MODERN FET STRUCTURES

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O S C S

Chapter 19.

MODERN FET STRUCTURES

Sung June Kimg [email protected] http://helios.snu.ac.krp

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CONTENTS CONTENTS

• Small-Dimension Effects

• Select Structure Survey

MODERN FET STRUCTURES

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Small-Dimension Effects

•

To achieve higher speeds and increased

packing densities FET device structures

packing densities, FET device structures

have become smaller and smaller.

(4)

•

Departure from long-channel behavior p g

− Significant upward slant in the post pinch-off portions of the I^D - V^D

p

300

200 250

V_G=2.6 V

V 2 4 V

µA]

150

200 _V

G=2.4 V

V_G=2.2 V

Current [µ

50 100

V_G 2.2 V

V_G=2.0 V

Drain C

L=0.6 µm

0 1 2 3 4

0

Drain Voltage [V]

(5)

•

In short-channel devices, V

^T

becomes a

function of the gate dimension and the

applied biases.

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Threshold Voltage Modification Threshold Voltage Modification

•

Short-channel

− |V^T| monotonically decreases with decreasing L.

decreasing L.

− The source and drain assist in depleting the region under the gate ⌫ less gate

the region under the gate. ⌫ less gate charge for inversion ⌫ V^T ↓

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) channel long

( V )

channel short

( V

V_T = _T − − _T −

∆

– First order quantitative expression

) Q

Q C (

1

) g

( )

(

BL BS

T T

T

−

= , Q_BS : Q_B for short-channel

Q Q f l h l

C_o Q_BL : Q_B for long-channel

⎞

⎛ 2W

W r

N ⎟⎠

⎞

⎜⎜

⎝

⎛ + −

−

= 1

r 1 2W L

r C

W qN

j j T

o T

A [ homework ]

– Examing ∆V_T/ V_T(long channel), short-

h l ff t d d b d i

channel effects are decreased by reducing xo, reducing rj, and increasing N_A.

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•

Narrow channel (skip) ( p)

− |V^T| monotonically increases with decreasing Z. ( Opposite to the L-dependence)

Z. ( Opposite to the L dependence)

− The gate-controlled depletion region extends to the side lying in part outside the Z-width of to the side, lying in part outside the Z-width of the gate; that is, the effective charge /cm²

being balanced by the gate charge↑ ⌫ V^T↓ being balanced by the gate charge↑ ⌫ V^T↓

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• If the lateral regions are assumed to beIf the lateral regions are assumed to be quarter-cylinders of radius W^T

L W ZLW

N π ² _⎞

⎜⎛

ZL

L 2W

ZLW qN

) width narrow

( Q

2 T T

A

B ⎠

⎜ ⎞

⎝

⎛ +

−

=

Z W 1 2

W

qN_A _T π ^T

⎟⎠

⎜ ⎞

⎝⎛ +

−

=

(^narrow ^width) ^qN ^W ^W

V ^A ^T π ^T

∆ =

∴ ( )

Z 2 width C

narrow V

o

∆ T =

∴

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Parasitic BJT Action Parasitic BJT Action

•

MOSFET bears a physical resemblance to a lateral BJT

•

Punch-through g

− The depletion regions around the source and drain to touch.

to touc

− The gate loses control of the subgate region and the I^D flows beneath the surface through the

the I flows beneath the surface through the touching depletion regions.

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− Punch-through can be suppressed by increasing N^A and decreasing the depletion widths.g p

increasing parasitic capacitances.

perform a deep-ion implantation to selectively increase the doping of the subgate region.

increase the doping of the subgate region.

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• Carrier multiplication and regenerative feedback

I h t h l d i th i lti li ti

− In short-channel devices, the carrier multiplication coupled with regenerative feedback can dramatically increase I^D and places a reduced limit on the

increase I^D and places a reduced limit on the maximum V^D.

− The added electrons drift into the drain. The source

e added e ect o s d t to t e d a e sou ce PN junction is forward biased.

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Hot-Carrier Effects Hot Carrier Effects

•

Oxide charging ( charge injection and )

trapping in the oxide )

− In the vicinity of the drain, some of carriers gain a sufficient amount of energy to surmount the Si-

SiO²barrier.→ A charge build-up within the oxide

− A larger percentage of the gated region is affected in the smaller devices.

− Significant changes in V^T and g^m.

− Oxide charging limits the useful “life” of a device.g g

− To minimize hot-carrier effects, the LDD ( Lightly Doped Drain ) structure is used

Doped Drain ) structure is used.

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•

Velocity saturation Velocity saturation

− The carrier drift velocities inside Si at T=300K approach a maximum value of 7

when the electric fields are large.

sec /

cm 10

v_dsat ≅ ⁷

g

− I^Dsat is significantly reduced. Approximately,

dsat T

G o

Dsat ZC (V V )v

I ≅ −

( )

( ) ^l ^h ^l

channel -

short

;

2 T G

Dsat

V V

I

V V

I ∝ −

(

_G _T

)

²

^; ^long ^- ^channel

Dsat

V V

I ∝ −

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(a) Experimental (b) Theoretical (a) Experimental

characteristics

(b) Theoretical

characteristics including velocity saturation

velocity saturation

(c) Theoretical

characteristics ignoring velocity saturation

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•

Ballistic Transport

− If very small dimension structures have L< l ( the average distance between scattering ( the average distance between scattering events ), a large percentage of the carriers

t l f th t th d i ith t

travel from the source to the drain without experiencing a single scattering event.

− It can lead to super-fast devices.

vdsat

>

carriers of

velocity average

Qthe

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Select Structure Survey

LDD (Li htl D d D i ) T i t

• LDD (Lightly Doped Drain) Transistors

– The reduced dimension devices are more susceptible to hot-carrier effects.

Lightly doped drain region (n^- region) between – Lightly doped drain region (n^- region) between

the end of the channel and the drain

– Electric field ←The voltage drop in the n^- region lowers the maximum

→Carrier injection into the oxide O id h i

→Oxide charging

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DMOS

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Buried-Channel MOSFET

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SiGe Devices

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SOI Structure

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MODFET (HEMT)

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MODERN FET STRUCTURES

O S C S