Introduction to MOS Transistors

Section 6.1-6.4

Outline

• Similarity Between BJT & MOS• Introductory Device Physics• Small Signal Model

BJT & MOS Transistor

[Chapter 4,5]

[Chapter 6,7]

Analogous Devices

• Terminals– “C”↔”D”– “E” ↔”S”– “B” ↔”G”

• Analogous Devices– NPN ↔NMOS– PNP ↔PMOS

Similarity in the Small Signal Equivalent Circuit

Terminal Resistance

NPN to NMOS

Introductory Device Physics

A Crude Metal Oxide Semiconductor (MOS) Device

P-Type Silicon is slightly conductive.

Positive charge attractnegative chargesto the interface between insulator and silicon.

A conductive path is createdIf 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!

NMOS in 3D

n+ diffusion allowselectrons movethrough silicon.

(provide electrons) (drain electrons)

W

Typical Dimensions of MOSFETs

These diode mustbe reversed biased.tox is made really thin

to increase C, therefore, create a strong control of Q by V.

A Closer Look at the Channel Formulation

Need to tie substrate to GNDto avoid current through PN diode.

Positive charges repel the holescreating a depletion region, a region free of holes.

Free electrons appear at VG=VTH.

VTH=300mV to 500 mV(OFF) (ON)

Change Drain Voltage

Resistance is determined by VG.

Change Gate Voltage

A higher VG leads to a lower channel resistance, therefore a larger slope.

Length Dependence

The resistance of a conductor is proportional to the length.

fixed VD

fixed VG

Dependence on Oxide Thickness

Q=CVC is inversely proportional to 1/tox.

Lower Q implies higher channel resistance.

fixed VD fixed VG

Width Dependence

The resistance of a conductor is inversely proportional to the crosssection area.

A larger device also has a larger capacitance!

Channel Pinch Off• Q=CV– V=VG-VOXIDE-Silicon

• VOXIDE-Silicon can change along the channel! Low VOXIDE-Silicon implies less Q.

VG-VD is sufficiently largeto produce a channel

VG-VD is NOT sufficiently largeto produce a channel

No channel

Electronsare sweptby E to drain.

Drain can no longer affect the drain current!

Regions

No channel

(No Dependence on VDS)

Assumption: