View
10
Download
0
Category
Preview:
Citation preview
Berkeley
Active Devices for Communication IntegratedCircuits
Prof. Ali M. Niknejad
U.C. BerkeleyCopyright c© 2014 by Ali M. Niknejad
Niknejad Advanced IC’s for Comm
Overview of Key Device Parameters
Niknejad Advanced IC’s for Comm
Generic Three Terminal Device
Niknejad Advanced IC’s for Comm
Large Signal Equations
Niknejad Advanced IC’s for Comm
Generic Device Behavior
slope is output resistance of deviceresistor (triode) region (very small output voltage)non-linear resistor region (small output voltage)“constant” current region (large output voltage, current nearlyindependent of output)
Niknejad Advanced IC’s for Comm
Large Signal Models
+Vi
−
I = f(Vi)
Resistors and capacitors are non-linear
Rπ and Ro depend on bias pointRg (intrinsic) depends on channel inversion levelRb can change due to current spreading effectsCgs varies from accumulation to depletion to inversionJunction capacitors vary with bias
Niknejad Advanced IC’s for Comm
Small Signal Models
Cπrπ
Cµ
+vπ
−gmvπ
rx
rz
ry
ro
In small signal regime, R and C linear about a bias point:
ForBJT:rx = rb
For aFETinput:
Cgs
Rg = Rnqs +1
dRpoly
d = 3 ↔ 12
Rnqs ∼ 1
5gm
Niknejad Advanced IC’s for Comm
Various Figures of Merit
Intrinsic
Voltage
Gain (a0)
Power Gain
Unilateral Gain
Noise
Noise figure (NF and Noise Measure M)Flicker noise corner frequency
Unity Gain Frequency
fT
Maximum Osc Freq fmax
Gain (normalized to current): gm/I
Gain Bandwidth
Niknejad Advanced IC’s for Comm
Other Important Metrics
Complementary devices
Device with same order of magnitude of fT / fmax
Lateral pnp a “dog” compared to vertical npn
Availability of Logic
Low power/high densityUseful with S/H (sample hold) and SC (switch capacitor)circuitsImportant for calibration
Breakdown voltage
Power amplifiers, dynamic range of analog circuitry
Thermal conductivity
Power amplifiers
Quality and precision of passives
Inductors, capacitors, resistors, and transmission lines
Niknejad Advanced IC’s for Comm
Current Gain
ii
io = gmvgs =gmvgs
jω(Cgs + Cgd)
Ai =ioii
Since id = gmvgs , and vgs jω(Cgs + Cgd ) = is , taking the ratiowe have
Ai =idis
=gm
jω(Cgs + Cgd )
Solving for |Ai = 1|, we arrive at the unity gain frequency
ωT = 2πfT =gm
Cgs + Cgd
Niknejad Advanced IC’s for Comm
Why fT?
vi
It is not at first obvious why fT plays such an important rolein analog (and digital) circuits. To see this, consider thesimple cascade amplifier, where a single stage amplifier drivesan identical copy. The gain of this first stage is given by theintrinsic gain of the amplifier, and the 3-dB bandwidth islimited by the pole at the high impedance node
ω0 =1
ro,1((1 + |A2|)Cgs,2 + Cd1,tot)
Niknejad Advanced IC’s for Comm
Cascade Example
Here Cd1,tot is the total drain capacitance(Cgd + Cds + Cwire + · · · ) and the effect of Millermultiplication is captured by boosting the second stage inputcapacitance by the gain A2. If the second stage is a cascodewith a small voltage gain between the gate/drain, then A2 canbe made smaller than unity to minimize its impact. So if weneglect the Miller effect, we have
ω0 =1
ro(Cgs + Cd ,tot)
Niknejad Advanced IC’s for Comm
Gain/Bandwidth Product
In most applications, we intend to embed this amplifier in afeedback loop, so the gain-bandwidth product is of interest.
