Thick-field threshold voltages depend on factors such as conductor material, oxide thickness, crystal orientation, doping levels, and processing conditions. Processes may quote a minimum value from worst-case scenarios. Some processes determine separate thresholds for each conductor-diffusion combination. Designers may consider the body effect for threshold adjustment. The body effect increases the transistor’s apparent threshold when the back gate-source junction is reverse-biased, but its effectiveness is limited, especially in lightly doped back gates.
What is the mechanism behind the formation of channels without conductors?
Answer: Channels can form without conductors through a mechanism called charge spreading. Charge spreading occurs when static electrical charges accumulate on insulator surfaces or interfaces. Although details are not fully understood, the basic principles involve the movement of static charges under electrical fields. Charge spreading is accelerated by higher temperatures and moisture. Channels can form in integrated circuits, and the rate of charge spreading depends on temperature and contaminants.
How does charge spreading affect standard bipolar and CMOS processes differently?
Answer: Standard bipolar processes appear more susceptible to charge spreading than CMOS due to less stringent cleanliness requirements. CMOS processes, which minimize ionic contamination, exhibit a degree of immunity to charge spreading. Charge spreading leads to parasitic PMOS transistors, especially in P-regions operating above the PMOS thick-field threshold. CMOS and BiCMOS processes, with fewer mobile ions, show a degree of immunity. Failures, often after prolonged high-temperature operation, can be partially reversed by baking the units to disperse accumulated static charges, but this isn’t a permanent cure.
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