Device & circuit scaling [Fonstad + Spectrum 05/12] Transistors - PowerPoint PPT Presentation

Device & circuit scaling [Fonstad + Spectrum 05/12]

� Transistors . In order to increase speed, let us shorten the gate by a factor s � and in order to have a constant K � In order not to spoil the performances of the FET, we also reduce the vertical dimension by s � So, higher doping is needed � and also a thinner gate oxide � This, however, increases K � These would give higher I and � , => possible breakdown � So, it’s better, to reduce the voltages. Let’s suppose � then

� And it is found that the power per transistor decreases with s 2 (supposing f increases by s ) � But we have a higher transistor density (by s 2 ) => power density remains roughly constant � (Normally, we are not able to scale V with 1/ s , so power density tends to increase) � Metal connections . Lateral dimension will be scaled by 1/ s , but as currents scale with 1/ s , height must not be scaled in order not to increase current density (otherwise, electromigration occurs) � Capacitive parasitic effects . If we do not scale the interlevel oxide thickness (as average length is supposed to remain constant) C decreases - this is good, as f has increased

voltage scaling

* Technology node=Nominal feature size = DRAM (or minimum metal interconnect) half pitch

� As transistors shrink, the problem of chip variability grows � Chips have improved because their transistors and connecting wires have kept getting smaller, but now they’re so small that random differences in the placement of an atom can have a big impact on electrical properties.

� For a long time, most variation was global , caused by slight alterations in the manufacturing process. Such changes differentiate one chip from another or all the chips on one wafer from those on another � A second source of variation is often called local process variation or process variability. � One of the most dramatic sources of local process variation comes from dopants. Transistor channels once contained tens of thousands of dopant atoms. Nowadays chipmakers produce transistors that can accommodate only a few hundred of them