ece 7366 advanced process integration set 10b: the bipolar transistors and bicmos dr. wanda wosik...
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ECE 7366 Advanced Process Integration
Set 10b: The Bipolar Transistors and BiCMOS
Dr. Wanda Wosik
Text Book: B. El-Karek, “Silicon Devices and Process Integration”
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BJT: High-Level Injection EffectCurrent Gain Degradation
Base Conductivity Modulation: • RB(int) decreases
• emitter efficiency g decreases too
• majority carriers create E-field – drift current of minority carriers
Low injection levels High injection levels – starts sooner for low doped base – HBT has advantage over Si BJT
Dnp<<pp Dnp>>pp
The effect of base modulation is stronger if combined with emitter crowding effect.
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Kirk Effect
High current densities increases base width i.e. base Gummel number b
At high injection level:Electric field in the low doped C – peak field at CB junction moves to n+/n region. Electrons accumulate in B at the C-B depletion layer dl in B shrinks = C expandsBase widens=Kirk effect.
Voltage drop here
Use higher doping in the collector region (SIC) but watch for capacitance increase.
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Base Push Effect (Kirk Effect)
Current through the depletion layer of B-C
•High injection aggravated also by the increase of the Base Gummel number at high VBE
• Ohmic voltage drop on C-epi consumes some of the applied VCE
•Onset of High Level Injection Dn@ base edge ≈ ND
•Increase ND in C to Kirk effect
• To prevent degradation of VCEO and maintain low capacitance use SIC (selective implanted collector) – higher than in n-epi ND≥1017cm-3
ND≥1016cm-3
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Frequency Response of Current Gain
Emitter delay tE
with current
E-B junction
Increase speed (E) : CjE, REext, rEint and CEparasitic
•C (junction): scaling (area), B doping , overlapping (Cparasitic) •Optimization of tE trade-off : speed, current-carrying capability, Early voltage, and punch-through
Base transit time
Emittertransit time
Base Transit Time tB of injected charges QnB
Injection&transport to the base
Increase speed (B):•Built-in E-field (graded doping)•µ(N(x)), D(N(x))
Base ~20-50 nm – high E-fields velocity overshoot and ballistic transport
tB~Wb
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Collector-Base Capacitance Delay, tCjC
CJC includes both SIC and non-SIC collector regions
Frequency Response - Collector Delay, tC
Transit Time through the Collector-Base Depletion Layer, txdC
Velocity saturation in C-B D.L. Vsat~107cm/s
Gain-Bandwidth Product, fT
(b≈1)
Figure of merit: fT=bf
adjustment 2-6 for grading in the base
Dominated by dynamic emitter resistance kT/IC
Wb shortening at higher voltage
Kirk effect dominates
Important for short base ~100nm – transport in base fast
Maximum Frequency of Oscillation, fmax (G≈1)
Power gain=Pout/Pin drops to 1 when f=fmax
Increase max speed: rB (intrinsic base by layout: Emitter width, multiple contacts, distance Bext/Bint), CjC
Go to slide 13
fT(IC)
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The Transistor as a Switch
ton=td+tr
toff=ts+tr
Switching uses large signals: • I(V) analyses very complex because
of nonlinear operation of the transistors
• Charge control method preferred for analyses
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Delay Time , td
VEC=ICRload
VEB=0 VEC≈VBC
From off on
If VBE=0 td=0 (reverse bias)
If VBE>0 td≠0 charging E-B junction
DQxdE=qNDDxdE
Charging (from reverse to zero):DQxdE/DVBECjE
&DQxdCCjC
For high speed C&E as small as possible
Current for this charging:
ts
tf
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Rise Time , tr
Time to bring IC from 10% to 90% of its final value (point AB)
Base electrically neutral: QnB=-QpB
Charging of E-B and C-B depletion layers
If there is recombination in the base
To bring IC to required level at point B
teff – includes 1/fT, RLCjC
For ultra thin bases - emitter charge storage has to be included
tr
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Storage Time , tsTime corresponding to maintain saturation after switching off the base signal IB (AB)
Large charge storage due to low doping level
t4- corresponds to return from saturation to point B
Minimize storage: minority t in C & distance Wepi b/w collector and B.L. Do not use AuClamp C-B with Schottky diode – small storage there in SBD + low voltage on C-B junction
