The circuit described below was used for the U6 model, the U12 circuit was the same, utilising the equivalent voltage sourcing and measuring terminals.
Voltage was sourced from the DAC0 digital to analogue converter output terminal and applied to the series circuit comprised of the DUT, a shunt resistor and a ground terminal. The shunt resistor was a known value resistor which was used in order to measure the current in the circuit, as the LabJack had no in built capacity for such measurement. This was done by measuring the voltage across it using the analogue input AIN1, with respect to ground. From this voltage, ohms law was then used to calculate current. The voltage across the DUT ...view middle of the document...
This was combined with programming methods discussed later in this report.
2.1 I-V Box
The circuit used for measurement was housed within an interfacing box (I-V box) built by Rob Tucker, as was the equivalent circuit for the U6 model housed in its own version of this box. Both boxes were an aluminium casing with BNC connectors for use with coaxial cables, which interfaced with the equivalent input interfaces of the LabJack circuits. Integrated in these circuits were four shunt resistors of orders of magnitude 100Ω to 100KΩ. A leadless chip mount and D25 port were features of the U12 I-V box only. However the U6 I-V box could interface with these features using the setup shown below in Figure 5. The device under test was mounted in the U12 I-V box, and coaxial cabling used to connect the BNC connectors on each box. The U6 then interfaced with the control computer via USB.
The purpose of this setup was to screen the device from electromagnetic noise through use of a Faraday cage and coaxial cabling (see Appendix 7.1). The chip mount provided convenient measurement of semiconductor devices mounted on ceramic packages. A similar mount provided on an extended piece of circuit board- known as a “cryowand”-could be interfaced with the D25 port to allow immersion of devices in liquid nitrogen. The integrated shunt resistors allowed digital control of shunt resistance without manual adjustment.
The I-V box BNC connectors featured both Hi and Lo connections for two channels, A and B. The two channel A connections corresponded with the high and low voltage wires shown surrounding the device in Figure 4.Channel B Hi was the interface for gate voltages and the corresponding Lo gave an additional ground.
Typically the device was wire bonded from gold contact pads on the device chip to the twenty pins on the leadless chip mount, which corresponded to the respective twenty BNC connectors on the lower half of the U12 I-V box. Often a single pin was used for the drain connections and multiple pins for the source connections, giving contact with various lengths of the device.
3 Volta Program
The program written to control the LabJack models was developed in the Scientific Python Development EnviRonment (Spyder) using the Python scripting language and utilising the dedicated LabJack PyLab control and Tkinter graphical user interface modules. The name decided on for the program was Volta.
Volta features a simple user interface window, allowing selection of either U12 or U6 model; as it incorporates the commands required for both devices , minimum and maximum voltage values for the sweep, the step size, which characteristics are returned and whether the sweep should be forward or reverse bias, or bipolar. Additionally a text file containing the data from a sweep is created with a user defined file name.
3.1 Integer Step Function
This function was designed to combat the lack of correlation between the voltage dropped over the device and that applied by...