Qualitative Discussion of MOS Transistors

Big Picture

• ES220 (Electric Circuits)– R, L, C, transformer, op-amp

• ES230 (Electronics I)– Diodes– BJT– Complementary Metal Oxide Semiconductor

(CMOS) Field Effect Transistor (FET)• Applications in Digital Integrated Circuits

• ES330 (Electronics II)– Explore Applications of CMOS in Analog Integrated

Circuits

Topics Covered in ES330

• Small Signal Model• Body Effect

A Crude Metal Oxide Semiconductor (MOS) Device

P-Type Silicon is slightly conductive.

Positive charge attractnegative chargesto 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!

An Improved MOS Transistor

n+ diffusion allowselectrons movethrough silicon.

(provide electrons) (drain electrons)

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)

MOSFET as a Variable Resistor

As VG increases, the density of electrons increases, the value ofchannel resistance changes with gate voltage.

You can build anattenuator circuit. (i.e. a voltage divider)

Change Drain Voltage

Resistance determined by VG.

Change Gate Voltage

Higher VG leads to a lower channel resistance, therefore 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 resitsance.

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)

Determination of Region

• How do you know whether a transistor is in the linear region or saturation region?– If VDS>(VGS-VTH) and VGS>VTH, then

the device is in the saturation region.– If VDS<(VGS-VTH) and VGS>VTH, then

the device is in the linear region.

Graphical Illustration

Limited VDS Dependence During Saturation

As VDS increase, effective L decreases, therefore, ID increases.

Pronounced Channel Length Modulation in small L

PMOS Transistor

IV Characteristics of a PMOS