Transcript ppt
IC Technology
and
Device Models
1.
2.
3.
4.
5.
The planar process for integrated circuit fabrication
Review of DC and AC Diode Models
Review of dc and ac JFET models
Review of dc and ac bipolar transistor models
Review of dc and ac MOS transistor models
1.1 The planar process for integrated circuit fabrication
The configuration is called the monolithic integrated circuit because it is formed
on a single silicon chip.
Wafer preparation Very high purity Silicon wafer cylinder 10, 12.5, 15 or 20
cm in diameter and 1 m in length. This crystal is then sliced (like a loaf of
bread) to produce wafers 200 micron thick and the surface is then polished to a
mirror finish. Impurities can then be added on purpose to the pure silicon in a
process known as doping, to allow controlled alteration of the electrical
properties of the silicon.
Oxidation It refers to the chemical process of silicon reacting with oxygen to
form SiO2. The oxide layer grown has excellent electrical insulation properties.
With dielectric constant of about 3.5, it can be used to form excellent capacitors.
It can also be used as a masking layer, allowing the introduction of the dopants
into the silicon only in regions that are not covered with oxide.
Diffusion The process of introducing impurity atoms (dopants) into silicon to
change its doping is known as diffusion. The two most common impurities used
as dopants are Boron (p-type) and Phosphorus (n-type). Both dopants are
effectively masked by thin SiO2 layer – the process is called Photolithography.
Photolithography refers to the process to produce mask for SiO2 on all layers
resulting in ‘windows’ in the layer for subsequent diffusion process.
Ion implantation It is a method superior than diffusion to introduce impurities
into Silicon with the aid of electric field. It is much more accurate process and
can be performed at room temperature.
Chemical vapor deposition It is a process by which gases or vapours are
chemically reacted leading to the formation of a solid on a substrate. It can
deposit the SiO2 layer at a faster rate and lower temperature.
Metallization The process of metallization is to interconnect the various
components of the IC to form the desired circuit. Metallization involves the
deposition of the metal (Al) over the entire surface of the Silicon. The required
interconnection is then etched selectively.
Packaging A finished silicon wafer may contain thousands of chips, which are
tested while in wafer form. The circuits are then separated from each other and
good circuits are then mounted in hermetically sealed plastic packages with
necessary connection legs.
Advantages of monolithic IC as compared with discrete components
Low cost (due to large quantities processed, no manual assembly)
Small size, low power consumption
High reliability (All components are fabricated simultaneously and there are no soldered
joints)
Improved performance (Because of low cost, more complex circuitry may be used to
obtain better functional characteristics)
1.2 Review of DC and AC Diode Models
If a negative voltage is applied to the diode anode, no current flows
and the diode behaves as an open circuit. Diodes operated in this mode
are said to be reverse-biased, or operated in the reverse direction.
On the other hand, if a positive current is applied to the anode, the
ideal diode behaves as a short circuit in the forward direction.
Simplified diode models
iD = 0, vD VD0
iD = (vD – VD0) / rD,
vD VD0
where VD0 is the intercept of line B on the voltage axis and rD is
the inverse of the slope of line B.
The small signal model
There are
applications in
which a diode is
biased to operate
at a point on the
forward i-v
characteristic and
a small ac signal is
superimposed on
the dc quantities.
For this situation
the diode is best
modeled by a
resistance equal to
the inverse of the
slope of the
tangent to the i-v
characteristic at
the bias point.
A dc voltage VD, represented by a battery, is applied to the diode; and a time varying
signal vd(t), assumed (arbitrarily) to have a triangular waveform, is superimposed on
the dc voltage VD. In the absence of the signal vd(t) the diode voltage is equal to VD,
and correspondingly the diode will conduct a dc current ID given by
ID = IseVD/VT
----------------------(1.1)
When the signal vd(t) is applied, the total instantaneous diode voltage vD(t) will be
given by
VD(t) = VD + vd(t) -------------(1.2)
Correspondingly, the total instantaneous diode current iD(t) will be
iD(t) = IsevD/VT
--------------(1.3)
Substituting for vD from eq (1.2) gives
iD(t) = Is e(VD + vd) / VT
which can be rewritten as
iD(t) = Is e(VD/VT) evd/VT
using equ. (1.1) we obtain
iD(t) = ID evd/VT
-----------(1.4)
Now if the amplitude of the signal vd(t) is kept sufficiently small such that
Vd / VT << 1
Then we may expand the exponential of eq 1.4 in a series and truncate the series
after the first two terms to obtain the approximate expression
iD(t) = ID(1 + vd / VT)
This is the small-signal approximation. It is valid for signals whose amplitudes are
smaller than about 10 mV.
