lecture20_March2.ppt

Grace Xing---EE30357 (Semiconductors II: Devices) 1

EE 30357: Semiconductors II: DevicesLecture Note #20 (03/02/09)

MOS Field Effect Transistors Grace Xing

Outline:

1. Last class: Compound semiconductor based devices2. Quick revisit of MOS capacitors and FETs (read on your own how

DRAM, CCD, flash memory etc. work, they are all based on MOS structures!)

1. Flat band voltage2. Effects of oxide charges (interface charges, fixed charges)


Chigh−freq =

11Cox'+

1CB'

CB' =

∈SiwB

=∈Si

2 ∈Si (2φf )qNA

'

φf =kTqlnNA'

ni

Oxide thickness

How to extract doping concentration from C-V measurements

VT

Depletion Weak inversion

Weak inversion is “weakly” defined term. The difference between high f and low f is true for MOS (gate-back ohmic contact) capacitors

However, there is no difference between high f and low f for MOS (gate-S/D ohmic contact) capacitors since the minority charges are supplied from the top ohmic contacts and the majority carrier are still supplied from the substrate ohmic contacts.

GateOxide

p-substrate

In a MOSFET, n+ ohmic

contacts can be grounded.

p+

n+n+

GateOxide

p-substrate

Ohmic contacts

p+

In a MOS-CAP,

p+ ohmic contact is grounded.


When the semiconductor energy band is flat, we call it flat band condition and the voltage (VGS) needed the flat band voltage VFB.

Q: if there is no charge in the oxide or at the oxide-semiconductor interface, i.e. an ideal MOS capacitor, what is the electric field in the oxide at Flat Band?

What is VFB?

A: zero since there is no charge anywhere;VFB = Vbi = ms/q

Q: what if oxide charge is not zero?

Flatband Voltage


Donor-like traps:Neutral when filled

Positive when empty

Gate

Oxide

Channel


VGate−Bulk +Vbi =VT + (−

MS

q) =φox +φs

MS =φM −φS

I) without oxide charges and interfacial charges

VFB =MS

q

II ) With these positive charges, VFB will decrease(more negative) :

Flat band voltage

VFB =MS

q−φox(0) =

MS

q−QfCox'−Qit(0)

Cox'

q(VFB+Vbi)

qox

m

S

Evac

Flatband Condition(s= 0)

Can you reason for VFB then?

Concept-Graph-Equation

QB=0

Area enclosed by is Vbi+VFB not Vbi!

Do not confuse VFB (the external applied bias) with the potential drop in the MOS!



MS

q) =φox +φs

MS =φM −φS

I) without oxide charges and interfacial charges

VT =MS

q+φox(VGB) +φs(VGB)

=MS

q−QB(2φ f )

Cox'

+ 2φ f

note:QB(2φ f ) < 0 here

q(VT+Vbi)q(2f)

qox

m

ΦS

Evac

Threshold Condition: I (w/o oxide & interface charges)

(s=2f)

Induced mobile charges (electrons in this example) <<

ionized dopants (acceptors here) Can be ignored


QB(2f)

Inaccuracy: the slope of Evac should be 1/3 smaller in Si than in SiO2

Area enclosed by is Vbi+VT not Vbi!


Donor-like traps: Neutral when filled and Positive when empty


MS

q) =φox +φs

MS =φM −φS

II ) With these positive charges, VT will decrease:

VT =MS

q+ (φs =2φ f ) + φox =−

QfCox'−Qit(2φ f )

Cox'

−QB(2φ f )

Cox'

⎛

⎝⎜

⎞

⎠⎟

q(VT+Vbi)q(2f)

qox

m

S

Evac Threshold Condition: II(with oxide/interface charges)

(Still true: s=2f)


QB(2f)

Positive oxide charges or interfacial charges

Smaller charger at the gate

Smaller field inside the oxide

Smaller total band bending in Evac

Area enclosed by is Vbi+VT not Vbi!


Boron – acceptors: ionized acceptors are negatively charged

Threshold voltage control There is a n-channel Si MOSFET. Due to some mishaps during fabrication, it ended up being a depletion mode FET with Vth = -0.1V. Our target Vth is 0.5V.

1.What type of interface charges can help us tune it to the right Vth? (negative)2.How many charges are necessary if Cox’ = 0.8uF/cm2? (the grey area = 0.5 – (-0.1) = 0.6V = Qii/Cox’)3.Does this process change Vbi of the device? (No)

QB(>2f) QB(<2f)

• Is E-field in Si equal to 1/3 of E-field in SiO2?• Why not?

QB(=2f)

0 V 0 V 0.5 V

Qii


Ci = Cox Cd = CB

1 ox LF ox HFit

ox LF ox HF

C C C CD

q C C C C

Also see Fig. S3.14 in Anderson


Field effect transistors – Voltage controlled barrier for current flowInput current (DC) = zero

Capacitive actions –Need two plates of charges separated by an insulating layer

Choices of insulating layer: oxide and depletion region

Compare with current controlled barrier devices - BJTs

2

0

( / )( / )

( / )between parallel plater

C cmV cm

F cm


SourceDrain

During writing operations,


Source Drain

During reading operations,

lecture20_March2.ppt

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