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To: Bill Jackson who wrote (64452)	11/28/2001 12:08:22 AM
From: Pravin Kamdar	Read Replies (1) \| Respond to of 275872

Bill, Pravin, thicker, do you not mean thinner? No, I mean thicker. Look. Permittivity is just a proportionality constant between electric potential energy (let's call it voltage) and electric field strength. That is, the higher the permittivity of a material between a potential difference, the higher the electric field. But electric field is also a measure of charge and it's separation -- that's where capacitance comes into this mess. Consider a parallel plate capacitor -- just two metal plates sandwiched around a thin sheet of dielectric (insulator). Now, apply a fixed voltage across the capacitor. Charge will flow from the power supply that is generating the voltage onto the plates of the capacitor. This charge creates an electric field across the dielectric. Now, if I keep the voltage the same, there are two ways to increase the amount of charge, and hence electric field and capacitance, on the plates. One way is to decrease the spacing of the plates with a thinner dielectric layer. The other way is to increase the permittivity of the dielectric layer (due to the amount of electric dipole polarization that is going on within the dielectric -- but let's not get into that). So, apply this to a MOS transistor gate -- which is just a parallel plate capcitor, with the top plate being the polysilicon gate, and the bottom plate being the silicon surface under the gate dielectric. If you are migrating to a smaller geometry, source and drain become closer together and you must lower the operating voltage to avoid punchthrough. But you still need a high electric field across the gate dielectric to invert the channel. So, you would typically just reduce the gate oxide thickness. But, this is leading to gate leakage and gate oxide breakdown, so the choice now becomes using a higher-k gate insulator in order to increase the electric field and allow the gate insulator to be thicker than it would otherwise have to be. Of course, for the exact same capacitance physics, you want to use low-k dielectrics between metal paths on the chip, in order to reduce the amount of charging a signal must do as it tries to speed along a path. Pravin Disclaimer. I'm really tired and haven't cracked a device physics book in quite some time, but it all makes sense to me.