1.3 Review of dc and ac JFET models
The figure shows the basic structure of the nchannel JFET. It consists of a slab of n-type
silicon with p-type regions diffused on its two
sides. The n region is the channel, and the ptype regions are electrically connected
together and form the gate. Current flow
between two of the device terminals is
established by a voltage applied to the third
terminal (gate terminal). The operation of the
device depends upon the reverse biasing
applied between the gate and the channel. It
is a unipolar transistor, in which current is
conducted by charge carriers flowing through
one type of semiconductor only. It is
characterized by very high input impedance
and therefore is implemented in the input
stage of an integrated-circuit op-amp. Since
gate to channel junction is almost always
reverse biased, very small leakage current (of
the order of 10-9 A) will flow in the gate
terminal, giving very high input impedance.
Operation of n-channel JFET with vDS small
iD
VGS= 0
-V1
-V2
Consider the n-channel JFET with small vDS applied. With vGS = 0,
the application of a voltage vDS causes current to flow from the drain
to the source. When a negative vGS is applied, the depletion region
of the gate-channel junction widens and the channel becomes
correspondingly narrower; thus the channel resistance increases and
the current ID (for a given vDS) decreases. Because vDS is small, the
channel is almost of uniform width. The JFET is simply operating as
a linear resistance rDS whose value is controlled by vGS. That means
the JFET is operating as a voltage controlled resistance. If we keep
increasing vGS in the negative direction, a value is reached at which
the depletion region occupies the entire channel. At this value of vGS
the channel is completely depleted of charge carriers (electrons); the
channel has in effect disappeared. This value of vGS is called the
pinch off voltage(Vp) or the threshold voltage of the device, Vt.
-V3
vDS
Operation with vDS increased
Now consider the case where vGS is
constant at a value greater than Vt, and
vDS is increased. Since vDS appears as a
voltage drop across the length of the
channel, the voltage increases as we
move along the channel from source to
drain. It follows that the reverse-bias
voltage between gate and channel
varies at different points along the
channel and is highest at the drain end.
Thus a channel acquires a tapered
shape and the ID-vDS characteristic
becomes nonlinear.
When the reverse bias at the drain end,
vGD, falls below the threshold voltage Vt,
the channel is pinched off at the drain
end and the drain current saturates.
IDSS = ID with vGS = 0 and vDS = -Vp
Static characteristics
Triode region - JFET operates as a linear resistance
Pinch off region - JFET operates beyond pinch-off voltage
These two regions are separated by a parabolic boundary given by vDS = vGS - Vp.
The JFET operates in the triode region for
vDS vGS -Vp
and the corresponding drain current is given by
iD = K[2(vGS – Vp)vDS – v2DS]
or, iD = IDSS[2(1-vGS/Vp)(vDS/-Vp) – (vDS/Vp)2]
for small vDS, iD 2IDSS/-Vp (1-vGS/Vp) vDS
giving rDS = vDS/ID = [2(IDSS /-Vp) (1 – vGS/Vp)]-1
At the boundary between triode and pinch off region, vDG = -Vp giving
VDS = vGS -Vp,, iD = IDSS (vDS / Vp)2 (parabola)
The JFET operates in saturation (pinch-off) for VDS vGS - Vp
The drain current is given by
iD = IDSS (1 – vGS / Vp)2 (1 + vDS)
where 1/VA is a positive constant included to account for the dependence of iD
on vDS in pinch off.
Small Signal Analysis
It is important to note that the small-signal model parameters gm and ro
depend on the dc bias point of the JFET.
gm = 2IDSS / |Vp| (ID / IDSS) gm (max) = gmo = 2IDSS / |Vp|
[IDmax = IDSS at VGS = 0]
The drain current of a practical JFET depends on vDS in a linear manner.
Such dependence is modeled by a finite resistance ro between drain and
source, whose value is given approximately by
ro |VA| / ID
where ro is in the range 10 to 1000 k.
Since gate to channel junction is reverse biased, depletion capacitances
exist between gate and source, and between gate and drain. Typically
Cgs = 1 to 3 pF and Cgd = 0.5 to 1 pF.
1.4 Review of dc and ac bipolar transistor models
The figure shows the signal
components of the BJT amplifier circuit.
It shows the expressions for the current
increments ic, ib and ie obtained when a
small signal vbe is applied. These
relationships can be represented by a
circuit. Such a circuit should have
three terminals , C, B, and E, and
should yield the same terminal currents
indicated in fig 1.12. The resulting
circuit is then equivalent to the
transistor as far as small-signal
operation is concerned, and thus it can
be considered an equivalent smallsignal circuit model.
The hybrid- Model
voltage-controlled current source
ic =gmvbe, ib = vbe / r, ie = vbe / re
current of the controlled source
ic = ib
1.5 Review of dc and ac MOS transistor models
Metal-oxide semiconductor field effect transistor (MOSFET) can be
enhancement or depletion type. Depending upon the semiconductor used for
forming channel, it is named “p channel” or “n-channel” MOSFET.
Enhancement type MOSFET
(OFF until turned ON)
Four terminals gate (G), Drain
(D), Source (S) and body
(substrate) (B) are brought out of
the device.
The substrate forms pn junctions
with the source and drain
regions. In normal operation
these pn junctions are kept
reverse-biased at all times.
Operation with no gate voltage
With no bias voltage applied to the gate, two back-to-back diodes exist in series
between drain and source. These back-to-back diodes prevent current
conduction from drain to source when a voltage vDS is applied.
