Download Transistor Structure

Transistor Structure
DMT 241 – Introduction to IC Layout
Outline
– Transistor structure
BJT
PMOS
NMOS
CMOS
BJT
• Bipolar Junction transistor is a 3 – terminal
elements that obtains its electrical
characteristics from the properties of pn
junctions.
• 2 types of BJT
– npn
– pnp
• The current flowing through an npn transistor is
due mostly to electrons, while that through a pnp
devices is due to hole
BJT- symbol
BJT
• Reverse active bias
– VBE > 0 and VBC<0
– Allows amplification
– Used for analog circuit
• Saturation
– VBE > 0 and VBC>0
– Large current can flow through the
devices but the transistor does not
control the value
• Cutoff
– VBE < 0 and VBC<0
– Small leakage currents can flow
– Model as open swotch
• Forward active bias
– VBE < 0 and VBC>0
– Used only a few special cases
MOS transistor
• MOS: Metal – Oxide – Semicomductor
• Majority carrier device in which the current
in a conducting channel between the
source and drain is controlled by a voltage
applied to gate
• nMOS: majority carrier are electrons
• pMOS: majority carrier are holes
Chapter 6 page 263
MOS Behavior
• Examined an isolated
MOS structure with a
gate and body but no
source and drain
• Top layer : polysilicon
• Middle layer: silicon
dioxide
• Bottom layer: silicon
body
polysilicon gate
silicon dioxide insulator
Vg < 0
+
-
p-type body
(a)
0 < V g < Vt
+
-
depletion region
(b)
V g > Vt
+
-
(c)
inversion region
depletion region
MOS Behavior
• Accumulation
– Negative voltage is applied to
gate
– Mobility positively charged hole
are attracted to the region
beneath the gate
• Depletion
– Low positive voltage is applied to
gate
– Resulting some positive charge
on the gate
– The hole in the body are repelled
from the region directly beneath
the gate, resulting in a depletion
region forming below the gate
polysilicon gate
silicon dioxide insulator
Vg < 0
+
-
p-type body
(a)
0 < V g < Vt
+
-
depletion region
(b)
V g > Vt
+
-
(c)
inversion region
depletion region
MOS Behavior
• Inversion
– A higher positive potential
exceeding a critical
threshold voltage applied,
attracting more positive
charge to the gate
– The holes a repelled further
and a small number of free
electrons in the body are
attracted to the region
beneath the gate
polysilicon gate
silicon dioxide insulator
Vg < 0
+
-
p-type body
(a)
0 < V g < Vt
+
-
depletion region
(b)
V g > Vt
+
-
(c)
inversion region
depletion region
nMOS transistor
• Four terminals: gate, source, drain, body
• Gate – oxide – body stack looks like a
capacitor
– Gate and body are conductors
– SiO2 (oxide) is a very good insulator
– Called metal – oxide – semiconductor (MOS)
capacitor
Source
Gate
Drain
– Even though gate is
no longer made of metal
n+
n+
p
bulk Si
Polysilicon
SiO2
nMOS operation
• Mode of operation depends on Vg, Vd, Vs
– Vgs = Vg – Vs
– Vgd = Vg – Vd
– Vds = Vd – Vs = Vgs - Vgd
• Three regions of operation
– Cutoff
– Linear
– Saturation
nMOS Cutoff
• No channel
• Vgs < Vt
• Ids = 0
Vgs = 0
+
-
g
+
-
s
d
n+
n+
p-type body
b
Vgd
nMOS Linear
• Channel forms
Vgs > Vt
+
-
– Vds = Vgs-Vgd
– If Vds=0 (i.e. Vgs=Vgd)
+
-
s
d
n+
n+
• No electrical field tending
to push current fr. d to s.
• Small positive potential Vds
• Current flows from d to s
– e- from s to d
• Ids increases with Vds
• Similar to linear resistor
g
Vgd = Vgs
Vds = 0
p-type body
b
Vgs > Vt
+
-
g
s
+
d
n+
n+
p-type body
b
Vgs > Vgd > Vt
Ids
0 < Vds < Vgs-Vt
nMOS Saturation
• Channel pinches off
• Ids independent of
Vds
• We say current
saturates
• Similar to current
source
Vgs > Vt
+
-
g
+
-
Vgd < Vt
d Ids
s
n+
n+
p-type body
b
Vds > Vgs-Vt
Nmos region of operation
1.
Cutoff region:
VGS < VT, any value of VDS
ID = 0
2.
Linear (or Resistive, or Triode) region:
VGS > VT, VDS < (VGS – VT)
V 

I D   VGS  VT  DS VDS
2 

W
where    nCox
L
3.
Saturation region:
VGS > VT, VDS > (VGS – VT)
I DSAT 

2
VGS  VT 2
where    n Cox
W
L
“CUTOFF” region: VG < VT
pMOS transistor
• Similar, but doping and voltages reversed
–
–
–
–
Body tied to high voltage (VDD)
Gate low: transistor ON
Gate high: transistor OFF
Bubble indicates inverted behavior
Source
Gate
Drain
Polysilicon
SiO2
p+
p+
n
bulk Si
pMOS operation
• Behave as nMOS but with the signs reverse and I-V
characteristic in the third quadrant
• Mobility of holes in silicon is typically lower than
electrons. This means pMOS transistor provide less
current than nMOS transistor of comparable size and
hence are slower
CMOS inverter
Outline
nMOS Operation
Cutoff
Vgsn < Vtn
Vin < Vtn
Linear
Vgsn > Vtn
Vin > Vtn
Vdsn < Vgsn –
Vtn
Vout < Vin - Vtn
Vgsn = Vin
Saturated
Vgsn > Vtn
Vin > Vtn
Vdsn > Vgsn –
Vtn
Vout > Vin - Vtn
VDD
Vdsn = Vout
Vin
Idsp
Idsn
Vout
pMOS Operation
Cutoff
Linear
Vgsp > Vtp
Vgsp < Vtp
Vin > VDD + Vtp Vin < VDD + Vtp
Vdsp > Vgsp –
Vtp
Vout > Vin - Vtp
Vgsp = Vin - VDD
Vdsp = Vout - VDD
Saturated
Vgsp < Vtp
Vin < VDD + Vtp
Vdsp < Vgsp –
Vtp
Vout < Vin - Vtp
VDD
Vtp < 0
Vin
Idsp
Idsn
Vout
Operating Regions (Summary)
Region
nMOS
pMOS
Output
A
Cutoff
Linear
Vout = VDD
B
Saturation
Linear
Vout > VDD/2
C
Saturation
Saturation
Vout drops sharply
D
Linear
Saturation
Vout < VDD/2
E
Linear
Cutoff
Vout = 0
NMOS off
PMOS res
2.5
Vout
2
NMOS s at
PMOS res
1
1.5
NMOS sat
PMOS sat
0.5
NMOS res
PMOS sat
0.5
1
1.5
2
NMOS res
PMOS off
2.5
Vin
The end