A Study of Evaporated Thin-Film Voltage-Controlled Tunable Distributed RC-Filters

Abstract

<p> The analysis, construction and performance of a new type of evaporated thin-film filter is described. It has the capability of frequency tuning by means of a single bias voltage.</p> <p> The device is basically a uniformly distributed resistance-capacitance network (URC) with a thin semiconductor film incorporated in the structure. The resistive film, insulator and semiconductor films constitute a metal-insulator-semiconductor system (MIS) whose capacitance is bias dependent. The device differs from other proposed tunable thin-film filters in the fact that the capacitance can be altered independently of the distributed resistance.</p> <p> A theory is developed to account for the effects of MIS-losses and parasitic inductance on the performance of URC-null networks. These parasitic effects manifest themselves in four different ways: (i) A shift of the null from the position predicted by the ideal theory. (ii) A change in the notch parameter α for an optimum null. (iii) A detuning effect in the case of tunable notch filters. (iv) A reduction in the available tuning range; the so-called "tuning range compression".</p> <p> Experimental units were made by vacuum evaporation of Nichrome, Y2O3, CdS and Al onto alumina substrates. An attempt is made to relate the device characteristics to the material properties. In this respect, the relative dielectric permittivity and thickness of the insulator, the doping density of the semiconductor, the semiconductor-insulator interface states and the bulk states in the semiconductor are found to be the most important parameters.</p> <p> Filters were built which operated in the frequency range 600 kHz to 6 MHz. The best notch filter has a tuning capability of 30% with a ±15V bias voltage. The notch depth is over 60 dB in most cases. A tunable bandpass amplifier which was constructed, has a centre frequency which can be shifted between 6 MHz and 7 MHz with a ±3.5V bias voltage. The Q of this particular device was measured to be 742.</p>