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Fall Time , tf
Time to decrease IC from 90% to 10% of its final value (point BA)
Excess minority carriers recombine so the charge is removed from the base
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Bipolar Junction Transistors and a Switch
SchottkyDiode used in n-p-n BJTs forfaster speed
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Silicon-Germanium TransistorHeterojunction Bipolar Transistor (HBT)
Epitaxy: Ultra High Vacuum CVD ~450°C Low pressure of
atmospheric pressure CVD ~600°CDoping: B2H6, AsH3 or PH3 in the gas
4.2% lattice mismatch
For x<20% the SiGe lattice constant:
Pseudomorphic conditions – adapt to substrate
Defect generation possible
If still metastable – film can relax to smaller Si lattice if annealed @high T
Bandgap engineering
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Energy Bands: Bandgap Lowering
Change in the bandgap mainly in the valence band
Note: base in Si BJTs had bandgap narrowing due to doping
Density of States
Strain should change bandgapni2 but also shape
(E(k)mn*&mp*). Effective masses decrease NC&NV
Barrier for holes injection to E Current gain
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Carrier Mobility
Low field Mobility in Strained SiGe
Strain in the crystal causes:
• distortion of the bands (split&shift)~ DE≈.6x (x- Ge fraction)
• change of effective mass of carriers
• distribution in the bands Piezoresistance
Energy Split ≈0.166xAll holes occupy the light-hole bands – high mobility there
reminder
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Carrier Mobility in BJT Base
Normal to the base
Mobility parallel to the base degrade under compressive strain (d.ue to alloy scattering)
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Transistor Parameters
BJT are made with SiGe base, poly-Si emitters with oxide interface
In Siin SiGe
Improvement in current gain comes from• mobility in the direction of e-flow
but • mainly from higher npSiGe because of
lower bandgap in the base
Current Gain
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Transistor ParametersCurrent Gain: Forward Active Mode
Bandgap changes along the base due to graded Ge incorporation
doping composition
Effect of mobility and density of states
VBESiGe<VBESi
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Base Transit Time
Reduce transit time tB to increase device speed
Bandgap lowering SiGe reduces tB by:
• increasing doping in the base• reducing B thickness but keeping Rs low and b high – can reach velocity
saturation• increasing transport by grading base with Ge – built-in E-field.• transport through the base will be negligible while that through collector will
dominate
For uniform doping and bandgap
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Transistor Optimization
•Digital designs: high speed, high gain, small size, high packing density.
•Analog designs: High Early voltage, bVA high product, low noise (size not so important – mainly C, R, L in the circuits).
• Mixed analog-digital designs – compatibility with CMOS so limited flexibility in optimization
•Power transistors: high voltage and current capabilities and power management
• All cases: transistors must have high reliability and yield and minimized leakage and power.
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Base Profile Optimization
High base transit time: thin base but highly doped for low RBint and small depletion layers in the base
Accelerating E-fieldDecelerating
field
E-B junction shift too high field, tunneling & surface trapping
B fast diffusion out from the Ge region (E & C) h-injection to c-Si degradation of b, speed, Early voltage – use spacers.
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Suppression of Boron Diffusion by Carbon (0.5-1%)
As and Sb diffusion enhanced by C
B and P diffusion retarded by C ( of interstitials’ concentrations)
Base Base
Base Base
Emitter Emitter
Emitter Emitter
50 nm base width possible.
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Collector Profile Optimization
Higher collector concentrations• increase current density for onset of Kirk effect•Reduce Rc and• transit time through C-B junction• reduced base-width (typically caused by B outdiffusion into C)
Net improvement of cutoff frequency fT but CjC and BVCEO
Product fTxBVCEO ~200GHz-V but for SiGe HBT ~ 500 GHz-V.
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Interaction between Process and Device Parameters Summary.
