martes, 25 de agosto de 2015

Measuring volt-ampere characteristic of materials

The present invention relates to measuring the volt-ampere characteristic of semiconductors and small currents in general.
Measuring the volt-ampere characteristic of semiconductors, p-n junctions and metal-semiconductor junctions is important in order to determine characteristics as density of carriers, height of schottky barrier, or conductivity.
 There are in the market some devices that actually do this kind of measures. Including features like measuring by pulses and measuring capacitance-voltage characteristic. But they are expensive and sometimes include features that are not necessary in a specific investigation.
Measuring by pulses has the advantage that it avoids heating of samples, and changes in the structure of junctions or contacts by the flux of carriers or generation of heat.
To measure volt-ampere characteristic of materials, in the laboratory was applied the simple use of multimeters and continue power sources. This method provides high precision and good results, limited only by the resolution of multimeters, the generation of heat, and superficial modifications on the junctions by the continue flux of carriers.
A device capable of measure on the basis of pulses of 10 ms with a frequency of 1 kHz was designed. Using cheap electronic components and easy to find in the electronic stores.
The ranges of measuring for current and tension are:    
10 mA – 1 mA
0.02 V – 2.20 V  

Figure 1 shows the electric diagram of the device.
The blocks that compose the device are:
A generator of pulses of 10
ms. of duration, with a frequency of 1 kHz.
A MOS-FET  transistor as modulator (to vary the voltage, it means the height of the pulse).
Two operational amplifiers, one configured as a non-inverting to amplify tension, the other as a trans-impedance amplifier to amplify current and to convert it in tension.
And two envelope detectors for each operational amplifier, to obtain a continue signal for the multi meters.
A virtual ground generator, and two voltage regulators (-12 and +12 Volt) were used to isolate the measures from the noise of the external power sources.

A program to acquire and process data was written also, using the LabVIEW graphical language.
The values measured by the multimeters are converted to real values by an adjusting function, obtained by a calibration process, adjusting curves in the program Origin version 5.0.

Figure 2  shows the diagram of the LabVIEW program.

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