Qualitative Discussion of MOS Transistors

Big Picture

• ES230– Diodes– BJT– Op-Amps

• ES330– Applications of Op-Amps– CMOS• Analog applications• Digital applications

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.

Dependence on Oxide Thickness

Q=CVC is inversely proportional to 1/tox.

Lower Q implies higher channel resitsance.

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

Transconductance

• As a voltage-controlled current source, a MOS transistor can be characterized by its transconductance:

• It is important to know that

What Happens to gm/ID when W and ID are doubled?

Body Effect

The threshold voltage will change when VSB=0!

Experimental Data of Body Effect

The threshold voltage will increase when VSB increases.

What if we drive the base with a small signal?

Input and Output

Vout, m=46 mVVin, m=1 mV

Replace the transistor by its small signal equivalent circuit

(EQ 5.157)

Comparision:ADS Simulation: 46EQ 5.157: 49.33

Small Signal Model for NMOS Transistor

PMOS Transistor

IV Characteristics of a PMOS

Small Signal Model of PMOS

Small Signal Model of NMOS