For a feedback system, we know that we can approximatelytrade gain for bandwidth. So if we place the amplifiers in afeedback loop, we have
Gain = A0
BW = ω0
Gain × BW = A0ω0 < gmro1
ro(Cgs + Cd ,tot)
=gm
Cgs + Cd ,tot≈ ωT
Niknejad Advanced IC’s for Comm
fT in Digital Circuits
Even in a digital circuit, we find that fT plays a key role.Consider the time constant of a simple gate, such as aninverter. If the fan out of an inverter is unity, in other wordsthe inverter drives an identical copy of itself, then the load ofthe inverter is approximately Cgs + Cd ,tot .
Niknejad Advanced IC’s for Comm
fT in Digital Circuits (cont)
The discharge current takes a complicated form, but to firstorder the transistor acts like a switch with on-conductancegds = gm (in triode region), so the discharge time is given bythe product
τT =Cgs
gm=
1
ωT
For larger fan-out (FO) circuits (and/or gates), we just scalefT by the FO.
Niknejad Advanced IC’s for Comm
Bipolar Junction Transistors
Niknejad Advanced IC’s for Comm
BJT Cross Section
n+ buried layer
p+
n+ poly contact n+ emitter base collector
field oxide
p-type substrate
n
Most transistor “action” occurs in the small npn sandwichunder the emitter. The base width should be made as small aspossible in order to minimize recombination. The emitterdoping should be much larger than the base doping tomaximize electron injection into the base.
A SiGe HBT transistor behaves very similarly to a normalBJT, but has lower base resistance rb since the doping in thebase can be increased without compromising performance ofthe structure.
Niknejad Advanced IC’s for Comm
Bipolar Small-Signal Model
roCπ
Cµ
gmvin
+vin
−Rπ Ccs
B
E
Crb
rc
re
The resistor Rπ dominates the input impedance at lowfrequency. At high frequency, though, Cπ dominates.
Cπ is due to the collector-base reverse biased diodecapacitance.
Ccs is the collector to substrate parasitic capacitance. In someprocesses, this is reduced with an oxide layer.
Cπ has two components, due to the junction capacitance(forward-biased) and a diffusion capacitance
Niknejad Advanced IC’s for Comm
Bipolar Exponential
Due to Boltzmann statistics, the collector current is describedvery accurately with an exponential relationship
IC ≈ ISeqVBE
kT (1)
The device transconductance is therefore proportional tocurrent
gm =dICdVBE
= ISq
kTe
qVBEkT =
qICkT
(2)
where kT/q = 26mV at room temperature. Compare this tothe equation for the FET. Since we usually havekT/q < Vgs − VT , the bipolar has a much largertransconductance for the same current. This is the biggestadvantage of a bipolar over a FET.
Niknejad Advanced IC’s for Comm
Control Terminal Sensitivity
BJT:
10 = exp
(q∆VBE
kT
)∆VBE =
kT
q× ln 10 ≈ 60mV
MOSFET:
10 =
(VGS ,1 − VT
VGS ,2 − VT
)2
√10 =
(VGS ,1 − VT
VGS ,1 + ∆VGS − VT
)∆VGS =
1−√
10√10
(VGS ,1 + VT ) ≈ 1V
Niknejad Advanced IC’s for Comm
Bipolar Unity Gain Frequency
The unity gain frequency of the BJT device is given by
ωT =gm
Cπ + Cµ=
gm
Cdiff + 2Cje0 + Cµ(3)
where we assumed the forward bias junction has Cje ≈ 2Cje0
Since the base-collector junction capacitance Cµ is a functionof reverse bias, we should bias the collector voltage as high aspossible for best performance.
The diffusion capacitance is a function of collector current,Cdiff = gmτF
ωT =gm
gmτF + 2Cje0 + Cµ=
1
τF +2Cje0+Cµ
gm
(4)
Niknejad Advanced IC’s for Comm
Bipolar Optimum Bias Point
fT (GHz)
IC
10−310−4 10−2
0
40
160
220
80
No Kirk Effect
ωT →1
τF
We can clearly see that if we continue to increase IC , thengm ∝ IC increases and the limiting value of fT is given by theforward transit time ωT ≈ 1/τF
In practice, though, we find that there is an optimum collectorcurrent. Beyond this current the transit time increases. Thisoptimum point occurs due to the Kirk Effect. It’s related tothe “base widening” due to high level injection. (Not StarTrek!)
