CHAPTER 6: MOSFET CHAPTER 6: MOSFET & RELATED DEVICES& RELATED DEVICES

Part 2

MOSFET

MOSFET

MOSFET FUNDAMENTALS

Perspective view of a metal-oxide-semiconductor field-effect transistor (MOSFET).

IGFET : insulating-gate field-effect transistor

MISFET : Metal-insulator-semiconductor field-effect transistor

MOST : Metal-oxide-semiconductor transistor

Basic device parameters:

• Channel length, L, (distance between two metallurgical n+ -p junction)

•Channel width, Z

•Oxide thickness, d

•Junction depth, rj

•Substrate doping, NA

BASIC OPERATION

2) Low Drain Voltage :When a small drain voltage applied, electrons will flow from the source to the drain through the conducting channel. The channel acts as a resistor, and the drain current ID

proportional to the drain voltage. Current flow can be controlled by using gate voltage. This create a linear region in IV curve

1) When VG=0 :Source and drain electrodes correspond to two p-n junctions connected back to back. Zero current flow from source to drain ideally. This create an inversion layer which will be a threshold voltage, Vt prior current flow. When small positive voltage applied to gate, VG the central MOS structure is inverted and a channel is formed. Current begin to flow.

3) Onset On Saturation :If VD increases, the inversion layer width reduces as the VD is larger than VG. At one point, the inversion layer will disappear as the VD is enough to compensate the inversion layer strength. This point is called pinch off point and the VD saturates.

4) Beyond saturation :Beyond pinch off point, maximum number of electron will flow from source to drain. Therefore, the current saturation occur. The current flow now controlled by VG and independent of VD. The major changes is the reduction of channel length.

Derivation of basic characteristics under following conditions:

a) The gate structure correspond to an ideal MOS diode – no interface traps, fixed-oxide charges or work function differences.

b) Only drift current considered

c) Carrier mobility in the inversion layer is constant

d) Doping in the channel is uniform

e) Reverse leakage current is negligibly small

f) The transverse field created by the gate voltage in the channel is much larger than the longitudinal field created by the drain voltage

g) The gradual-channel approximation – the charges contained in the surface depletion region of the substrate are induced solely from the field created by gate voltage

Figure 6.16. (a) MOSFET operated in the linear region. (b) Enlarged view of the channel. (c) Drain voltage drop along the channel.

oSGS CyVyQ )()(

QS : Total charge induced in the semiconductor per unit area

y : distance from the source

S(y) : surface potential at y

Co=ox/d : gate capacitance per unit area

)()()()()(

yQCyVyQyQyQ

scosG

scsn

Qn : charge in the inversion layer per unit area

Qs: total of Qn

Qsc : charge in surface depletion region per unit area

we can obtain Qn

)(2)( yVy Bs Refer fig. 16.6(c)

)(22)( yVqNWqNyQ BAsmAsc

Conductivity of the channel at y: )()()( xxqnx n

ii xx n dxxqnL

ZdxxLZg

00)()(

)(yQZdy

gLdydR

nn

)(yQZdyIdRIdV

nn

DD

Channel conductance (constant mobility) :

nn Q

LZg

Channel resistance of an elemental section by:

Voltage drop across the elemental section:

ID independent of y

Figure 6.17. Idealized drain characteristics of a MOSFET. For VD VDsat,

the drain current remains constant.

2/32/3 22

232

22 BBD

o

AsD

DBGonD V

CqN

VVVCLZI

For a given VG : ID increases linearly with VD then saturated

The dashed line : locus of the drain voltage VDsat (ID approach a max value)

2

2 TGon

Dsat VVLCZI

2

2 2112

GBGDsat

VVV

TGonD VVCLZI TGD VVV

B

o

BAsT C

qNV

2

22

TGonVD

DD VVC

LZ

VIg

G

constant

DonVG

Dm VC

LZ

VIg

D

constant

Consider linear and saturation regions (for small VD)

for

Threshold voltage VT :

Channel conductance gD :

Transconductance gm :

Pinch-off point : VD increased to a point that charge Qn(y) in the inversion layer at y=L number of mobile ē at the drain are reduced drastically

At this point VD & ID VDsat & IDsat

VD > VDsat saturation region

2

2 TGon

Dsat VVLCZI

2

2 2112

GBGDsat

VVV

TYPES OF MOSFETCross section, output, and transfer characteristics of four types of MOSFETs.

TRESHOLD VOLTAGE

Figure 6.20. Calculated threshold voltage of n-channel (VTn) and p-channel (VTp) MOSFETs as a function of impurity concentration, for devices with n+-, p+–polysilicon, and mid-gap work function gates assuming zero fixed charge. The thickness of the gate oxide is 5 nm. NMOS, n-channel MOSFET; PMOS, p-channel MOSFET.

MOSFET applied in the semiconductor industry for :a) VLSI Circuitsb) Memory Devices ( DRAM, SRAM, Nonvolatile Memory)c) CMOS Digital Circuitd) Microprocessors

Advantages of MOSFET :a) In CMOS, perfect zero current when input switch off. Good logic ‘1’ and logic ‘0’ condition.b) Ability to scale down in size. (Channel length, width, area)c) Ability to control threshold voltage when device shrink. Quick switching.d) Oxide layer between gate and channel prevent DC flow – Power Consumption.e) Low power consumption allow more components per chip surface area.

MOSFET APPLICATION

DISADVANTAGES OF MOSFET

Reduces VT makes MOSFET could not switched off – weak inversion layer.

Interconnect capacitance between wires – device miniature reduces.

Heat production impact – shrink device size, increasing device quantity.

Thin oxide requirement – gate oxide tunneling leakage problem.

Increased process fabrication steps – complex circuit design.