range. At present it is used to amplify the signal of a photodiode coupled to a CaF, scintillating crystal which will be used in an experiment on doublebeta decay of 48 Ca. 40n rms V/rHz In 400p IOOHz 1kHz IOk xlz Fig. 5. Series noise at 4 K and 77 K of the circuit of fig. 3. 1OOkH2

5 D. V. Cumin et al. / Front-end in GaAs [ T = 4K 180~ 150~ r.m.s. e A r Fig. 6. Equivalent noise charge at 4 K of the first GaAs charge sensitive preamplifier developed. Further versions have been developed to match detector capacitances of 80 pf [14] and 400 pf [11] and to reach pulse amplitudes of 1 V at the receiving-end of a 50 SZ coaxial cable terminated at both ends. A slew rate of 100 V/Ws has been measured. In both cases transistors in parallel at the input have been used to increase the matching detector capacitance at the expense of increasing the power dissipation. The input resistance of the first 400 pf version was 22 fl and the factor of merit was about 7 MHz/mW. The ENC at 77 K was 5500 electrons rms at 100 ns unipolar Gaussian shaping and 104 electrons rms at a 100 ns peaking time bipolar shaping. 64 channels of the first prototype of the LAr Accordeon calorimeter have been equipped with this instrument. The circuit configuration of a very recently developed version of charge-sensitive preamplifier is indicated in fig. 7. It uses five transistors in parallel at the input and a single cascode stage. 144 channels of the Accordeon LAr calorimeter prototype have been equipped with these preamplifiers which have been mounted onto double sided hybrids of 15 x 17 mm 2. Fast signals have been read-out using 20 ns peaking time bipolar shaping [15]. Only one MESFET type, NE25139-U71 have been used in the realization of this project. One inconvenience of this choice was that some channels have shown slow response and this occurred because the selected device changes its do output characteristics when large drain-source voltage excursions are applied at cryogenic temperatures. This effect was later-on investigated and attributed to the hot-electron trapping in the region between drain and gate, possibly in the interface between the channel and the passivation layer [16]. Rb IN Fig. 7. Circuit diagram of a GaAs charge-sensitive preamplifier optimized for the LAr Accordeon calorimeter prototype. Fig current response of the bread-board prototype circuit of fig. 7. The manufactured hybrid preamplifiers have shown less than 10 ns rise time. VII. FRONT-END ELECTRONICS

6 390 D.V. Camin et ai / Fïùnt-end in GaAs dissipation is 54 mw. The factor of merit F is 20 MHz/mW for the prototype and 10 MHz/mW for the hybrids. For large signals, a slew rate of 120 V/ps was measured Use of monolithic array of MESFETs Fig. 9. S current response of the preamplifier based around a monolithic 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 where noise is the prime factor, and CF 739 where large voltage excursion are expected. The latter transistor type behaves correctly at cryogenic temperatures even for voltage excursions in excess of 12 V. The response to a 8 current is shown in fig. 8. The rise time is 5 ns in the prototype and about 10 ns in the hybrids manufactured. As the feedback capacitance is CF = 33 pf, the input resistance results lower than 10 fl. The power The high resistivity of the semi-insulating substrate, 109 fl cm, makes it possible to have very high isolation between devices in GaAs monolithic structures if care is taken to limit the backgating effect. As a first approach towards a monolithic integration of GaAs low-noise preamplifiers, a charge-sensitive version based on a monolithic array of MESFETs have been developed. The schematic diagram is similar to that of fig. 7, but the devices are this time the array of MESFETs 16GO20 made by Gigabit Logic. The 1.26 x 1.24 mm 2 die contains 11 MESFETs of 22 GHz transition frequency. It is mounted in a rather large case (10 x 10 mm2 36 pin leadless chip carrier) which is required as every MESFET electrode is connected to a pin. Prototypes of this preamplifier have been realized by mounting the chip onto a 4 layer, 15 x 11 mm 2 hybrid circuit. Using a feedback capacitance of 33 pf, a rise time of 9 ns was obtained at 77 K when detector capacitance was 400 pf, fig. 9. The input resistance is therefore 8 f. Pre tipo G - Shaper Ortec 450 Tau =100ns NS (e/pf) 15.5 v e a w a+ h V U Z Fig. 10. ENC of the charge-sensitive preamplifier based on the monolithic array of MESFETs. Shaping is unipolar Gaussian with 7 = 100 ns.

7 D. V. Camin et al. / Front-end in GaAs Pre tipo G - tp=20ns bipolar shaping NS(el/pF) CD (pf) Fig. 11. Idem as fig. 10 but using bipolar shaping with a peaking time of 20 ns. The noise as a function of detector capacitance for 300K and 77 K are shown in figs. 10 and 11. The larger decrease in noise at longer shaping time corresponds to the rapid decrease of the 1/f noise when MESFETs are cooled down. In the same figure it can be observed that 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 be calculated. The next step foreseen is the customization of the MESFET array by bonding the individual components inside the chip carrier itself. In this way it will be possible to reduce size and parasitic capacitances. Fig. 12 shows the different versions of hybrid charge-sensitive preamplifiers which have been described above. 5. Conclusions Different verions of low-noise preamplifiers based on GaAs MESFETs have been developed and tested with success either at 4 K with bolometric detectors and photodiodes, and at 87 K with a prototype of the LAr Accordeon calorimeter. For the latter, series of about 200 units have been manufactured with a yield higher 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 the figure. VII. FRONT-END ELECTRONICS

8 392 D.V. Camin et al. / Front-end in GaAs cooling down. The power dissipation can be kept low without sacrificing speed or charge sensitivity, thanks to the high electron mobility of GaAs devices. A first step towards monolithic integration have been taken by developing a version based on a monolithic array of MESFETs which will be customized in the near future. Results obtained so far are quite satisfactory. For bolometric detectors, effort will be put in reducing the 1/f noise still further as it will be of benefit for large mass, very high resolution detectors. For accelerator applications, work still have to be done to test how noise is affected in particular by gamma radiation as the resistance to neutron fluence have been proved. References [1] A. Alessandrello, D.V. Camin, A. Giuliani and G. Pessina, Proc. Workshop on Low Temperature Devices for the Detection of Low Energy Neutrinos and Dark Matter, ed. G. Pretzl (Springer, Berlin-Heidelberg 1987) p [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. [5] S.M. Sze, Physics of Semiconductor Devices (Wiley, 1981) pp [6] D.C. Look, Electrical Characterization of GaAs Material and 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 [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) [15] F. Gianotti, these Proceedings, Proc. 5th Pisa Meeting on Advanced Detectors : Frontier Detectors for Frontier Physics, 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.