The MOSFET Transistor

Transcription

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. Copyright 1999 Leslie Smith 31R6 - Computer Design Slide 17 Page 1 1

2 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. 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. For details see any book on VLSI design. 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 Page 2 2

3 Layouts (1) Each part of the process... placing diffusion, placing gate oxide, placing gates, and others too is carried out using a mask the mask defines where (e.g.) which regions of the chip will have N-type ions diffused on to it The process of designing these masks is known as generating a layout. Here is the layout for an N-channel MOS transistor (an NMOS transistor) Copyright 1999 Leslie Smith 31R6 - Computer Design Slide 20 Layouts (2) N-type diffusion is shown in green Poly is shown in red In manufacture, the polysilicon deposition precedes diffusion, so that there is no diffusion under the poly gate. The transistor is symmetrical which is the source and which the drain depends on how the transistor is wired up The electrical characteristics are strongly influenced by the gate dimensions W width L length and the gate oxide thickness is also crucial to performance. Copyright 1999 Leslie Smith 31R6 - Computer Design Slide 21 Page 3 3

4 PMOS transistors Schematic: Only difference from nmos FET is circle on gate. 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 again quite switch-like. Copyright 1999 Leslie Smith 31R6 - Computer Design Slide 22 Layout of a PMOS transistor If the underlying substrate is n-type, and the diffusion is p-type then a pmos transistor is formed P-type diffusion is shown in brown Generally, the silicon substrate is p-type, and a well of n-type silicon needs to be formed (by diffusion) before a pmos transistor can be formed. Copyright 1999 Leslie Smith 31R6 - Computer Design Slide 23 Page 4 4

5 Depletion mode transistors Depletion mode transistors have a threshold voltage set so that when the gate is connected to the source (I.e. Vgs = 0) the transistor is on They are used as resistors Schematic symbol: Layout: Effect of implant is to change the characteristics of the substrate. Copyright 1999 Leslie Smith 31R6 - Computer Design Slide 24 Page 5 5