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ECE 7366 Advanced Process Integration

Set 10a: The Bipolar Transistor - Basics

Dr. Wanda Wosik

Text Book: B. El-Karek, “Silicon Devices and Process Integration”

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Bipolar Junction Transistors for Digital and Analog Applications

RFAMS

Why BJTs? Performance of SiGe BJTs superior than CMOS by several generation(ex. 115 nm ~385GHz SiGe vs. 20 nm CMOS ~389GHz)

Bipolar Junction Transistors

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Bipolar Transistors

E-B junction is forward biased=injects minority carriers to the baseBase (electrically neutral) is responsible for electron transport via diffusion (or drift also if the build in electric field exists) to collectorC-B diode is reverse biased and collects transported carries

VBE>0 VBC<0

IE=IEn+IEp IC=aIEa<1

IB=IEp+Irec

IE IC

IB

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Bipolar Junction Transistors

n-p-nIntegrated circuit BJT

p-n-p Individual device

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Bipolar Junction Transistors

Currents’ Components

small

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Bipolar Junction Transistors

n-p-n p-n-p

n+-p p+-n n-pp-n

BJT – injection and transport of carriers as well as capacitance and resistances optimized for gain, speed, and power.

Doping asymmetry; see F-level in Emitter in n-p-n and p-n-p transistors.

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Notation and Biasing for Bipolar Junction Transistors

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Charge Distributions in p-n-p Transistors Under Bias

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BJT Operation

Common emitter

Common base

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p-n-p BJT Energy Band Diagram

Thermal equilibrium

Forward bias condition

Holes are minority carriers injected from E to B

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Idealized BJT Structure

• Base is short: Ln>>WB i.e. no recombination• Emitter is transparent: Lp>>WE • Low field in the depletion layers: no ionization i.e no breakdown• Leakage currents low• No recombination in the E-B space charge region

•Doping asymmetry•Doping uniformity in all regions

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Idealized BJT StructureForward Biasing Condition

Emitter efficiency

Base transport factor:

Current gain in CB configuration

Current gain in CE configuration

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Planar TransistorStructure and Doping

n+

NE>>NB>>NC

E-field

Intrinsic base: short (Wb<<Lp); pinch-off base

Short emitterThe role of contact

Arsenic used in the buried layer• slow diffusion into epilayer• collector voltage not degraded by n+ diffusion

As vs P

Process integration of collector plug doping

Dopant distributions to ensure high injection g and transport a

As

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Low Level Injection ParametersElectron Injection to the BaseBuilt-in E-field

Electron component of Emitter region

No recombination in the base

Gummel base number

For high doping – bandgap narrowingStrong effect in emitter

Note: Base is short

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Note: Base is short but emitter is long

Electron current

Injection of Minority Carriers

Hole current

NA(x) <<NDE(x)

µpE<<µnB

niE2(x)>>niB

2(x) DEg(N) Bandgap narrowing

tpE<<tnB SRH & Auger≅f(N)

Emitter injection efficiency next

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Emitter injection efficiency

Emitter Injection Efficiency and Current Gain

Current gain CB configuration

CE configuration

Need largeGE

Need smallGB

Recombination in the SCR

At high VBE High injection levels:

•Dnp≈pp(x)•ohmic voltage drop• base width modulation

Gummel Plots

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Transport of Minority Carriers

Diffusion length vs. base width

Recombination in the base

No built-in E-field – diffusion current only

Base Current Components • Hole injection into emitter (to decrease, use high doping levels in E)• Generation current in C-B junction (no defects)• Recombination in the neutral base base (insignificant – base short)• Recombination in the E-B space charge region and the junction surface (depletion layer – defects)

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Collector-Base Reverse Characteristics

Common Emitter configuration

Open base

Early voltage

Can lead to second breakdown

BJT are not symmetrical reverse operation conditions• injection at C-B junction is small (collector doping the lowest)• transport in the base poor (E-field retards carrier drift)

b with Ic due to high injection levels (also Kirk effect)

bR<<bF

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Bipolar Junction TransistorsForward Operation Mode Early Effect

Early Voltage

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Bipolar Junction TransistorsBreakdown Voltages

Common BaseCommon Emitter

Collector-Base junction

IB=0

Multiplication factor

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Emitter Base Reverse Characteristics

High dopants’ concentrations in E-B junction• Surface concentrations are the highest • Hot carriers possible to increase recombination-generation there

• current gain decreases.

