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Unformatted text preview: 5 – Slide 15 EE105, Fall 2013 Prof. Ming C. Wu Flatband Voltage, VFB §༊ The built in potenNal can be “cancelled out” by applying a gate voltage that is equal in magnitude (but of the opposite polarity) as the built in potenNal. This gate voltage is called the ﬂatband voltage because the resulNng potenNal proﬁle is ﬂat. ρ ( x) x
tox 0 V ( x)
x
tox 0
EE105, Fall 2013 There is no net charge (i.e. ρ(x)=0) in the
semiconductor under for VGB = VFB. Lecture 15 – Slide 16 Prof. Ming C. Wu 8 Voltage Drops across a MOS Capacitor VGB − VFB = Vox + VS V ( x) tox 0 x Xd §༊ If we know the total charge within the semiconductor (Q̕S) , we can ﬁnd the electric ﬁeld within the gate insulator (Eox) and hence the voltage drop across the gate insulator (Vox): ⎛ྎ − QS ⎞ྏ
ʹȃ
− QS
ʹȃ
− QS
Vox = Eoxtox = ⎜ྎ
E ⋅ dS = Eox A =
⎜ྎ Aε ⎟ྏtox = C
⎟ྏ
∫
ε ox
ox
⎝ྎ ox ⎠ྏ
where QS is the areal charge density in the semiconductor [C/cm2] and Cox ≡ ε ox t ox is the areal gate capacitance [F/cm2] Lecture 15 – Slide 17 EE105, Fall 2013 Prof. Ming C. Wu VGB < VFB (AccumulaNon) §༊ If a gate voltage more negaNve than VFB is applied, then holes will accumulate at the gate insulator/semiconductor interface. ρ ( x)
tox x
0 V ( x)
tox x
0 EE105, Fall 2013 Areal gate charge density [C/cm2]: QG = Cox ⋅ (VGB − VFB )
Lecture 15 – Slide 18 Prof. Ming C. Wu 9 VFB < VGB < VTH (DepleNon) §༊ If the applied gate voltage is greater than VFB, then the semiconductor surface will be depleted of holes. –...
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This note was uploaded on 01/22/2014 for the course EE 105 taught by Professor Kingliu during the Fall '07 term at Berkeley.
 Fall '07
 KingLiu
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