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

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

GaAs HEMTs Overview

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)

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

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

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)

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

RF (Radio Frequency) Characteristics

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

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

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

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

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

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/