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Copyright 1999 Leslie Smith 31R6 - Computer Design Slide 18 How to build an nmos transistor nmos transistors are built by starting with p-type silicon placing two n-type areas (which will become the source and the drain) close together, but not touching growing some silicon dioxide over the p-type silicon between them placing the gate conductor (usually polycrystalline silicon) on top of this silicon dioxide The process by which this is achieved is complex, but is rather like printing, in that making areas n-type is achieved by diffusion through a mask impurities which will make the p-type substrate n-type are embedded into particular parts of the silicon substrate some silicon dioxide is grown over the area between two n-type diffused areas the gate oxide the gate itself is placed over this gate oxide area gate is generally made up of polycrystalline silicon (poly) Copyright 1999 Leslie Smith 31R6 - Computer Design Slide 19 2ģ Layouts (1) Each part of the process.

A positive gate potential (Vgs > 0) electrostatically induces a negative charge at the surface of the area below the gate this turns the area below the gate into n-type the diodes disappear, and the transistor conducts. Copyright 1999 Leslie Smith 31R6 - Computer Design Slide 17 1Ģ What is an nmos transistor? (and how does it work?) (see handout for diagram) Bulk of transistor is p-type Source and Drain are n-type pn junction is a diode that is, current flows in one direction only electrons flow from n to p or current flows from p to n but not the opposite way - so if the gate is not connected, no current flows. 1 The MOSFET Transistor The basic active component on all silicon chips is the MOSFET Metal Oxide Semiconductor Field Effect Transistor Schematic symbol G Gate S Source D Drain The voltage on the gate controls the flow of electrons from the source to the drain Copyright 1999 Leslie Smith 31R6 - Computer Design Slide 16 Electrical properties of nmos transistor nmos = n-channel MOS transistor Electrical properties: When the voltage on the gate is 0 (Vgs = 0), the transistor is off When the voltage on the gate > threshold, the transistor is on the behaviour is quite switch-like.