Niknejad Advanced IC’s for Comm
Base Transmit Time
Ci = Cj + Cdiff ≈ Cdiff = τF · gm
Base transmit time
Current gain unity freq.
fT =gm
2πCi≈ 1
2πτF
fT ≈2µn
2πW 2B
kT
q
fT ∝1
W 2B
Niknejad Advanced IC’s for Comm
CMOS FET Transistors
Niknejad Advanced IC’s for Comm
CMOS Cross Section
p-type substrate
n+ n+ n+ p+ p+ p+
S D G S D G
n-well
PMOS NMOS
LLov
VDD
Modern CMOS process has very short channel lengths(L < 100nm). To ensure gate control of channel, as opposedto drain control (DIBL), we employ thin junctions and thinoxide (Tox < 5nm).
Due to lithographic limitations, there is an overlap betweenthe gate and the source/drain junctions. This leads to overlapcapacitance. In a modern FET this is a substantial fraction ofthe gate capacitance (up to half).
Niknejad Advanced IC’s for Comm
CMOS DNW Device
NW NWPWPW PW
P-Substrate
In a standard CMOS process, only the PMOS is isolated bythe n-well. All NMOS devices share a common body (psub).This means that the body-source cannot be tied together(unless grounded) and moreover, digital substrate noisecouples to sensitive analog and RF circuitsIn many processes, a deep n-well (“DNW”) can be used toisolate the p-well. This is like a triple-well process, allowingisolated NMOS transistors. The DNW is formed from a ringof NW and an overlapping DNW region.
NW NWPWPW PW
P-Substrate
DNW
Niknejad Advanced IC’s for Comm
FET Small Signal Model
Cgs gmvgs gmbvbs ro
+vgs
−
CgdRg
Cdb
RS CsbCgb
G
S
D
B
The junctions of a FET form reverse-biased pn junctions withthe substrate (well), or the body node. This is another formof parasitic capacitance in the structure, Cdb and Csb.
At DC, input is an open circuit. The input impedance has asmall real part due to the gate resistance Rg (polysilicon gateand NQS) and Rs,d account for junction and contactresistance.
Niknejad Advanced IC’s for Comm
FET Simplified Models
In the forward active (saturation) region, the inputcapacitance is given by Cgs
Cgs =2
3W · L · Cox + Cpar
Don’t forget that layout parasitics increase the capacitance inthe model, sometimes substantially (esp in deep submicrontechnologies). Ro is due to channel length modulation andother short channel effects (such as DIBL).
Cgs gmvgs gmbvbs ro
+vgs
−
Cgd
Cdb
CsbCgb
G
S
D
B
Cgs gmvgs ro
+vgs
−
Cgd
Cdb
G
S
D
For low frequencies, the resistors are ignored. But theseresistors play an important role at high frequencies.If the source is tied to the bulk, then the model simplifies.
Niknejad Advanced IC’s for Comm
FET Extrinsic Model
G
DS
B
Rge
Cgse Cgde
Rse Rde
Cbse Cbde
Rs,bs Rs,bd
Rbe
Substrate parasitics, gate resistance, source/drain resistances,extra metal and contact resistance and capacitance. At veryhigh frequencies, the distributed inductance of the leads.
Niknejad Advanced IC’s for Comm
Intrinsic Voltage Gain
Important metric for analog circuits
Av ,mos = gmro =2IDS
Vdsat
VA
IDS= 2
VA
Vdsat
Av ,bjt = gmro =qICkT
VA
IC= 2
VAkTq
Communication circuits often work with low impedances inorder to achieve high bandwidth, linearity, and matching.
To achieve high fT , the Vdsat is relatively large so the currentis increased to obtain sufficient gain.
