Measurement Techniques, Vol. 57, No. 3, June, 2014
RADIO MEASUREMENTS METHODICAL ERROR IN MEASURING NONLINEAR CAPACITY V. A. Sergeev1 and I. V. Frolov2
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We obtain the dependencies of methodical error on harmonic test signal amplitude in the measurement of nonlinear capacitance using various types of AC/DC converters. Using a semiconductor diode as an example, we show that the smallest methodical error in measuring diffusion capacitance is achieved for average-value rectifiers and that the error in measuring barrier capacitance is the same for all types of converters. Keywords: nonlinear capacitance, voltage converter, semiconductor diode, diffusion capacitance, barrier capacitance.
The capacitance of many elements of electric circuits, in particular, the equivalent capacitance of semiconductor devices (SD), are essentially nonlinear, i.e., are strongly dependent on the applied voltage. Volt–Faraday characteristics (VFC) are important characteristics of SD, which enable the determination of various properties and device parameters, such as the profile of the dopant distribution, contact potential differences, lifetime of minority charge carriers, and others [1–4]. To use VFC for this purpose and to diagnose the quality of SD requires a high measurement accuracy of these characteristics. The nonlinearity of the capacitance of the controlled elements leads to the appearance of methodical measurement error dependent on the level of the test signal. Moreover, it is clear that measurement accuracy increases with a decrease in the test impulse. Major manufacturers of equipment for measuring SD parameters are seeking to lower the amplitude of the test signal. Thus, the Keithley 4200-SCS device for measuring SD characteristics permits establishing a minimal 10 mV effective value for the test signal; the Agilent precision meter RLC E4980A enables a minimal value of 100 µV. However, the dependencies connecting measurement error due to capacitance nonlinearity and the test signal amplitude when using different types of analog-to-digital converters, have not been presented in the literature. The analysis of methodical error in measuring nonlinear capacitance will consider as an example the equivalent capacitance of a semiconductor diode using a capacitive-resistive divider [5]. According to the requirements [5], the differential diode resistance for a given bias at the measurement frequency must exceed the capacitance by more than a factor of 10. This requirement is satisfied with reverse-bias diodes and with forward-bias diodes at small currents and high frequencies of the test signal [6]. In this case, we can assume that the effect of a parallel switched differential resistance can be ignored and that the diode conductivity is purely capacitive. Analysis of Methodical Error in Measuring Diode Capacitance Using Different Types of Detectors. Barrier and diffusion capacitance of a diode are small-signal parameters characterizing the change in charge dq with respect to a change in the applied voltage [4]. In the limit of small voltage increments dU, they are defined as a differential capacitance C(U) = dq/dU.
1
Ulyanovsk Branch, Kotelnikov Institute of Radio Engineering and Electronics, Russian Academy of Sciences, Ulyanovsk, Russia; e-mail:
[email protected]. 2 Ulyanovsk State Technical University, Ulyanovsk, Russia. Translated from Izmeritel’naya. Tekhnika, No. 3, pp. 65–69, March, 2014. Original article submitted October 21, 2013.
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To measure differential capacitance, a diode is supplied with a bias voltage UC(t) that contains a constant component U0, which gives an operating point, and a test signal in the form of a harmonic voltage of frequency ω and amplitude Um: UC(t) = U0 + Umsinωt.
(1)
The variable current IC(t) flowing through the diode creates a falling voltage UR(t) across a reference resistor with resistance Rref switched sequentially with the diode: UR(t) = RrefIC(t). To simplify the later analysis, we assume Rref
