Low Capacitance Detectors Research Articles

A Custom Low-Noise Silicon Photodiode Detector Designed for Use With X-Ray Capillary Optics

We present the first results of a custom low-noise silicon photodiode detector designed for use with X-ray capillary optics. This system is intended to perform as a smaller, lighter, and low-powered alternative to currently existing X-ray telescope technologies. The detector has an active area with a diameter of 100 μm, and its small size leads to a low capacitance of well under 1 pF and a leakage current of 1 pA or less at the depletion voltage. We tested the detector with both a discrete preamp and an ultralow-noise front-end application-specific integrated circuit (ASIC). The front-end ASIC, developed at Brookhaven National Laboratory, was designed for low-capacitance detectors but had not been previously tested with a detector. Here, we discuss the detector design, the experimental setup, and the resulting detector performance. Using radioactive laboratory sources, we measure a full-width at half-maximum (FWHM) of 211 ± 8 eV at 5.9 keV with the ASIC and are able to set the threshold as low as 0.6 keV.

Analytical Model of the Discharge Transient in Pulsed-Reset Charge-Sensitive Amplifiers

A study of the reset transient of charge-sensitive amplifiers operating with a pulsed switch in parallel to the feedback capacitance is presented. Analytical models have been developed for amplifiers in both linear and slew rate regimes during the reset phase. The models predict the time interval required to reset the amplifier that, in most cases, significantly differs by the simple discharge of the feedback capacitance through the switch resistance and strongly depends on the open-loop gain and bandwidth of the core amplifier and on the input capacitance. The models also quantitatively predict spurious voltage transients at the preamplifier's input and possible ringings at the output, as a function of the main general parameters of the circuit. A case application study of the presented model is proposed for low-capacitance detectors, such as semiconductor drift detectors or small pixel detectors.

Low capacitance CMOS silicon photodetectors for optical clock injection

We have studied the response of CMOS com- patible detectors fabricated in a silicon-on-sapphire (SOS) process, operated under short pulse excitation in the blue. These high speed, low capacitance detectors would be suit- able for very precise, surface-normal clock injection with silicon CMOS. We characterize the capacitance of the de- tector structure through a combination of experimental tech- niques and circuit-level and electromagnetic simulations. The transit-time-limited response of the detectors is vali- dated through pump-probe experiments. Detector response times of ∼35 ps have been measured, and devices have ca- pacitance as low as ∼ 4f F.

The double-gate p-JFET-inputted amplifier for low-capacitance detectors

The low-noise monolithic double-gate p-JFET-inputted amplifier for detectors with capacitance up to 30 pF is described.

CMOS preamplifier for low-capacitance detectors

We present a new CMOS preamplifier and shaper, optimized for charge measurements with detectors of 0.1–1 pF capacitance. A self-adaptive biasing scheme with nonlinear pole-zero cancellation allows us to use an MOS device operated in the triode region as the DC feedback element while eliminating nonlinearity and sensitivity to supply, temperature, and process variations and accepting up to several μA leakage current. The circuit is continuously sensitive and requires no external adjustments to set the feedback resistance. Secondary sources of noise are minimized subject to a power dissipation constraint. Implemented in a 1.2 μm CMOS process, the preamplifier achieves an ENC of 35 e − + 58 e −/pF at 23 μs shaping time at a power consumption of about 3.2 mW. The integrated preamp/shaper has 50 ns shaping time and the ENC is 120 e −. It has 0.3% nonlinearity over an input dynamic range of 0–5 fC.

