Evolution of SOI MOSFETs: from Single Gate to Multiple Gates
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Evolution of SOI MOSFETs: from Single Gate to Multiple Gates Jean-Pierre Colinge Department of Electrical and Computer Engineering University of California Davis, CA 95616, USA ABSTRACT To improve short-channel characteristics and increase current drive, SOI technology is shifting focus from "classical" single-gate MOSFET architectures to multiple-gate device structures. This paper traces the history of single- and multiple-gate SOI MOSFETs and summarizes the electrical characteristics of such devices.
HISTORY OF SOI MOSFETS In this section the history of single-gate and multiple-gate MOSFETs is described. Figure 1 shows the SOI MOSFETs "family tree". Single-gate SOI MOSFETs The first SOI transistors date back to 1964. They were partially depleted devices fabricated on Silicon-on-Sapphire (SOS) substrates [1]. SOS technology was successfully used for numerous military and civilian applications [2] and is still being used to realize commercial HF circuits in fully depleted CMOS [3,4,5]. Once the first SOI substrates (the insulator is now silicon dioxide) were available for experimental MOS device fabrication, partially depleted technology the natural choice derived from SOS experience. Partially depleted CMOS continues to be used nowadays and several commercial IC manufacturers have SOI products and product lines such as microprocessors (e.g.: AMD, IBM, Motorola), memory chips (Honeywell). Variations on the partially depleted SOI MOSFET theme include devices where the gate is connected to the floating body. These devices, which have been called "voltage-controlled bipolar-MOS device" [6], "hybrid bipolar-MOS device" [7,8], "gate-controlled lateral BJT" [9], "multiplethreshold CMOS" [10], or "dynamic threshold MOS" [11], have ideal subthreshold characteristics, reduced body effect, improved current drive, and superior HF characteristics [12]. They are mostly used for very low-voltage (0.5 V) applications [13]. The first fully depleted SOI MOSFET date back to the early 1980's where it was quickly established that these devices exhibited superior transconductance, current drive and subthreshold swing [14,15]. Fully-depleted SOI technology is being used in commercial products as well (e.g.: OKI, Peregrine). In addition to the "regular", inversion-mode devices it
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was shown that accumulation-mode FD SOI MOSFETs can be fabricated. These possess characteristics comparable to those of inversion-mode devices. [16] Double-gate SOI MOSFETs The first publication describing a double-gate SOI MOSFET dates back to 1984. The device received the acronym XMOS because of the resemblance of the structure with the Greek letter Ξ [17]. This initial paper predicted the good short-channel characteristics of such a device. The first fabricated double-gate SOI MOSFET was the "fully DEpl
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