Transcript GaAs RF Transistors - Purdue College of Engineering

GaAs HEMTs Overview & applied techniques to
improve high-speed performance
Feb 15th 2017
Jinhyun Noh
Contents
• GaAs HEMTs overview
•
•
•
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High Electron Mobility Transistors
GaAs conventional HEMTs
GaAs PHEMTs
GaAs mm HEMTs
• RF (Radio Frequency) characteristics
• Applied techniques to improve high-speed performance on
fabrication steps
• Mesa isolation
• Ohmic contact
• Gate formation
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GaAs HEMTs Overview
High Electron Mobility Transistors (HEMTs)
• Comparison with MESFETs
• MESFETs
Using Bandgap difference
For mobility
• 3-terminal device (gate, source, drain)
• Control Tr. by depletion region.
Lattice buffer
Floating and insulating
• HEMTs
(a)
• Upgrade MESFETs using heterojunction
structure
• 2DEG (2 dimensional electron gas) channel
• Electrons stuck in 2DEG (~1012 cm-2)
(b)
Fig. 1. (a) GaAs MESFET (b) Idealized
MESFET cross section
Fig. 2. (a) Conventional GaAs HEMT
schematic (b) Energy band diagram
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GaAs HEMTs Structure
• Role of layers
• Buffer layer : defect isolation, smooth surface creation.
• 2DEG : result from the band gap difference between AlxGa1-xAs and GaAs
 A sheet of nearly-free electrons
• The spacer layer : separates the 2DEG from ionized donors generated by
n+ active layer.
• Interaction decreases with increasing separation between impurities and 2-DEG
(mobility ↑)
• drawback: The sheet carrier concentration in the channel is reduced as the
spacer layer ↑
Fig. 3. GaAs HEMTs structure
Fig. 4. GaAs HEMTs energy
band diagram
• Delta doping : higher current concentration
• Uniform carrier distribution: improve gate leakage current and the breakdown
voltage
• Donor layer : source of electrons.
• n+ GaAs : low-resistance Ohmic contacts.
Fig. 5. Spacer thickness vs.
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sheet carrier concentration
GaAs P(Pseudomorphic)HEMT
• What is PHEMT?
• Using strained updoped InGaAs channel
• Advantages (over conventional HEMTs)
• High Indium mole fraction(~15%)
• High transconductance  deeper quantum-well
• High mobility  lower effective mass
• Applications
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•
•
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Fig. 6. PHEMT material system in energy gap vs. lattice
Low-noise HEMT
Power HEMT
Digital applications
High frequency
Fig. 7. Different between HEMT & PHEMT in energy band diagram
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GaAs mm(Metamorphic) HEMTs
• InP InGaAs/InAlAs HEMTs
• Very high Indium mole fraction (53%)
• Very low noise and very fast
• But, expensive substrate (X5~7 than GaAs sub.)
• GaAs mm HEMTs
• As high as possible Indium mole fraction (3~40%)
• High mobility
• Maximize conduction bandgap discontinuity
Fig. 8. InP InGaAs/InAlAs HEMTs & mm HEMTs
material system in energy gap vs. lattice
• Deeper quantum well  high transconductance
Fig. 9. In mole fraction vs. mobility
Fig. 10. mm HEMT material system (△Ec)
Fig. 11. Comparison of GaAs PHEMT, InP HEMT and mm 6HEMT
GaAs mm HEMTs Buffer
• Graded buffer layer
• Linear grading
• Smooth surface, good to
accommodate lattice mismatch
• Hard to grow
(a)
• Step grading
• Easy to grow (over linear grading)
• More dislocations
(b)
Fig. 14. (a) Linear grading (b) step grading buffer
Fig. 13. Critical thickness depending on
grading rate
 No general agreement on which
approach is superior (considering
convenience and/or practicality)
Fig. 15. The epitaxial structure example of mm HEMTs
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RF (Radio Frequency) Characteristics
RF(Radio Frequency) Characteristics
• ft : Unity-Current-Gain Frequency
•
•
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ft 
Frequency where current gain is 1
Low power circuit
Intrinsic parameter
In the full formula,
gm
vs

