DESIGN AND CONSTRUCTION OF AN ELECTROMETER FOR I-V CHARACTERISTICS OF SEMICONDUCTOR DEVICES

Abstract

An electrometer is an electrical instrument for measuring electric charge or electrical potential difference. There are many different types, ranging from historical hand-made mechanical instruments to high-precision electronic devices. Electrometers based on vacuum tube or solid state technology can be used to make voltage and charge measurements with very low leakage currents. Previous electrometers were cumbersome, expensive and have limitation in the range of measurement. Hence, this project focuses on the design of a low cost digital current electrometer for the measurement of current flowing through a metal-oxide semiconductor devices ranging from nA – pA with a data logger. It consists of seven different sections: the power supply, the microcontroller, the analog to digital converter (ADC), the real time clock, the data logger, the current to voltage converter and the Liquid Crystal Display sections. The power supply comprises a transformer, rectifier, filter and voltage regulator which supplies the required voltage needed to power the device. The electrometer circuit requires positive and negative 5 V, therefore LM7805 and LM7905 were used to give +5 V and -5 V respectively before it is fed to the circuit and the microcontroller. The microcontroller unit controls the whole circuitry via the firmware uploaded on it. A 16-bit analog-to-digital converter (ADC) converts the analog input (voltage) for the microcontroller to process into a digital data output (current). The real time clock provides local time for the device measurement. The secured digital (SD) card was used to log the measured current of the metal oxide semiconductor device for immediate and future data analysis. The current-voltage converter circuit changes the input voltage to current through a 1 GΩ resistance. The liquid crystal display shows a digital visual display of the real-time operation of the low current electrometer. The standard electrometer, Keithley 2400, was used to calibrate the developed electrometer. In order to examine the performance of the electrometer, several electronic devices such as resistors, diode, Light Emitting Diode, Zinc Telluride Based schottky diode (ZnTe), among others, were employed as devices under test using Keithley 2400 programmable electrometer applied as a reference. From the measurements obtained using the developed electrometer, it was observed that the results obtained were similar to that of the Keithley 2400 programmable electrometer. The correlations obtained from the semiconductor under test give 0.906, 0.908, 0.99 and 1 for resistor, diode, LED and Schottky diode which show a better result.