0. GaAs HEMTs Overview & applied techniques to improve high-speed performance
Feb 15th 2017
Jinhyun Noh

1. Contents GaAs HEMTs overview RF (Radio Frequency) characteristics
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

2. GaAs HEMTs Overview

3. High Electron Mobility Transistors (HEMTs)
Comparison with MESFETs
MESFETs
3-terminal device (gate, source, drain)
Control Tr. by depletion region.
HEMTs
Upgrade MESFETs using heterojunction structure
2DEG (2 dimensional electron gas) channel
Electrons stuck in 2DEG (~1012 cm-2)
Fig. 1. (a) GaAs MESFET (b) Idealized MESFET cross section
Fig. 2. (a) Conventional GaAs HEMT schematic (b) Energy band diagram
Using Bandgap difference
For mobility
Lattice buffer
Floating and insulating
(a)
(b)

4. GaAs HEMTs Structure Role of layers
Fig. 3. 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 ↑
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. 4. GaAs HEMTs energy band diagram
Fig. 5. Spacer thickness vs. sheet carrier concentration

5. 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
Low-noise HEMT
Power HEMT
Digital applications
High frequency
Fig. 6. PHEMT material system in energy gap vs. lattice
Fig. 7. Different between HEMT & PHEMT in energy band diagram

6. GaAs mm(Metamorphic) HEMTs
Fig. 8. InP InGaAs/InAlAs HEMTs & mm HEMTs material system in energy gap vs. lattice
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
Deeper quantum well  high transconductance
Fig. 11. Comparison of GaAs PHEMT, InP HEMT and mm HEMT
Fig. 9. In mole fraction vs. mobility
Fig. 10. mm HEMT material system (△Ec)

7. GaAs mm HEMTs Buffer Graded buffer layer
Linear grading
Smooth surface, good to accommodate lattice mismatch
Hard to grow
Step grading
Easy to grow (over linear grading)
More dislocations
 No general agreement on which approach is superior (considering convenience and/or practicality)
Fig. 13. Critical thickness depending on grading rate
Fig. 14. (a) Linear grading (b) step grading buffer
(a)
(b)
Fig. 15. The epitaxial structure example of mm HEMTs

8. RF (Radio Frequency) Characteristics

9. RF(Radio Frequency) Characteristics
ft : Unity-Current-Gain Frequency
Frequency where current gain is 1
Low power circuit
Intrinsic parameter
In the full formula,
Cgd, Cgs ,rs+rd ,rds degrades ft
fmax : Maximum Oscillation Frequency
Frequency where power gain is 1
Viewpoint of power
Needs high ft
Extrinsic parameter(parasitic capacitance, resistance & bonding pad capacitance) degrades fmax.
Low resistive loss  high fmax
Keys of Ft, Fmax engineering
Lg ↓  ft, fmax ↑
ri ↓, feedback cap. ↓  fmax ↑
Fig. 16. HEMTs AC model
Fig. 17. RF characteristics of devices

10. Applied Techniques to Improve High-speed Performance on Fabrication Steps

11. 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

12. 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)
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. 19. Ohmic contact
Fig. 20. Ohmic contact energy band diagram (a) theoretical (b) realistic

13. 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.
Effect
Removing current flow in capping layer  Channel current is only controlled by gate voltage.
High Transconductance
Increasing gate breakdown voltage
Recess Length
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. 21. Gate recess formation
Fig. 22. Gate recess length (LR)
Fig. 23. Example of Gate recess formation

14. 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
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
Fig. 24. T-Gate process
Fig. 25. T-Gate structure SEM image

15. Thank you!

16. Reference
Ali, Fazal. HEMTs and HBTs: devices, fabrication, and circuits. Artech House Publishers, 1991.
gs/mpags/ex5/devices/hetrojunction/ohmic/