Niknejad Advanced IC’s for Comm
Voltage Gain for Tuned Amplifiers
Inductive loads are also common to tune out the loadcapacitance and form a resonant circuit. The gain is thusgiven by
Av = gmRp = gmω0QL
In a given process, there’s a maximum Q that we can obtain,usually Q ∼ 10 at 1 GHz in a typical 90nm process with thickmetal options. The inductance cannot be increased withoutbound due to area limitations and ultimately due to thetuning requirements
ω0L =1
ω0(Cgs + Cdb + Cpar )
ω0L ≤1
ω0Cgs
Av = gmω0QL = gmQ1
ω0Cgs=
gm
Cgs· Q · 1
ω0= Q · fT
f0
Niknejad Advanced IC’s for Comm
DC Gain for Short Channel Devices
1e-07 2e-07 3e-07 4e-07 5e-07 6e-07 7e-07 8e-07 9e-07 1e-06
A 0
L (m)
Vgs=1.2V
0 0.2 0.4 0.6 0.8 1 1.2
A 0
Vds
L=100nmL=200nmL=250nmL=500nm
L=1000nm
CMOS is running out of steam because the DC gain per deviceis dropping with smaller L. This makes design very difficult,especially since the DC gain drops with supply voltage.
Most fast transistor come with the penalty of lower speed ofoperation.
Niknejad Advanced IC’s for Comm
Normalized Gain
For a bipolar device, the exponential current relationshipresults in a high constant normalized gain
gm
IC=
q
kT≈ 1
26mV
For a square law MOSFET, in saturation we have
gm
IDS=
2
VGS − VT
In weak inversion, the MOSFET is also exponential
gm
IDS=
q
nkT≈ 1
26mV
1
n
The factor n is set by the ratio of oxide to depletioncapacitance
Niknejad Advanced IC’s for Comm
MOSFET in Subthreshold
In sub-threshold, the surface potential varies linearity with VG
The surface charge, and hence current, is thus exponentiallyrelated to VG
Niknejad Advanced IC’s for Comm
I-V Curves Vgs
0 0.2 0.4 0.6 0.8 1 1.2
I ds
Vgs
Vds=50mVVds=250mVVds=650mVVds=1.25V
The I-V curve of a given transistor with fixed dimension (Wand L) reveals most of the salient features. For example, aplot of Ids versus Vgs for a family of Vds quickly revealscurrent drive capability. If a device is biased in weak ormoderate inversion, then the logarithmic plot is more useful asit expands this regime of operation.
Niknejad Advanced IC’s for Comm
I-V Curves Vgs (log)
0 0.2 0.4 0.6 0.8 1 1.2
log(
I ds)
Vgs
Vds=50mVVds=250mVVds=450mVVds=750mVVds=850mVVds=1.25V
The leakage currents (sub-threshold slope) is important inanalog applications that use the transistor as a switch. If thedevice cannot be turned off, then it’s very difficult to buildhigh precision discrete time circuits.
Niknejad Advanced IC’s for Comm
I-V Curves Vds
0 0.2 0.4 0.6 0.8 1 1.2
I ds
Vds
Plotting the current versus drain-source voltage, or a plot ofIds versus Vds for a family of Vgs shows the current saturationbehavior of a device. The output conductance is more easilyobserved using small-signal parameters as discussed shortly,but certain trends can be observed even from the raw I-Vcurves.
Niknejad Advanced IC’s for Comm
MOS Transconductor Efficiency
Since the power dissipation is determined by and large by theDC current, we’d like to get the most “bang for the buck”.
From this perspective, the weak and moderate inversionregion is the optimal place to operate.
The price we pay is the speed of the device which decreaseswith decreasing VGS
Current drive is also very small.
