ECE 7366 Advanced Process Integration Set 10a: The Bipolar Transistor - Basics Dr. Wanda Wosik Text Book: B. El-Karek, “Silicon Devices and Process Integration” 1

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 2

Bipolar Transistors E-B junction is forward biased=injects minority carriers to the base Base (electrically neutral) is responsible for electron transport via diffusion (or drift also if the build in electric field exists) to collector C-B diode is reverse biased and collects transported carries V BE >0 V BC <0 I E =I En +I Ep IC=IEIC=IE  <1 I B =I Ep +I rec IEIE ICIC IBIB 3

Bipolar Junction Transistors n-p-n Integrated circuit BJT p-n-p Individual device 4

Bipolar Junction Transistors Currents’ Components small 5

Bipolar Junction Transistors n-p-np-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. 6

Notation and Biasing for Bipolar Junction Transistors 7

8 Charge Distributions in p-n-p Transistors Under Bias

9 BJT Operation Common emitter Common base

10 p-n-p BJT Energy Band Diagram Thermal equilibrium Forward bias condition Holes are minority carriers injected from E to B

11 Idealized BJT Structure Base is short: L n >>W B i.e. no recombination Emitter is transparent: L p >>W E 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

12 Idealized BJT Structure Forward Biasing Condition Emitter efficiency Base transport factor: Current gain in CB configuration Current gain in CE configuration

13 Planar Transistor Structure and Doping n+ N E >>N B >>N C E-field Intrinsic base: short (W b <<L p ); pinch-off base Short emitter The 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   and transport   As

14 Low Level Injection Parameters Electron Injection to the Base Built-in E-field Electron component of Emitter region No recombination in the base Gummel base number For high doping – bandgap narrowing Strong effect in emitter Note: Base is short

15 Note: Base is short but emitter is long Electron current Injection of Minority Carriers Hole current N A (x) <<N DE (x) µ pE <<µ nB n iE 2 (x)>>n iB 2 (x)   E g (N) Bandgap narrowing  pE <<  nB  SRH & Auger ≅ f (N) Emitter injection efficiency next 

16 Emitter injection efficiency Emitter Injection Efficiency and Current Gain Current gain CB configuration CE configuration Need large  G E Need small  G B Recombination in the SCR At high V BE  High injection levels:  n p ≈p p (x) ohmic voltage drop base width modulation Gummel Plots

17 Transport of Minority Carriers Diffusion length vs. base width Recombinatio n 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)

Collector-Base Reverse Characteristics 18 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)    with I c due to high injection levels (also Kirk effect)  R <<  F

Bipolar Junction Transistors Forward Operation Mode  Early Effect Early Voltage 19

Bipolar Junction Transistors Breakdown Voltages Common Base Common Emitter Collector-Base junction 20 I B =0  Multiplication factor

21 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 V A Reverse Punch-Through Voltage Ex. N A (0)=2.5E18cm -3, N A (Wb)=5E16cm -3 VPT≈3.3V – before V EB0

22 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

23 Interface Oxide (IFO) In c-Si long emitters recombination in the bulk PolySi emitter Transport of carriers into poly (L p >x jE ) 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 E  holes reach contact (leakage) For thin oxide 1nm I B limited by tunneling

24 Polysilicon Emitters: SIMS Results n-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

25 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

26 Transistor Resistances Scaling: Frequency Response, Conductance And Switching Speed Emitter-Base Collector-Base Emitter Resistance  1/A E Poly-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)

27 Measurements of Emitter Resistance Measurements: ac (watch for parasitic capacitances), dc widely used – here floating collector I B (V CEsat ) When E-B and C-B junctions become forward biased  Parasitic transistors affect E resistance High impedance voltmeter

28 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

29 Geometry of BJT – Extrinsic Base Resistance R Bint >>R Bext  V BE (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 P E /A E  Base spreading resistance – nonuniform along the emitter junction Intrinsic base resistance high

30 Effective Base Resistance Assume: R Bint =24k  /sq Apply bias: R bint <R B0 Probe the transistor base here R Bint ≈R B0 /3 at low I C. At high I C, R Bint  and R B  R Bex b/c of emitter crowding effect (current crowding) npn transitor with double base contact- pinch-off resistor

31 Breakdown Voltages – Influence Of Base Resistance

32 Collector Resistance Resistance of the Collector Collector resistance extracted from: test structures, SIMS, SRP versus depth. The “fly-back” (R E method not good – injected carriers in reverse operation - affect measurements)