Experimental and Theortical Results on the Performance of Elementary Amorphous Silicon thin film Integrated Circuits
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EXPERIMIENTAL AND THEIORFICAL RESULTS ON THE PERFRMANCE OF?ELEM[NTARY AIWA SILICON THIN FILM IMICRATED CIR•1ITS
M. BOWg, S. SALAMON, Z. KISS Chronar Corp., P.O. Box 177, Princeton,
New Jersey 08542,
USA
ABSTRACT Elementary amorphous silicon logic integrated circuits have been manufactured on glass substrates by rf glow discharge. Circuits presented include two different inverters, a 6-transistor addressable static memory cell, a 3-transistor addressable dynamic memory cell, and a 9-stage ring oscillator.
All of the circuits use n-channel enhancement type load TFTs instead of ohmic resistors. The channel length of the driver transistors is
10 Wn and
15
pro,
experimental geometry ratios range from 0=5 to 0=33. The influence of the geometry ratio on static and dynamic characteristics is examined. Circuits operating at supply voltages of less than 5 V have been demonstrated. INTRODUCTION
Presently,
amorphous
silicon technology is expanding into the field of
microelectronics. Certain applications of a-Si field effect transistors in large area electronics, e.g. in active liquid crystal display panels [1,2], optical page readers, and electronic copying [3] are at the edge of commercialization. Because of frequency limitations [4], a-Si TFTs in commercial products are to date still used as switching elements only, while crystalline silicon chips are required for the peripheral driver circuitry. Along with a better understanding of the material and the development of novel and faster device structures [e.g. 5,6] it appears, however, that it may be only a matter of time until a-Si technology can also be employed for the fabrication of on-substrate integrated circuits, e.g. multiplexers and decoders. While cut-off frequencies of about 1 MHz can be achieved with conventional planar TFTs without requiring submicron structures, multi-MHz operation becomes feasible for vertical structures. In this paper we report results of an exploratory effort to systematically study the basic building blocks of a-Si thin-film integrated circuits
for potential applications in battery powered consumer products. For these applications, in addition to speed, a low supply voltage is an important issue, too. The design of inverters is briefly examined and experimental results on the performance of elementary logic circuits are discussed. Mask layouts for several circuits are presented. Finally a ring oscillator is investigated, and dynamic device properties are evaluated. IVRE
DESIGN
The elementary logic circuit is the inverter. A transistor connected in series with a resistor represents a simple inverter. In a thin film circuit an ohmic resistor can be integrated in the form of a doped n*-layer of appropriate geometry. With regard to reproducibility and yield, however, a load transistor (fig. 1) is preferred. This approach also eliminates an additional n*-patterning step, since otherwise an nt-layer is solely needed to improve the contact characteristics. Therefore, it can be patterned along with the source and drain metallization.
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