Browsing by Author "Mhamilawa, Edward Eliah"

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    Automatic measuring system for impendance investigation of Mos structures
    (University of Dar es Salaam, 1985) Mhamilawa, Edward Eliah
    A case is presented to explain the observed frequency dependency of some reported tinuous capacita Cp and conductance G versus bias voltage Vb characteristics of a MOS transistor junction, measured at high frequencies. Based on a discussion on the appropriate electrical model of a device at high frequencies, a series model is chosen for the design of an instrument for measuring parameters of MOS structures. The design principle of the instrument is based on a vector ratio method in which a high frequency signal is transduced by a probe using a special current to voltage transformer and a pair of synchronous detectors, For high accuracy, high frequencies used are down converted to suitable value of 10 kHz using a superheterodyne phase lock loop system and a pair of mixers. The realized instrument is versatile and of low cost. It has the capability of automatic plotting of continuous capacitance Cs with its series resistance Rs versus bias voltage Vb characteristics of semiconductor devices such as MOS capacitors, varactors and similar structures on an X-Y Y recorder. The main features of the instrument include the capability of measuring capacitances from 1 pF to 100 pF and resistances Pram 0 ohm to 1000 ohms in steps of four scale ranges with an accuracy of better than 2 per cent. A bias voltage ranging from -110 V to 1l0 V with a variable sweep range up to ± 10 V and different sweep speeds included together with selectable frequencies of 25, 50 and 100 MHz. Using the developed instrument a set of CS (Vb), Rs(Vb) characteristics of some semiconductor structures are measured and analysed. From the data the frequency dependency of the measured characteristics is observed to be low in agreement with the expected theoretical results. Typically the change of capacitance and its loss resistance of a MOS transistor junction, when frequency is changed from 25 MHz to 100 MHz, have dropped from 48% and 652% to 14% and 40% respectively.