New Analysis to Measure the Capacitance and Conductance of MOS Structure

Dr. W. F. Mohamad & Dr. L. S. Ali Electrical Engineering Department College of Engineering University of Mosul Abstract : In this research thin film layers have been prepared at alternate layers of resistive and dielectric deposited on appropriate substrates to form four – terminal RY-NR network. If the gate of the MOS structures deposited as a strip of resistor film like NiCr, the MOS structure can be analyzed as R-Y-NR network. A method of analysis has been proposed to measure the shunt capacitance and the shunt conductance of certain MOS samples. Matlab program has been used to compute shunt capacitance and shunt conductance at different frequencies. The results computed by this method have been compared with the results obtained by LCR meter method and showed perfect coincident with each other.


1-INTRODUCTION
In the recent years, there have been rapidly growing interest and activity in thin film integrated circuits as an approach to microelectronics. Electronic circuits have been fabricated on the basis of replacing conventional lumped elements with their thin film equivalents.
The majority of the up-dated work, however has been concerned with the investigation of sandwiched three layer rectangular and exponential shaped structures. In these structures, alternate layers of resistive and dielectric films are deposited on appropriate substrates to form four terminal R-Y-NR network [1].
The method of analysis that was used to obtain the steady state ac response and the response to a unit step is rather straightforward. It is shown that the partial differential equation relating voltage, position, and time is of second order homogeneous ordinary linear differential equation [2]. If the gate of the MOS structure is deposited as a strip of resistor film like NiCr, the MOS structure can be analyzed as R-Y-NR network [3].

2-Open circuit voltage transfer function of exponential DP R-Y-NR subnetwork
The matrix parameter functions (MPFs) of a solvable DP R-Y-NR network are defined with the following symbols [2]: Employing the technique of subnetwork generation [4,5], the open circuit voltage transfer function T vo of the exponential distributed parameter two-port three layer subnetworks of Fig. (1) is obtainable in terms of the matrix parameter functions (MPFs).
The exponential distributed parameter R-Y-NR structure consists of two resistive layers with per unit length (PUL) series resistance R = R o exp(Kx) and NR=NR o exp(Kx) for the first and second resistive layers respectively.    [4] for the Subnetwork in Fig. (1a) is: and that for the subnetwork in Fig.(1b) is: where g and a are (MPFs) for the exponential distributed parameter (DP) R-Y-NR structure. For structure of length L and ac signal, they are identified as [5]: For N=0 which means that the second resistive layer is perfect conductive film, Equations (7) and (8) will respectively be abbreviated to: Substituting the matrix parameter functions in the Equations (11) and (12) will respectively give: Considering the uniform distributed thin film R-Y-NR network; that means the constant of exponential taper is zero (K = 0), and substituting in the Equations (13) and (14) leads respectively to get: Fig.(1a) . Fig.(1b) ... (16) where m is a complex angle per unit length and Then the complex angle is mL=m×L and for circuit connection in Fig.(1a) and for that in Fig.(1b), then:

3-EXPERIMENTAL RESULTS
For the sake of showing the accuracy of the proposed method, the shunt capacitance and shunt conductance measurements have been carried out on a certain MOS samples. These samples are accomplished by depositing a strip of NiCr resistor thin film as a gate contact and then depositing two dot aluminum points at the two ends of the strip for measurement purposes (see Fig.(1c)).
At the beginning, the transfer function of the device has been measured for both configurations shown in Fig.(1). The response of transfer function magnitude and its phase with respect to frequency have been plotted as shown in Figs. (2) and (3) respectively for positive gate biasing. For negative biasing, the transfer function magnitude and phase responses have been plotted as shown in Figs. (4) and (5) respectively. Matlab program has been used to compute shunt capacitance and shunt conductance for strip gate MOS structure at different frequencies. For a zero bias, the shunt capacitance and shunt conductance of the MOS structure at different frequencies have been computed. These results and those obtained using LCR meter method [6] have been plotted. As shown in Figs. (6) and (7), the results obtained from the two methods coincided with each other.

4-CONCLUSIONS
In this research the high frequency C-V and G-V device measurements were fulfilled using MOS structure as a thin film distributed R-Y-NR structure with four terminal two port network. This conclusion encourage using the proposed method as a tool for C-V and G-V plots at any frequency.