Creating a channel for
current flow
When a sufficient number of electrons
accumulate near the surface of the
substrate under the gate, an n region
is created, connecting the source and
drain regions. Current flows through
the channel if voltage is applied
between source and drain. The
induced n region thus forms a
channel for current flow from drain to
source.
Operation with small vDS
After the creation of a channel,
a small positive voltage is
applied between drain and
source which causes a current iD
to flow through the induced n
channel. Current iD depends
upon the electron density of the
channel and the magnitude of
vGS. Specifically, for vGS = Vt the
channel is just induced and the
current conducted is still
negligibly small. As vGS exceeds
Vt, more electrons are attracted
into the channel thus the
channel depth is increased. The
result is a channel of increased
conductance or equivalently
reduced resistance.
Operation as vDS is increased
Let vGS be held constant at
a value greater than Vt.
Now the appplied voltage
vDS appears as a voltage
drop across the length of
the channel. Since the
channel depth depends on
this voltage, we find that
the channel is no longer of
uniform depth; rather, the
channel will take the
tapered form, being deepest
at the source end and
shallowest at the drain end.
When vDS is increased to the value that reduces the voltage between gate and channel at
the drain end to Vt – that is, vGS – vDS = Vt or vDS = vGS – Vt – the channel depth at the
drain end decreases to almost zero, and the channel is said to be pinched off. Increasing
vDS beyond this value has little effect on the channel shape, and the current through the
channel remains constant at the value reached for vDS = vGS – Vt.
The drain current thus saturates at this value, and the MOSFET is said to have entered
the saturation region of operation.
Static characteristics
Triode region (ohmic region)
For the MOSFET to be operated in triode region, following conditions should be
fulfilled.
i) vGS >Vt ii) vGD > Vt iii) vDS < VGS - Vt
Then, iD = k [ 2(vGS –Vt) vDS –vDS2]
where k is a constant given by k = 1/2 n Cox (W/L)
Pinch-off region
For the MOSFET to operate in the pinch off region, the following conditions should
be fulfilled.
i) vGD Vt, ii) vGS Vt, iii) vDS vGS - Vt
Then, iD = k(vGS –Vt)2 which is a voltage controlled current source.
Taking the channel length modulation effect into consideration, the drain current
is given by
iD = k (vGS – Vt)2 (1 + vDS / VA)
At the boundary between triode region and pinch off region
iD = k vDS2
Depletion-type MOSFET (ON until turned OFF)
The depletion MOSFET has a permanently implanted channel. Thus an nchannel depletion type MOSFET has an n-type silicon region connection the n+
source and the n+ drain regions at the top of the p-type substrate. Thus if a
voltage vDS is applied between drain and source, a current iD flows for vGS = 0.
In other words, there is no need to induce a channel, unlike the case of the
enhancement MOSFET.
Positive vGS enhances the channel by attracting more electrons into it. A
negative gate voltage causes electrons to be repelled from the channel; and
thus the channel becomes shallower and its conductivity decreases. The
negative vGS is said to deplete the channel of its charge carriers, and this mode
of operation is called depletion mode. As the magnitude of vGS is increased in
the negative direction, a value is reached at which the channel is completely
depleted of charge carriers and iD is reduced to zero even though vDS may be
still applied. This negative value of vGS is the threshold voltage of the n-channel
depletion-type MOSFET. Thus a depletion-type MOSFET can be operated in the
enhancement mode by applying a positive vGS and in the depletion mode by
applying a negative vGS.
Triode region
Because the threshold voltage Vt is
negative, the depletion NMOS will
operate in the triode region as long
as the drain voltage does not exceed
the gate voltage by more than |Vt|.
That is
vDG < -Vt
vDS < vGS – Vt
then
iD = IDSS [2(1 – vGS/Vt) (vDS/-Vt) – (vDS/Vt)2]
Pinch-off region
For the device to operate in the
saturation, the drain voltage must be
greater than the gate voltage by at
least |Vt|.
vDG -Vt
vDS vGS – Vt
iD = IDSS ( 1 – vGS/Vt)2
Small-signal analysis of enhancement MOSFET
For a given MOSFET,
gm is proportional to
the square root of the
dc bias current.
At a given bias
current, gm is
proportional to
(W/L)1/2. Hence to
obtain relatively large
transconductance the
device must be short
and wide.
Transconductance of
the bipolar junction
transistor (BJT) is
proportional to the
bias current and is
independent of the
physical size and
geometry of the
device.
gm = (2nCox)1/2 (W/L)1/2 (ID)1/2
Example
Consider the MOSFET with ID = 1 mA and nCox = 20 A/V2.
Then transconductance of the device for different values of W/L
ratio is
gm = 0.2 mA/V for W/L = 1
gm = 2 mA/V
for W/L = 100
In contrast, a BJT operating at a collector current of 1 mA has
gm = 40 mA/V. However, in spite of their low gm, MOSFET have
many other advantages, including high input impedance, small
size, low power dissipation, and ease of fabrication.