Slide điện tử từ trường lecture 7 metal oxide semiconductor field effecttransistors (mosfets)
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Lecture 07
Metal-Oxide-semiconductor
Field-EffectTransistors
(MOSFETs)
圖片來自casemods.pointofnoreturn.org/pwm/mosfets.html
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topics
• FET’s physical operation
• MOSFET’s current-voltage
characteristic
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Why FET ?
•
•
•
•
Small size (5% BJT)
Little operation power
Simple Manufacturing process
Easy to implement VLSI
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• MESFET
• JFET
Classification
– Depletion
• N-channel
• P-channel
• MOSFET
– Depletion
• NMOS
• PMOS
• CMOS
– Enhancement
• NMOS
• PMOS
• CMOS
CuuDuongThanCong.com
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Microelectronic Circuit by
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Structure of JFET (n-channel)
gate
P-type
source
P-type
depletion
drain
N-type
depletion
P-type
+ VGS
−
− VDS
+
VGS = 0 ⇒ I ≡ I DSS
Reverse VGS ↗ Ỉdepletion region ↗Ỉchannel ↘ỈI↘
Reverse VGS ↗↗Ỉ depletion region ↗↗Ỉchannel ~0 ỈI=0 (pinch off)
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symbol of JFET
D
D
G
G
S
N-channel
S
P-channel
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meiling CHEN
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Structure of depletion-type MOSFET (n-channel)
VGS = 0V
I DSS
electrons
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depletion-type MOSFET (n-channel)
VGS = −V
++++++
Depletion
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symbol of depletion-type MOSFET
NMOS
Current direction
PMOS
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Structure of enhancement-type NMOS transistor
insulator
1010 ~ 1015 Ω
L = 0.1 ~ 3μm
tox = 2 ~ 50nm
W = 0.2 ~ 100 μm
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meiling CHEN
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symbol of enhancement-type MOSFET
NMOS
PMOS
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L = 0.1 ~ 3μm
tox = 2 ~ 50nm
W = 0.2 ~ 100 μm
S
if
I DS = 0
D
VGS = 0 , (VGS < Vt )
→ I D = 0, RDS ≈ 1012 Ω
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VGS > Vt
VDS = 0V
+
enhancement
Minority electrons
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Small VDS
iD
VGS > Vt
VDS ↑⇒ I D ↑
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Ohmic region
RDS ↓
iD
VGS > Vt
VDS ↑⇒ I D ↑
RDS ↑
Resistance
characteristic
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large VDS
Pinch-off
VDS = max
VDS = 0
iD
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Pinch-off
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Total capacitance at dx
Q = CV
⇒ dq = −CoxWdx[v GS −v( x) − Vt ]
n − electrons
Cox : capacitance per unit gate area
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dq dq dx
=
dt dx dt
dq = −CoxWdx[vGS − v( x) − Vt ]
i=
dx
dv( x)
= − μ n E ( x) = μ n
dt
dx
dv( x)
dx
dv( x)
iD = i = μ n CoxW [vGS − v( x) − Vt ]
dx
iD dx = μ n CoxW [vGS − v( x) − Vt ]dv( x)
⇒ i = μ n CoxW [vGS − v( x) − Vt ]
∫
L
0
iD dx = ∫
v DS
0
μ nCoxW [vGS − v( x) − Vt ]dv( x)
1 2
W
[(vGS − Vt )vDS − vDS
] Ohmic
2
L
= vGS − Vt Saturation region
iD = μ nCox
∴ vDS
region
1
W
[(vGS − Vt ) 2 − (vGS − Vt ) 2 ]
2
L
1
W
iD = μ nCox (vGS − Vt ) 2 Saturation region
2
L
⇒ iD = μ nCox
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Structure of enhancement-type CMOS transistor
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EMOSFET operation mode
•
•
•
Ohmic (triode) region
Saturation region
Cutoff region
vDS ≤ vGS − Vt
vSD ≤ vSG − Vt
vDS ≥ vGS − Vt
vSD ≥ vSG − Vt
vGS ≤ Vt > 0
vSG ≤ Vt , Vt < 0
+
+
−
+
−
−
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NMOSFET current-Voltage characteristic
Ohmic region
ID =
μ n Co w
2L
2
[2(vGS − Vt )vDS − vDS
]
ID =
μ n Co w
2L
(vGS − Vt ) 2
Cutoff region
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圖片來自en.wikipedia.org/wiki/MOSFET
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Microelectronic Circuit by
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ID =
μ n Co w
2L
(vGS − Vt ) 2
Saturation region large-signal model
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