Large area, low capacitance Si(Li) detectors for high rate X-ray applications

Large area, single-element Si(Li) detectors were fabricated using a novel geometry which yields detectors with reduced capacitance and hence reduced noise at short amplifier pulse-processing times. A typical device employing the new geometry with a thickness of 6 mm and an active area of 175 mm/sup 2/ has a capacitance of only 0.5 pF, compared to 2.9 pF for a conventional planar device with equivalent dimensions. These low capacitance detectors, used in conjunction with low capacitance field effect transistors, will result in X-ray spectrometers capable of operating at very high count rates while still maintaining excellent energy resolution. The spectral response of the low capacitance detectors to a wide range of X-ray energies at 80 K is comparable to typical state-of-the-art conventional Si(Li) devices. In addition to their low capacitance, the new devices offer other advantages over conventional detectors. Detector fabrication procedures, I-V and C-V characteristics, noise performance, and spectral response to 2-60 keV X-rays are described. >

Position resolution and linearity of resistive, unidimensional radiation detectors employing the rise time read-out method

An investigation has been made of the possible improvement in resolution employing the rise time method, if the filter bandwidth is increased beyond that required for position linearity. To this end, the computed resolution and linearity of position information have been plotted as a function of the filter time-constant for RC and double-delay line shaping. High and low capacitance detectors have been considered. The plots yield an estimate of the improvement in resolution obtainable, if position linearity is disregarded. Preamplifier noise was assumed to be negligible.

Hypercryogenic Detector-Fet Unit--Core of High-Resolution Spectrometer

A hypercryogenic high-resolution x-ray and ?-ray spectrometer is described. The spectrometer consists of a lithium-drifted germanium detector and a germanium-junction field-effect transistor (JFET) preamplifier. The detector is operated at its optimum temperature, which is in the range of 10-30° K. The JFET's are operated at liquid helium temperature. This results in 0.28 keV FWHM preamplifier resolution with a slope of 0.018 keV/pF. Resolutions of the order of 0.4 keV are obtained with low capacitance detectors for low-energy x rays. The ? rays of 57Co are measured with 0.68 keV FWHM. The described spectrometer was used to study some aspects of the trapping effects in germanium detectors.

A regrown p + contact for lithium-drifted germanium gamma-ray detectors

A method for applying regrown GaIn contacts to lithium-drifted germanium gamma-ray detectors, either before or after drifting, has been developed. Since alloying of the regrown contact and lithium diffusion can be accomplished during the same temperature cycle, it is possible to rearrange contacts and thereby construct thin, deep, low-capacitance detectors from large-area shallow ones. These in turn can be combined in one package to make deep, large volume-dectors.

U-Junction Ge(Li) Drift Detectors

A U-junction drifting configuration has been used which permits the fabrication of large volume, low-capacitance detectors in much less time than previously required. For example, it is now possible, with good material, to fabricate in approximately one week an 18-cm3 diode with 3-pF capacitance. The U-junction structure is partially wrapped around three sides of a parallelepiped of the desired dimensions. The drifting then proceeds toward the remaining side of the detector. At the completion of the drift, only a narrow strip of P-type material remains along one edge. The portion of the lithium junction used for the rapid drift is now lapped off, leaving the low-capacitance junction structure for the collection of the charge. No further processing is required, and the diode is ready for mounting. Initial tests on two 6-cm3 U-junction detectors indicate that the diodes have the expected capacitance of about 2 pF and a relatively thin window. A resolution of 705 eV for the 60-keV gamma ray of Am241 has thus far been obtained, and the measured Fano factor of 0.157 is consistent with the currently accepted value.

Amplifiers for Use with P-N Junction Radiation Detectors

The signal on radiation pulse detectors is in the form of a charge on a capacitor. An ionizing particle produces a charge pair for each 3.6 ev dissipated in silicon. The capacitance of a p-n junction is proportional to the area and inversely proportional to the square root of the bias potential. Three similar preamplifier circuits are described. Negative charge feedback is employed so that the output amplitude is essentially independent of the value of the detector capacitance. The amplifiers have a high open loop gain, a fast response, and show little tendency to oscillate. Best overall performance, a noise line width of 5 kv, is achieved using WE417A or equivalent tubes in cascode at the input. RCA 7586 tubes give a line width of 8-10 kv with low capacitance detectors. The line width with transistors is 20-30 kv.