Device for Automated Measurement of Volt-Ampere Characteristics

Abstract

The device, which allows automated measurement of volt-ampere characteristics of semiconductor devices, is developed. The device is implemented using the microcontroller of STM32F4 series, DAC, voltage–current transform circuit, voltage and current sensor and a laptop with LabVIEW programming environment. A wide range of current measurement is possible - from 25 μA to 800 mA with current control up to 0,4 μA. The digital voltage value from the microcontroller is transmitted to the AD5624R DAC via the SPI interface. AD5624R is a 12 bit four channel DAC with built-in voltage reference of 2,5 Volts. Using two DAC channels allowed to increase the resolution of voltage control to up to 12 μV. The voltage-current transform circuit consists of two precision operational amplifiers AD8510 and OP97 with low offset voltage, n-channel and p-channel field-effect transistors with low leakage current and voltage. In the device it is important to control the offset voltage in order to provide a minimum initial current through the sample tested. The schematic is discussed for expanding the measurement range and compensating for the error of the zero code from the DAC. In the circuit, coarse control of the output voltage is implemented with a step of 1,2 mV, and fine control - with a step of 12 uV. Two channels of the DAC channel C and D were used for this purpose. The device circuit also provides compensation for the error of the zero DAC code. Among the resistive current sensors, we chose the INA226 chip, which can be purchased on the so-called breakout board. This sensor is one of the most accurate among analogues, containing a 16-bit ADC and providing digitized values of current and voltage at the output. The developed device has a linear dependence of the code to current. The linearity of current control is confirmed to have the correlation coefficient 99,9%. The approach to calibrate the device is described, which will minimize the error and expand the range of measurements. In order to correct the INA226 readings, correction coefficients are used, so that the approximated line equation for the precise voltmeter readings and the INA226 readings match. It is possible to use the approximated values of the current for obtaining the volt-ampere characteristic in the subthreshold region when the voltage on the sample is less than the voltage and current values are low (less than 1 mA), and the correted values from INA226 for building the volt-ampere characteristic in the upper threshold region, when the voltage on the sample exceeds the and current values are higher. Ref. 12, fig. 8.