Integrated Circuit Devices

Professor Ali Javey

Summer 2009

MOSFETsReading: Chapters 17 & 18

The First Transistor

1956 Physics Nobel Prize

Invention of the Field-Effect Transistor

In 1935, a British patent was issued to Oskar Heil. A working MOSFET was not demonstrated until 1955.

Today’s MOSFET Technology

Gate oxides as thin as 1.2 nm can be manufactured reproducibly.Large tunneling current through the oxide limits oxide-thicknessreduction.

Introduction to the MOSFET

Basic MOSFET structure and IV characteristics

Introduction to the MOSFET

Two ways of representing a MOSFET:

Complementary MOSFETs (CMOS)

When Vg = Vdd , the NFET is on and the PFET is off. When Vg = 0, the PFET is on and the NFET is off.

NFET PFET

Qualitative discussion: n-MOSFETVG > VT ; VDS 0ID increases with VDS

VG > VT; VDS small, > 0ID increases with VDS , but rate of increase decreases.

VG > VT; VDS pinch-offID reaches a saturation value, ID,sat The VDS value is called VDS,sat

VG > VT; VDS > VDS,sat

ID does not increase further, saturation region.

Threshold voltage for NMOS and PMOSWhen VG = VT, s = 2 F; we get expression for VT.

FSi

A

ox

SioxFT 2

22

Nq

xVIdeal n-channel(p-silicon) deviceboth terms positive

|2|2

2 FSi

D

ox

SioxFT

NqxV

Ideal p-channel(n-silicon) deviceboth terms negative

Si / ox = = 11.9 / 3.9 3

](bulk)[1

FiF EEq

F > 0 means p-type F < 0 means n-type

How to Measure the VT of a MOSFET

Vt is measured by extrapolating the Ids versus Vgs (at low Vds) curve to Ids = 0.

tgsdsnstgsoxedsat VVVVVCL

WI )(

Vds = 10mV

Ids

Vgs

Vt

Quantitative ID-VDS Relationships “Square Law”

2

2DS

DSTGoxn

DV

VVVCL

ZI satDS,DS0 VV TG VV ;

ID will increase as VDS is increased, but when VG – VDS = VT, pinch-off occurs, and current saturates when VDS is increased further. This value of VDS is called VDS,sat. i.e., VDS,sat = VG – VT and the current when VDS= VDS,sat is called IDS,sat.

2TGox

satD, 2VV

L

CZI

satDS,D VV TG VV ;

Here, Cox is the oxide capacitance per unit area, Cox = ox / xox

ID-VDS characteristics expected from a long channel (L << L) MOSFET (n-channel), for various values of VG