2Ci 2Lg
vs : Electron saturation velocity
Lg : Gate length
• Cgd, Cgs ,rs+rd ,rds degrades ft
• fmax : Maximum Oscillation Frequency
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•
Frequency where power gain is 1
Viewpoint of power
Needs high ft
Extrinsic parameter(parasitic capacitance, resistance & bonding pad
capacitance) degrades fmax.
f
1
f r
• Low resistive loss  high fmax
f 

2 ri / rds  2 f t rg C gd 2 r
t
t
Fig. 16. HEMTs AC model
ds
max
i
• Keys of Ft, Fmax engineering
• Lg ↓  ft, fmax ↑
• ri ↓, feedback cap. ↓  fmax ↑
Fig. 17. RF characteristics of devices
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Applied Techniques to Improve High-speed
Performance on Fabrication Steps
Mesa Isolation : Applied Techniques
• Purposes
• To isolate devices
• To restrict current flow
• To reduce parasitic capacitances and resistances
• General process in GaAs HEMTs
• Phosphoric acid wet etching(InGaAs,InAlAs) : Etching to middle of buffer layer
for isolation
Fig. 18. w/ and w/o mesa isolation in GaAs HEMTs
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Ohmic Contact : Applied Techniques
• Purpose
• To allow electrical current to flow into or out of semiconductor
(minimize contact resistance)
• Theoretical basis
• Theoretically, junction between metal and semiconductor
(WFm<WFs)
• In real (WFm>WFs), Making tunneling dominant (minimize potential
drop)
Fig. 19. Ohmic contact
• Fabrication example (to make tunneling dominant => high
doping => Alloy) - Ge, Au, Ni based Ohmic metal
• Heating the surface of wafer
• Ga(Ⅲ) diffuses into the metal -> AuGa
• Ge(Ⅳ) diffuses into the wafer and acts as a dopant
• Optional Nickel – to help diffuse
• Overcoat of “thick” Gold(contact resistance ↓)
Fig. 20. Ohmic contact energy band
12 diagram (a)
theoretical (b) realistic
Gate Recess : Applied Techniques
• Recessed Gate
• Gate is placed in an etched slot to locate it slightly below the surface of the
semiconductor
• Purpose
• Highly Doped Cap. Layer -> Ohmic Contact, Low Contact Resistance
• For Schottky Contact, contact with non-doped Barrier is needed.
Fig. 21. Gate recess formation
• Effect
• Removing current flow in capping layer  Channel current is only
controlled by gate voltage.
• High Transconductance
• Increasing gate breakdown voltage
• Recess Length
Fig. 22. Gate recess length (LR)
• Narrow recess (LR is small)
• Rs↓  gm ↑, fT ↑
• Wide recess (LR is large)
• Cgd ↓, rd ↑  fmax ↑, BV ↑
Trade off Relation  Optimize using double & asymmetric recess !
Fig. 23. Example of Gate recess formation
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T-Gate : Applied Techniques
• Why T-Gate?
• Lg ↓  higher gain & lower noise. 
• But Lg ↓  the higher gate resistance 
• Solution
• Large cross-sectional area at the top of gate
• Remaining a short gate length in contact with the wafer
 Called T-Gate or Mushroom Gate!
• Gate metal
• Requirements of gate metal
• Good adhesion, thermal stability, electrical conductivity
• Overlay metal
• Enhances electric conductivity  Au
Fig. 24. T-Gate process
• Barrier metal
• Prevents diffusion(by heat) between Schottky metal and Gold Pd, Mo, Pt
• Examples of gate metal system for GaAs
• TiPtAu, TiPdAu, CrPdAu, MoAl
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Fig. 25. T-Gate structure SEM image
Thank you!
Reference
• Ali, Fazal. HEMTs and HBTs: devices, fabrication, and circuits. Artech
House Publishers, 1991.
• https://www2.warwick.ac.uk/fac/sci/physics/current/postgraduate/re
gs/mpags/ex5/devices/hetrojunction/ohmic/
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