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Introduction to MOS Transistors Section 6.1-6.4. Outline Similarity Between BJT & MOS Introductory Device Physics Small Signal Model.

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Presentation on theme: "Introduction to MOS Transistors Section 6.1-6.4. Outline Similarity Between BJT & MOS Introductory Device Physics Small Signal Model."— Presentation transcript:

1 Introduction to MOS Transistors Section 6.1-6.4

2 Outline Similarity Between BJT & MOS Introductory Device Physics Small Signal Model

3 BJT & MOS Transistor [Chapter 4,5] [Chapter 6,7]

4 Analogous Devices Terminals – “C”↔”D” – “E” ↔”S” – “B” ↔”G” Analogous Devices – NPN ↔NMOS – PNP ↔PMOS

5 Similarity in the Small Signal Equivalent Circuit

6

7

8 Terminal Resistance

9

10 NPN to NMOS

11 Introductory Device Physics

12 A Crude Metal Oxide Semiconductor (MOS) Device P-Type Silicon is slightly conductive. Positive charge attract negative charges to the interface between insulator and silicon. A conductive path is created If the density of electrons is sufficiently high. Q=CV. V2 causes movement of negative charges, thus current. V1 can control the resistivity of the channel. The gate draws no current!

13 NMOS in 3D n+ diffusion allows electrons move through silicon. (provide electrons)(drain electrons) W

14 Typical Dimensions of MOSFETs These diode must be reversed biased. tox is made really thin to increase C, therefore, create a strong control of Q by V.

15 A Closer Look at the Channel Formulation Need to tie substrate to GND to avoid current through PN diode. Positive charges repel the holes creating a depletion region, a region free of holes. Free electrons appear at V G =V TH. V TH =300mV to 500 mV (OFF)(ON)

16 Change Drain Voltage Resistance is determined by V G.

17 Change Gate Voltage A higher V G leads to a lower channel resistance, therefore a larger slope.

18 Length Dependence The resistance of a conductor is proportional to the length. fixed V D fixed V G

19 Dependence on Oxide Thickness Q=CV C is inversely proportional to 1/t ox. Lower Q implies higher channel resistance. fixed V D fixed V G

20 Width Dependence The resistance of a conductor is inversely proportional to the cross section area. A larger device also has a larger capacitance!

21 Channel Pinch Off Q=CV – V=VG-V OXIDE-Silicon V OXIDE-Silicon can change along the channel! Low V OXIDE-Silicon implies less Q.

22 VG-VD is sufficiently large to produce a channel VG-VD is NOT sufficiently large to produce a channel No channel Electrons are swept by E to drain. Drain can no longer affect the drain current!

23 Regions No channel (No Dependence on VDS) Assumption:

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