Friday, January 29, 2010

Home made C-V measurment system

My dual degree project is fixed and I am going to work on Nano devices. So surely I will be doing some HEMT fabrication and there characterization. Most common of electrical characterization is C-V measurement at different frequencies. So first challenge is measuring Capacitance (C) of the device. We had C-V measurement system in our lab but unfortunately it is shifted to electrical department and I am not in mood to go to EE department every time I want to do my measurements. Setting up a new CV setup is costly, A LCR meter from Agilent itself cost Rs 9 Lakh and we are not in mood of spending money on it. So I decided to make our own C-V measurement setup. Now I know some of methods which are used to measure capacitance.
  1. Charge the capacitor with constant current source and then see how the voltage changes across the capacitor. Assuming ideal capacitor we can find capacitance equal to current / (time rate of change of voltage ). Well this method is simple but not accurate and many multimeter uses this method and there are very cheap so I am not going to this method.
  2. Apply an AC signal across the RC circuit. Measure the voltage across the resistor or capacitor. The phase shift , amplitude difference between these signals will give as value of the capacitance. This is the method which is widely used in the more sophisticated instruments which cost lakhs. So I am going to use this method.
Now after deciding my method I am going to need some instruments. I made some circuits for both signal generator and aquaring data of voltage drop across the resistor. But since I need more sophisticated instrument I decided to use the Tektronix AFG3000 signal generator and Tetronix TDS1012B oscilloscope both of the instrument is available in my lab. I am going to use matlab for the data controlling the signal generator and acquiring the data from the oscilloscope. A Graphical user Interface will be made so that user can choose range of capacitance measurement and other tasks. This is what I have done so far , I will update the blog as I will go through the project

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