Rade S. Popovic

QUANTIZED CURRENT JUMPS IN SILICON PHOTOCONDUCTORS AT ROOM TEMPERATURE

We report on the observation of quantized jumps due to single-carrier trapping and detrapping at defect states in silicon photoconductors of 103 μm3 in volume. A specifically designed electrical test structure in a low-doped (2×1014 cm−3) silicon crystal was fabricated. It consists in four substrate resistances connected in a Wheatstone bridge. After the exposure to light, the bridge offset voltage recovers its equilibrium value with steps of 5–10 μV, corresponding to the emission or capture of a single carrier. Such structures also display random telegraph signals in the dark, with steps of similar amplitude. This behavior is observed with structures processed on Czochralski-grown substrates and not with those processed on float-zone substrates. Simple calculations based on quantized free carrier concentration variations corroborate the above measurements.

Compensation of the temperature-dependent offset drift of a Hall sensor

Abstract We present a method to cancel the offset of a Hall sensor and compensate its temperature-dependent drift. We adapt and improve a compensation technique based on a correction using the input voltage. A more accurate compensation for the temperature drift is obtained by an offset calibration at two different temperatures. To decrease the calibration time, we propose a procedure for fast heating of the sensor. It consists of forcing a current pulse through a p-n junction of the sensor. Since the resistance of a forward-biased diode is small, this principle is compatible with low-voltage applications. After correction, the resulting offset is less than two percent of the initial offset over the temperature range −10 to +60°C. The corresponding residual equivalent offset is lower than 250 μT.

High accuracy analog Hall probe

The Hall probe we developed is a miniaturized analog system, consisting of two silicon vertical Hall devices, current stabilizers, amplifiers and error-correcting circuits. The output signal is 1 V/T, with an error less than 10/sup -4/ in the ranges /spl plusmn/2 T, 15/spl deg/C to 35/spl deg/C, and DC to 10 kHz. The probe features high long-term stability, low offset drift and low noise.

A differential relaxation oscillator as a versatile electronic interface for sensors

Abstract A simple and versatile electronic interface circuit for sensors is presented. The novel interface circuit is based on a relaxation oscillator in differential configuration. In such a configuration, the sensitivity is strongly increased and compensations are made possible. It can be applied to resistive, capacitive and inductive sensors or detectors. Experimental and simulation results confirm the theory built up. High sensitivity is measured. Non-idealities of electronic components set the limit of attainable sensitivity.

First integrated inductive proximity sensor with on-chip CMOS readout circuit and electrodeposited 1 mm flat coil

The first fully integrated inductive proximity sensor has been realized with a 75 nH, 1 mm side gold electrodeposited squared coil and with on-chip 1 μm CMOS readout circuit. The measured nominal frequency output is of 8.95 MHz and increases up to 11.60 MHz with an aluminum target placed at 50 μm of the coil. The on-chip electronic interface is based on a differential relaxation oscillator designed to be fast and to have a frequency output increase when approaching a conductive non-ferromagnetic target, thus enhancing strongly the sensor sensitivity. The new sensor microsystem has been successfully applied for angular position measurements.

A method for spark rejection in ultraviolet flame detectors

A novel method is presented to render ultraviolet (UV) flame detectors insensitive to ignition spark radiation. The method involves isolating the signal due to the sparks from the UV sensor output and subtracting a DC signal proportional to it from the output of the flame detector sensor system. A practical demonstration of the method is given using a commercially available UV flame detector with the addition of an analog circuit to perform the necessary signal processing. A selectivity improvement to spark radiation of greater than 130 has been obtained. The method is robust in that it is independent of the distance between the light source and the flame detector and has been designed to work with different spark generators found on the market. The analog circuit is simple, requiring few components, thus ensuring rugged, fail-safe operation and low cost.

Die stress drift measurement in IC plastic packages using the piezo-Hall effect

Abstract A method for encapsulation stress drift measurement based on the piezo-Hall effect is proposed. Accuracy of ±0.25 MPa for the sum of in-plane normal stresses is achieved. Using this method, the drift of die stress in IC plastic packages has been measured after temperature cycling. Stress relaxation with the time constant of about one day has been observed.

Metallic profile and coin imaging using an inductive proximity sensor microsystem

Abstract We have realized metallic profile and coin imaging using a miniaturized inductive proximity sensor. The sensor is a microsystem composed of a 1 mm diameter coil and a new ASIC interface based on the principle of a differential relaxation oscillator. We successfully acquired wellrecognizable images of various coins with different geometrical, electrical and magnetic properties. The integration of such sensor microsystems on a line will open the way to fast and safe coin pattern recognition.

Chopping of a weak magnetic field by a saturable magnetic shield

Abstract A method for measuring a weak magnetic field with a Hall sensor is described. This method consists of chopping the magnetic field to be measured with a magnetic shield surrounding the Hall sensor. The magnetic shield is periodically driven into saturation by means of an excitation coil. Thus, the Hall device is alternately exposed to/shielded from the d.c. or slowly varying external field to be measured. During the time intervals when the magnetic shield is saturated, the external field passes and is detected by the Hall sensor. When it is not saturated the Hall sensor is shielded from the external field. This chopping method yields a magnetic measurement unaffected by 1/ƒ noise and offset errors of the Hall sensor, therefore improving its detectivity.

Temperature compensation of an integrated low power inductive proximity microsensor

Abstract A simple temperature compensation method for inductive proximity microsensors based on the differential relaxation oscillator has been developed and successfully tested. With this compensation and for the temperature range from −20°C to +80°C, an accuracy better than ±10 μm at 1 mm distance to an aluminum target has been measured. The microsensor has been integrated with a 3-V, 1-μm CMOS read-out circuit using a gold bumping layer to form a 3.8-mm side flat coil. The power consumption of the whole compensated microsystem is lower than 10 mW. To achieve this, the temperature behaviors of the whole microsensor and of its building elements, namely the sensing coil (nearby a target) and the read-out circuit, have been studied and a compensation method has been developed. The inductance of the integrated coil is temperature-independent in the frequency range up to 12 MHz, whereas its resistance depends mainly on the temperature coefficient of the conductor resistivity. The resonance frequency of the coil is not affected by temperature. In its principle, the electronic circuit has a temperature-dependent drift in the sensing distance range. This drift can, however, be compensated using a negative temperature coefficient resistor. Analytical derivations and simulation tools have been used for the choice of the optimal coefficient for a specific sensing distance range.