GaAs MESFETs and monolithic circuits in cryogenic environments

W e present the latest results o f a research work started in 1986 in Milano, aiming at the optimisation o f GaAs MESFETs and circuits for cryogenic readout o f particle detectors used in experiment o f high energy physics. For bolometric detectors we have been looking for low l / f noise and good dynamic performance at 4 K. W e want also fast speed with low white noise and high radiation resistance for the readout o f liquid Argon caloiimeters. An ion-implanted MESFET process was selected for the realisation o f FETs and monolithic preamplifiers for the mentioned applications. The noise and dynamic parameters o f the process have been determined for the first time at 4 K and 77 K. SPICE parameters have also been extracted. As a result o f this work, monolithic low noise preamplifiers were designed and fabricated. The chips have been tested with a LAr detector at CERN, confirming the expected performance. Voltage-sensitive preamplifiers for 4 K have also been fabricated and will soon be evaluated. The performance o f FETs and monolithic preamplifiers at cryogenic tempertaures are reported. In experimental particle physics, a certain number of particle detectors must operate at cryogenic ~emperatures. Bolometric detectors, used in experiments of rare events in non-accelerator physics [ I ] , are cooled down to less than 10 mK. Their front-end electronics are in many cases located at 4 K to reduce the noise contribution due to stray capacitances, microphonics and radio frequency interference [ 2 ] . Signals produced by the detectors have their energy concentrated in the very low frequency region, typically below 1 kHz. Cryogenic liquid calorimeters, which are being developed for the future generation o f particle accelerators [3], are based on the collection of the charge created by the interaction o f high energy particles in the detector. LAr is the cryogen adopted as the charge drifting medium. LKr has also being considered [4]. Signals are filtered by shaping amplifiers which have narrow bandpass centered in a frequency between 1 MHz and 1 0 MHz. Severe requirements regarding noise, speed, power dissipation and resistance to radiation (photons and neutrons) are demanded to the readout preamplifier. This has to be located close to each detector cell, immersed in the LAr, and therefore must operate at 87 K. In principle, some classes o f solid state devices, like SiJFETs and MOSFETs, could be used for the realization o f cryogenic low-noise readouts for the mentioned applications [5] . Despite of the very different signal bandwidth and operating temperatures required for those detectors, we have found GaAs MESFET technology appropriate for the realization o f both types o f readout. In fact, a GaAs MESFET conducts even at the lowest temperature, due to the abscence o f carrier freeze-out, and its high roomtemperature l / f noise decreases strongly ~ipon cooling down. In addition, the high transition frequency allows fast operation and low thermal noise in the FET channel. In this paper we will overview the characteristics o f GaAs MESFETs at cryogenic temperatures pointing in particular to those aspects which affect the performance of low-noise preamplifiers for very low and medium frequency applications. W e will also report on the latest results obtained in monolithic GaAs preamplifiers realized with a monolithic process. Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jp4:1994625 C6-160 JOURNAL DE PHYSIQUE IV 2. GaAs MESFETs at Cryogenic Temperatures. Nature has given GaAs fundamental physical parameters which make it an attractive option for the realization of low noise, fast, cryogenic pulse 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 cal-sies even at 4 K. Some fundamental parameters of GaAs are indicated in Table I 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 [6], 171. TABLE I GaAs Si So far, only MESFETs have been used for the realization of 111-V based cryogenic low noise preamplifiers for particle detectors, although other devices like AlGaAsJGaAs High Electron Mobility Transistors (HEMTs) may be used to take advantage of their impressive electron mobilities which, from a value of 6000 crn2 / V sec at room temperature can reach 3 x 105 cm2 / V sec at 77 K and 2 x 106 cm2 1 V sec at 4 K. The selection of a suitable MESFET process is done looking for a 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 and dominates at low frequencies. The distribution of the spectral power density is proportional to l/f due to the presence of multiple traps, each one contributing with a lorentzian term z / (1+ w2 T 2, whese 2 is the caracteristic time constant of the trap. At low temperatures, this dominant l/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 is normally observed. An additional factor of five in noise reduction generally occurs when cooling to 4 K 181. electron 300K mobility 77K 4K Electric field at peak velocity Ionization energy of dopant impurities