Niknejad Advanced IC’s for Comm
Transconductance (Vgs)
0 0.2 0.4 0.6 0.8 1 1.2
g m
Vgs
L=100nmL=200nmL=250nmL=500nm
L=1000nm
The value of gm increases with Vgs . For the diffusioncomponent of current, at low overdrive, the increase in gm isexponential due to the exponential dependence of current ongate voltage.For the drift component of current, Ids is proportional to theamount of charge in the channel and the carrier velocity. Thechannel charge scales in proportion to the gate bias, so for afixed mobility, we expect the gm to increase linearly.
Niknejad Advanced IC’s for Comm
gm vs. Vgs (cont)
0
200
400
600
Effective FieldM
obilit
y
Triode CLM DIBL SCBE
Rou
tkΩ
Vds (V)
2
4
6
8
10
12
14
0 1 2 3 4
In fact, for low fields, due to Coulomb scattering, the mobilitywill experience an enhancement due to screening provided bythe inversion layer, and the increase in gm is faster than linear.
On the other hand, due to high field effects, we know themobility will eventually degrade as carriers are pushed closerto the silicon-insulator interface, where surface scatteringcauses the mobility to drop.
Niknejad Advanced IC’s for Comm
gm vs. Vds
0 0.2 0.4 0.6 0.8 1 1.2
g m
Vds
L=100nmL=200nmL=250nmL=500nm
L=1000nm
The trend for gm as a function of Vds can be divided into tworegions. In the triode region of operation, increasing Vds
increases the current, so the overall gm increases.
As the device nears saturation, one would expect the gm tosaturate.
Niknejad Advanced IC’s for Comm
Output Conductance
0 0.2 0.4 0.6 0.8 1 1.2
g ds
Vds
L=100nmL=200nmL=250nmL=500nm
L=1000nm
gds in triode region is very large since the drain voltage has adirect impact on the channel charge. In fact, to first order,gds = gm since varying the gate has the same impact asvarying the gate due to the presence of the inversion layer,making the drain terminal just as effective as the gate-source.
In a long channel device, in the so-called pinch-off region, thedevice terminal no longer controls the channel charge directly,and the output conductance drops dramatically.
Niknejad Advanced IC’s for Comm
Output Resistance
0 0.2 0.4 0.6 0.8 1 1.2
r o
Vds
L=100nmL=200nmL=250nmL=500nm
L=1000nm
The drain still modulates the depletion region width, whichindirectly impacts the device through channel lengthmodulation (CLM).
In short channel devices, both because of the increase in therelative magnitude of the change in channel length δL relativeto L, but also due to Drain Induced Barrier Lowering (DIBL).
Niknejad Advanced IC’s for Comm
Output Resistance Broken Down
Triode CLM DIBL SCBE
Rou
tkΩ
Vds (V)
2
4
6
8
10
12
14
0 1 2 3 4
Niknejad Advanced IC’s for Comm
FET Unity Gain Frequency
Long channel FET:
Note that there is apeak fT sinceeventually themobility of thetransistor drops dueto high vertical fields
Short channel limit:
fT =1
2π
gm
Cgs + Cgb + Cgd
assuming Cgs Cgb + Cgd
fT =1
2π
gm
Cgs=
1
2π
3
2
µn
L2(VGS − VT )
IDS ≈ vsatQinvW = WCox (VGS − VT )vsat → gm = WCoxvsat
fT =1
2π
gm
Cgs=
3
2
WCoxvsat
WLCox∝ 1
L
Niknejad Advanced IC’s for Comm
Scaling Speed Improvements
0
1
10
100
1980 1985 1990 1995 2000 2005
long-c
hannel regio
n
short-channel re
gion
year
fT
CMOS transistors have steadily improved in performance justas predicted by theory. In the short channel regime theimprovements are linear with scaling.
At the same time, the decreasing supply voltage has led to areduced dynamic range. Also the maximum gain has notimproved as much· · ·
Niknejad Advanced IC’s for Comm
fT versus Dimension
1e-07 2e-07 3e-07 4e-07 5e-07 6e-07 7e-07 8e-07 9e-07 1e-06
f T
L (m)
Vgs=1.2V
The well known improvement in fT with channel scaling isshown here. Since both gm and Cgs depend linearity on thewidth, only the channel length L matters.
