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GaAs HEMTs Overview & applied techniques to improve high-speed performance Feb 15th 2017 Jinhyun Noh Contents • GaAs HEMTs overview • • • • 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 1 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 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 ↑ 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. 4 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 • • • • 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 5 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 7 RF (Radio Frequency) Characteristics RF(Radio Frequency) Characteristics • ft : Unity-Current-Gain Frequency • • • • ft Frequency where current gain is 1 Low power circuit Intrinsic parameter In the full formula, gm vs 2Ci 2Lg vs : Electron saturation velocity Lg : Gate length • 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. 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 9 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 11 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 13 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 14 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/ 16