Lecture 23

Lecture 23 - EEE 352: Lecture 27 Field Effect Transistors *...

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Unformatted text preview: EEE 352: Lecture 27 Field Effect Transistors * Three terminal devices * Field effect action ⇒ Threshold voltage ⇒ Current saturation A p-channel MOSFET For the p-channel MOSFET, negative voltage is applied to the drain, the gate, and these result in negative drain current. We use an n-type substrate and negative gate bias pulls positive charge to the surface. The first bit arises from pushing the electrons away, leaving positively charged donors. Beyond V T (now < 0), we create a p-type surface channel which connects between the two p-type regions at the source and drain, and current can now flow. Field Effect Transistor Action We can rewrite the current equation as but the direction in which the current is defined has to be reversed as well A major point in today’s devices is that the oxide itself is very thin. A typical n-channel device Field Effect Action OXIDE THICKNESS 1.0 nm Today’s devices work down here! t ox ~ 1.0-1.5 nm V T ~ 0.2-0.4 V The thin oxide leads to tunnel leakage, which has made the case for high-K dielectrics—they can be thicker (higher K) but have the same capacitance. Cross-section of the transistor in the older 90 nm technology, 45 nm gate length. The very thin oxide is approaching the minimum that can be used. approaching the minimum that can be used....
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This note was uploaded on 10/28/2009 for the course EEE 352 taught by Professor Ferry during the Fall '08 term at ASU.

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Lecture 23 - EEE 352: Lecture 27 Field Effect Transistors *...

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