Reverse Early Voltage much larger than VA

Reverse Punch-Through Voltage

Ex. NA(0)=2.5E18cm-3 , NA(Wb)=5E16cm-3 VPT≈3.3V – before VEB0

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Polysilicon Emitter and Interface Oxide

Emitter: Arsenic poly-Si doped • implant into poly•anneal polySi+c-Si junction (<50nm)

Contact away from the junction (leakage) – poly also plays a role of sacrificial layer

•Emitter is self aligned to base (capacitance, resistance, speed) •Extrinsic base contact made in p+-poly-Si• Shallow trench isolation (STI) at junctions reduces capacitances

Double poly n-p-n Scaling of BJTs requires shallow junctions and small areas: use polySi+self aligned structures

This transistor results in a smaller base current: injection into c-Si, tunneling through oxide, recombination in the poly-Si

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Interface Oxide (IFO)

In c-Si long emitters recombination in the bulk

PolySi emitter• Transport of carriers into poly (Lp>xjE) • Interface poly/c-Si important for

recombination

Larger barrier height for holes that for electrons (δ≈4Å)

Degenerated semiconductor

Forward bias condition

For short E (20-50nm) no recombination in Eholes reach contact (leakage)

For thin oxide 1nm IB limited by tunneling

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Polysilicon Emitters: SIMS Resultsn-p-n transitor

p-n-p transitor

1nm oxide

•Oxide thickness control by CVD, RTO, Atomic Layer Deposition•No oxide: epitaxial growth•Oxide breaks-ups: local recrystallization –

junction nonuniformity (lower current gain)• Segregation of dopant at the interface

SIC- selective implanted colector

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Narrow-Emitter Effects

Reduction of current gain because:• E-Gummel number decreases and leads to lateral

injection of holes from the base (not big - if tunneling is dominating)

• Shadowing and aspect ratio effects reduce doping at the E perimeter

• Poly-Si grains columnar (postimplant anneal)– less diffusion in lateral direction – less perimeter doping due to this edge effect

• Self-aligned E-B junction: extrinsic base encroachment – compromise b/w resistance and injection.

• Width of emitter degrades further the injection

3D effects in small devices

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Transistor Resistances Scaling: Frequency Response, Conductance And Switching Speed

Emitter-Base

Collector-Base

Emitter Resistance 1/AEPoly-Si emitters: •Contact resistance b/w metal/silicide and poly•Poly/mono Si interface resistance (IFO)•Vertical resistance in poly (dopants, grains, thickness – watch for silicide penetration)•Vertical resistance of c-Si emitter (doping)

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Measurements of Emitter Resistance

Measurements: • ac (watch for parasitic capacitances),• dc widely used – here floating collector

• IB(VCEsat)

When E-B and C-B junctions become forward biased Parasitic transistors affect E resistance

High impedance voltmeter

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Base Resistance

Extrinsic and Intrinsic (active) base

Ideal diodes plots ~60mV/decade

Use 4 point probe for the sheet resistance in both regions

Also SIMS, Spreading Resistance measurements (depends on depth).

Heavy doping effects may be important – Kirk effect

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Geometry of BJT – Extrinsic Base Resistance

RBint>>RBext

DVBE(y) develops when base current flows

Nonuniform biasing at E-B junction – emitter crowding effect

Low injection

Make interdigitated geometry for emitters in high power devices PE/AE

Base spreading resistance – nonuniform along the emitter junction

Intrinsic base resistance high

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Effective Base ResistanceAssume:RBint=24kW/sq

Apply bias:Rbint<RB0

Probe the transistor base here

RBint≈RB0/3 at low IC. At high IC, RBint and RBRBex b/c of emitter crowding effect (current crowding)npn transitor with double base contact- pinch-off resistor

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Breakdown Voltages – Influence Of Base Resistance

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Collector Resistance

Resistance of the Collector

Collector resistance extracted from: test structures, SIMS, SRP versus depth.

The “fly-back” (RE method not good – injected carriers in reverse operation - affect measurements)