Shorter channel lengths improve both the gm and lower thecapacitance of the device. In the velocity saturated limit, onlythe drop in capacitance plays a role.
Niknejad Advanced IC’s for Comm
Cgs versus Bias
Classical
Quantum
Poly Depletion
Cgg
Cox
VGBVFB VTH
The MOS capacitor of modern devices does not followclassical equations due to polysilicon depletion and quantumeffects ...
Niknejad Advanced IC’s for Comm
fT versus Bias
0 0.2 0.4 0.6 0.8 1 1.2
f T
Vgs
L=100nmL=200nmL=250nmL=500nm
L=1000nm
To go fast, you must burn power ...
It is important to note that fT is usually measured usingscattering parameters and so using the “DC” values of gm andC will fail to capture fT at high frequency. We will return tothis point in the RF section of this chapter.
Niknejad Advanced IC’s for Comm
High Frequency
Niknejad Advanced IC’s for Comm
Maximum Two-Port Power Gain
Measure or calculate two-port parameters at a particularfrequency. To obtain maximum gain Gmax , design an inputand output matching network to satisfy the followingconditions
YS = Y ∗in
YL = Y ∗out
where YS and YL are the source and load admittance seen bythe two-port and Yin and Yout are the input and outputadmittance seen looking into the two-port when loaded bythese matched admittances.This leads to two equations and two unknowns (4 real).
Niknejad Advanced IC’s for Comm
Maximum Power Gain fmax
fmax = max. freq. of activity = max. freq. of oscillation =freq. when power gain = 1
Gp ≈
(fTf
)24rx(go + gm
CµCπ
)+ 4rxgo
Gmax ≈fT
8πrxCµf 2max
= 1
fmax =
√fT
8πrxCµ
Cπ
Cµ
+vπ
−
gmvπ
rx
go
Niknejad Advanced IC’s for Comm
FET fmax
Ids
gds
Rs
Rg
Cgs
Rd
L
Source Gate Drain
Cgso Cgdo
WF
Csb
Rsb
Cdb
RdbRbb
Bulk
p-sub
fT ≈gm
2πCgg
fmax ≈fT
2√
Rg (gmCgd/Cgg ) + (Rg + rch + Rs )gds
Minimize all resistances
Rg –use many small parallel gate fingers, < 1µm eachRsb, Rdb and Rbb – substrate contacts < 1− 2µm from deviceRs , Rd – don’t use source/drain extensions to reduce L
Niknejad Advanced IC’s for Comm
SiGe HBT and CMOS FinFETs
Niknejad Advanced IC’s for Comm
SiGe Technology
Higher Performance: Demonstrations of fT > 500GHz andapproach 1THz.
Problem:
As WB decreases → rb increases
Solution:
SiGe base allows for higher fTwithout reducing WB
Niknejad Advanced IC’s for Comm
SiGe HBT Action
A SiGe BJT is often called an HBT (heterojunction bipolartransistor)
Ge epitaxially grown in base
Causes strain in crystalCauses extra potential barrier for holes (majority carrier) in thebase from flowing into emitter
Beneficial effects
WB decreases, NB increases, rb lowNE decreases, Cj decreases
Niknejad Advanced IC’s for Comm
GaAs/InP Technology
Electric Field (V/cm)
Car
rier
Dri
ft V
elo
city
(cm
/s)
10 101010 102 654310 5
108
10 7
10 6
Si
Ge
InPGaAs
Si
One of the primary advantages of the III-V based transistors isthe higher peak mobility compared to Si. With short channeldevices, most of the current flow is due to velocity saturation,which greatly limits the advantages of III-V over silicon.
The insulating substrate also allows higher Q passives.
The extra cost of these technologies limits it to nicheapplications such as very high frequencies, high performance,and power amplifiers.
