Tibor Mohácsy

Noise in piezoresistive Si pressure sensors

The principles of the construction of piezoresistive silicone pressure sensors are outlined. The fabrication of sensors with ion-implantation and common silicone wafer technology is described. The simulation of the devices showed that the membrane thickness has a major influence on the sensitivity, while the misalignment is less important. The low-frequency noise spectra of the piezoresistive elements are Lorenzian; the characteristic time constant is about 23 μs. The bias dependence of the spectra is in some degree less than it was expected from the regular V2 scaling. The plateau of the noise spectrum at working conditions is higher with almost 30 dB than the thermal noise. This excess noise is attributed to a trap level; however the origin of this G-R center is not clear yet. The figures of merits of the sensor were also estimated in numerical examples.

Low-frequency noise in porous Si LED

The current-voltage characteristics and the low-frequency noise spectra of p-type Si - Porous Si - Al light emitting diodes were investigated. over 1 V forward biases a reasonable fit was obtained in the Fowler-Nordheim plot. At lower biases, however, an additional current-component appears, which shows a saturating character. This current component is ascribed to trap-assisted tunneling. Any attempts of accurately fitting the I-V characteristic by other known transport mechanisms failed, as reported earlier. The measured noise spectra show 1/f character. While the biasing current was varied from a about 30 microAmps up to several mAs, the noise level remained constant within the measuring error, i.e. the voltage noise is independent of the bias. This is contradictory to the results obtained on uniform resistors, where the noise power scales with I2, or V2. On this reason the observed noise is attributed to the saturating trap-assisted tunneling.

ANOMALOUS BIAS DEPENDENCE OF THE NOISE IN POROUS SI LED

The current-voltage characteristics and the low-frequency noise spectra of p-type Si–Porous Si–Al diode like structures were investigated. Over 1 V forward biases a reasonable fit was obtained in the Fowler-Nordheim plot. Any attempts of accurately fitting the I-V characteristic by other known transport mechanisms failed. At lower biases, however, an additional current-component appears which shows a saturating character. This current component is ascribed to trap-assisted tunneling. The measured noise spectra show 1/f character; however the magnitude of the noise shows saturation with increasing biases instead of the usual case, where the noise power scales with I2, or V2. This finding is interpreted by a model of two parallel current paths. The noise arising from the smaller and saturating current determines the noise performance of the whole device.