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Here, HBT less susceptible to high T CMOS
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Harame, 2004
BiCMOS on SOI.
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Krishna, 2009
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BiCMOS Process Flow
Indicate bipolar/analog specific modules
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NPN Transistor in BiCMOS
Intrinsic Epi 0.4-0.6µm
0.1-1.0 µm
As or Sb N+ buried layer NBL
sinker
deep
shallow
trenches
SIC
NPN extrinsic baseSiGe grows as a crystal on Si and polycrystalline on oxide
NPN intrinsic base SiGe100-200nm GeH4+B2H6 epiSi cap at E/B
Poly-Si E
~500 nm
SOI
Sequence is marked by #s – Incorporated into the CMOS process flow. NPN & PNP have common processes also.
Sinkers done by multiple implant of As&P for tailoring profilessilicide
SIC is high low dose P or As implant ~1012-1013 cm-2
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Common with NPN
PNP Transistor in BiCMOS
P+ BL
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Intrinsic Base Regions for NPN and PNP Transistors
CVD: • SiGe ~100-200nm GeH4+B2H6
• Si cap @625°C for npn HBT
Very steep profiles of Ge and of B or As or P.
Acts as a buffer b/w E&B
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Bipolar Transistors in BiCMOS @ PNP Base Patterning
@ Poly E Patterning
Base implantedwith P or As
Poly-resistor can be implanted at this step – include R pattern in the mask
20-30nm SiO2 50-100nm Si3N4 • Isolate Bex and poly-Si E
• Etch E window NPN• SIC implant with P 1012-1013cm-2
@ 100-200keV• Grow IFO 0.5nm in RTO• Deposit poly-Si 150-200 nm• As implant 2x1016cm-2 , 1000°C• Pattern and etch poly-E NPN
• Nitride to stop PNP poly-Si etch• B implant for SIC @ 50-100keV• Grow IFO• Implant B (5-8)x1015cm-3
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Rucker et al., 2012
Example of 0.13mm SiGe HBT at 500 GHz
WE =0.12mmRBCBC RBi=2.6Ω/sqdsp ox ~25nm RB
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Common with NPN
Intrinsic Epi 0.4-0.6µm
0.1-1.0 µm
As or Sb N+ buried layer NBL
sinker
deep
shallow
trenches
SIC
Poly-Si E
~500 nm
silicide
•CMOS gate-stack, poly-R and capacitors patterned and etched together (directional).
•NMOS S/D used as conacts to PNP base, NPN collector, poly-R and capacitors.
•PMOS S/D serve as contacts to NPN base and PNP collector
Compare NPN and PNP transistors
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Passive ElementsResistors
Resistors
Inductor
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Passive ElementsCapacitors: Stability in Operation (V & T)
TCC includes TCC due to: thermal expansion, space charge region (surface depletion – Si and poly-Si), oxide e(T). Watch for leakage.
Plugs/Vias reduce resistance values
Protection of- and against Cu by Al2O3/Ta barriers.
MIM capacitors for decoupling use high K
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Passive ElementsVaractors
Variable capacitor when the voltage is applied to a p-n junction, Schottky diode or MOS structure.Used for tuning ex. radio receiver or voltage-controlled oscillator (VCO)
Junction Varactor n=1/2 for abrupt junction 1/3 for linearly graded junction
Depletion layer
p+-n junction
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Depletion layer with VR gives capacitance depending on dopant distribution
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Passive ElementsJunction Varactors
The most important parameters: • C/area
• capacitance sensitivity (the highest for hyperabrupt junction)
• tuning range
• quality factor
• low frequency noise
• breakdown voltage
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BiCMOS Applications Read out integrated circuits (ROICs) ex.
“ROIC are used for consumer, industrial, scientific and military needs such as automobile and maritime night vision, surveillance, medical and X-ray imaging, building diagnostics, fire fighting, gas detection, helmet mounted and weapons sights, smart munitions, micro-UAVs (unmanned aerial vehicles)”
See passive elements, pnp also available here and in general in BiCMOS
Arjun Kar-Roy et al, 2010 SPIE
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