The electrical properties of a conductive polymer material are altered by doping to control its resistivity. Therefore, it is important to accurately measure the electrical resistivity of a conductive polymer. The four-probe gauge is commonly used in the semiconductor industry to measure the resistivity of inorganic semiconductor materials. The conductive polymer belongs to the organic semiconductor material, and the conductive mechanism is different, and the resistivity interval span is large (10-3 to 1010 Ω·cm). The use of a four-probe gauge does not meet the application requirements. At present, when measuring higher resistivity, the measurement circuit can be built according to the national standard (GB3048.3-83). However, this method has strict requirements on the shape of the sample, and the circuit construction is time-consuming and labor-intensive; the excitation voltage is difficult to control, and the voltage is too small to affect the measurement accuracy. If the voltage is too large, a large current may be generated, which may affect the sample characteristics, and is too high. The voltage is very dangerous. In order to avoid pre-treatment processes such as tableting and molding, and to measure accurately and safely over a wide range, necessary improvements have been made here.
1 Measurement principle 1.1 Four probe resistance measurement methodThe four-probe method reduces the effects of contact resistance and wire resistance. The constant voltage excitation signal Vs is used instead of the constant current signal between the 1 and 4 probes. As shown in Figure 1.
Current through the sample:
Where: Rw is the wire resistance; Rct is the pin (1, 4) contact resistance; R14 is the sample resistance.
In order to obtain the sample current I, the total resistance Rx needs to be obtained by other means. Ratio measurement is introduced here. as shown in picture 2.
Where: Vin is the input reference voltage (corresponding to Vs in Figure 2); Vout is the output voltage of the amplifier circuit; Rf is the feedback resistor.
1.2 Measurement resolution The theoretical resolution of the resistance measurement:
When Vin and Rf are constant, the resolution sharply decreases as the resistivity of the sample to be tested increases. Maintain high resolution over the full scale range by:
(1) ration of a larger Rf when measuring a larger Rx;
(2) Change the voltage value of the excitation signal.
Through the program-controlled amplification technology, different gear positions are designed according to the range of resistance values, and different excitation signal voltage values ​​are selected for the gear positions. The corresponding relationship is shown in Table 1.
Semiconductor materials have more stringent requirements for current flowing through the sample when measuring resistivity:
(1) The current should not be too small to ensure that the voltage between the inner probes is measurable;
(2) The current should not be too large to reduce the effect of thermal effects on the resistivity of the sample;
(3) When measuring large resistivity samples, the injection current should be reduced to reduce the influence of minority injection.
The flow current affects the sample resistivity value, but usually the range of resistivity is not affected by the current. The safe operating current can be derived from the limits of this range.
The current flowing through the test sample in this method:
The system adopts LPC2148 ARM7 chip as the core unit of mathematical operation and control. The system structure is shown in Figure 3.
The control processing unit includes an LCD liquid crystal display circuit, a keyboard input circuit, a serial communication circuit, feedback control circuits 1 and 2, and a microcontroller. The preset current limiting resistor Rc is used to define the current through which the small resistivity material is measured, and the current flowing can be calculated by the equation (9). The measurement current is guaranteed to be less than 2 mA over the full range. The parameters and gear positions are shown in Table 1.
It includes a controllable constant voltage generating circuit, a differential amplifying circuit, a ratio measuring circuit, and an analog-to-digital (A/D) conversion circuit. The constant voltage source uses ADR01 to generate a reference voltage output of 10 V, and further obtains reference signals of 0.1 V and 0.01 V. The voltage between the inner two probes in Figure 2 is measured by AD620 differential amplifier circuit and high precision op amp. AD546, the feedback control circuit 2 adjusts the gain resistor RG to change the amplitude of the output signal; the analog-to-digital conversion circuit uses a 16-bit ∑-Δ type AD7705, and the measurement resolution reaches 52μV/LSB when using the 3.401V reference voltage. As shown in Figure 4.
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