Niknejad Advanced IC’s for Comm
FinFETs and Multigate Transistors
To combat the problems with scaling of MOSFETs below45nm, Berkeley researchers introduced the “FinFET”, adouble gate device. (Intel uses this technology at the 22nmnode)
Due to thin body and double gates, there is better “gatecontrol” as opposed to drain control, leading to enhancedoutput resistance and lower leakage in subthreshold.
Niknejad Advanced IC’s for Comm
FinFET Structure and Layout
Gate straddles thin silicon fin, forming two conductingchannels on sidewall
Multi-finger layout very similar to RF layout.
Niknejad Advanced IC’s for Comm
An Aside on Thermal Conductivity
GaAs
Semi-insulating substrateNot very good conductor of heatHigh quality passive elements (next topic)
Si
Semi-conducting substrateGood conductor of heatLossy substrate leads to lower quality passives
Niknejad Advanced IC’s for Comm
An Aside on Thermal conductivity (2)
Also depends on packaging
Example: in flip-chip bonding, thermal conductivity function ofnumber of bumps rather than substrateBack-side of die can lose heat through radiation or convectionthrough air but thermal contact is much more effective
Flip chip bonding
Wire bonding
Niknejad Advanced IC’s for Comm
FET/BJT Comparisons
Niknejad Advanced IC’s for Comm
High BJT Transconductance
gm =dICdVBE
= ISq
kTe
qVBEkT =
qICkT
For fixed current, BJT gives more gain
Precision
Important in multiplication, log, and exponential functionsMore difficult in FETs due to process/temp. dependenceIS process dependent in BJT ... use circuit tricks
Niknejad Advanced IC’s for Comm
Advantages of BJT
gm
IC=
q
kT=
1
25mV
gm
ID=
2
VGS − VT≈ 1
250mV
IC ∝ e−E/kt = eqVBE/kT
For high-speedapplications, need to biasin stronginversion...Results in ∼10xlower efficiency
For a BJT, this relationship is fundamental and related to theBoltzman statistics (approximation of Fermi-Dirac statistics)
For a MOSFET, this relationship is actually only valid for asquare-law device and varies with VT (body bias) andtemperature
Niknejad Advanced IC’s for Comm
Advantage of BJT over FET (2)
Better precision
About 4 decades(420mV) of linearity
Example:
VBE1 + VBE2 = VBE3
VBE = VT ln
(ICIS
)IC1 · IC2
IS1 · IS2=
IC3
IS3
log(IC)
VBE(V)420mV
Can build exp, log, roots, vector mag
Lower 1/f noise corner
Lower offset voltageVOS ∼ 1mV
Niknejad Advanced IC’s for Comm
Disadvantage of BJT
rb hurts gain (power), NF
SiGe allows fast transistors with low rb
Exponential transfer function (advantage and disadvantage)
Exponential → non-linear
Expensive in high volumes, cheaper in low volumes
Absence of a switch
Old CMOS gets cheaper!45nm ∼ $1M mask → 0.25µm $50k mask
Niknejad Advanced IC’s for Comm
Advantage of FET over BJT
Cheaper and more widely available (many fabs in US, Asia,and Europe)
Square law → less distortion
PMOS P-FET widely available, only ∼ 2× less performance.
Triode region → variable resistor
Widely available digital logic
Low leakage in gates
Sample and hold (S/H) and switch cap filters (SCF)
Dense digital circuitry / DSP for calibration
Offset voltages and mismatches can be compensated digitally
Dense metal layers allows MIM (“MOM”) capacitors for free
Niknejad Advanced IC’s for Comm
References and Further Reading
UCB EECS 142/242 Class Notes (Niknejad/Meyer)
UCB EECS 240 Class Notes (Niknejad/Boser)
Analysis and Design of Analog Integrated Circuits, Paul R.Gray, Robert G. Meyer. 3rd ed. New York : Wiley, c1993.
Manku, T. “Microwave CMOS-device physics and design,”IEEE Journal of Solid-State Circuits, vol.34, (no.3), March1999. p.277-85. 32
Niknejad Advanced IC’s for Comm
Recommended