This study compares the electrical properties of transistors in SOS and ISE (Isolated Silicon Epitaxy) SO1 wafers. Included in this study is the radiation response of transistors fabricated with both types of wafers. Processing of both types of wafers was identical and used the process flow from an existing SOS production process.The impurity profile of the SO1 and SOS wafers was essentially identical. Thus, the activation of the implant of both materials is essentially the same. The threshold voltages of N channel MOSFETs with the body grounded was only about 0.1V higher for the SOS material than for the ISE material. The hole mobility profile plots of the film generated using a 5 terminal body contacted transistor revealed that the SOS film has about 50% of the mobility of the SO1 film at the midpoint. The mobility tapers off slowly as the back interface is approached for both SO1 and SOS. This tapering off of the mobility may be due to both reduced crystallinity near the back and the increased impurity concentration at the back.The improved crystallinity of the ISE material over SOS material was found to affect several key transistor parameters. One was the on voltage of the body-source diode . Figure 1 shows plots of Log current versus voltage of diodes fabricated from the 2 materials. The lower I 1 on I t voltage (0.467V versus 0.655V @ I=O.luA) of the SOS material is a consequence of the higher density of recombination centers. The implication of this effect is that the "Kink effecttt and dynamic threshold variation due to gate to body capacitive coupling will be larger for ISE N channel transistors than for SOS transistors. The impact ionization current for the ISE material also proved to be higher. Figure 2 shows a composite plot of the drain and body currents of a SOS NFET and a ISE NFET with the body grounded. The SOS transistor has a peak body current that is a factor of 4 less than that of the ISE material. The higher impact ionization current and higher diode trontt voltage associated with the ISE NFET results in a severe subthreshold tlkinktl effect that is not seen in SOS transistors. Figure 3 shows Log drain current versus gate voltage for both types of transistors with the body floating. The curve of the ISE transistor indicates that at a gate voltage of about 0.15V the transistor switches from a low conducting state to a relatively high conducting state due to the strong impact ionization current-body effect interaction. The SOS transistor shows a more subdued effect.Another parameter which was investigated was the bipolar gain. For a base current of lOuA and a gate voltage of -lV the ISE NFET (L=4um) was found to have a beta of 3.5 compared with the SOS NFET beta of 0.00005. The high beta of the ISE NFET will be detrimental to the operation of some common CMOS circuits unless layouts include a body 39
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