Comment Re:Brief guide to SOI (Score 3) 69
Why SOI?
The gate of a MOS transistor is essentially a capacitor, and the speed at which it can operate is determined by how long this capacitor takes to charge/discharge.
Basic physics will tell you that the thicker the capacitor, the smaller its value, and the faster the transistor. By putting the transitor on an SOI wafer, the silicon-dioxide layer acts as extra thickness for the capacitor, reducing its value, and making the thing faster.
The gate of a MOS transistor is essentially a capacitor, and the speed at which it can operate is determined by how long this capacitor takes to charge/discharge.
Basic physics will tell you that the thicker the capacitor, the smaller its value, and the faster the transistor. By putting the transitor on an SOI wafer, the silicon-dioxide layer acts as extra thickness for the capacitor, reducing its value, and making the thing faster.
Erm, not exactly.
You're right that gate capacitance is important, but what's really affected by the insulator layer are two different things:
- junction capacitance - This is the capacitance of the source and drain, and it's formed by the parasitic PN juntion at the boundary between the source/drain and the bulk silicon. It includes both a sidewall component (formed by the vertical boundary between the source/drain and the bulk) and an area component (formed by the horizontal s/d-bulk boundary at the bottom of the source/drain). This latter component is eliminated in SOI, because there's no bulk silicon under the device.
- body effect - A posistive voltage difference between the source and the bulk causes the effective threshold voltage to rise, which means the transistor is harder to turn on. Since there isn't any bulk below the channel, there isn't a mechanism for the bulk bias to affect the threshold voltage. This is especially important when you have stacks of transistors, e.g. in the pulldown path of a NAND gate.
There's more on this at IBM's web site.
