High Electron Mobility Transistors (HEMTs)

Transcription

1 High Electron Mobility Transistors (HEMTs) Source Gate Drain W g 1 8 V G = 2 V V G = 1 V Active Region Source S. I. Buffer L g W g Gate d Open channel Drain I D (ma/mm) g m = 2 ms/mm V DS (V) Active Region S. I. Buffer L g Pinch off Similar to normally-on MOSFETs but no substrate doping. For accurate formula, refer to Sze: Physics of Semiconductor Devices

2 Output Power Calculation (AC, not DC) A I max I DS Q I SWING linear BIAS POINT P out, max = V SWING I SWING 8 Minimize V knee V knee V DS B V breakdown Maximize V breakdown Maximize I max V SWING Maximize n s µ or n s v s GaAs phemt AlGaN/GaN HEMT V knee (V) 1 5 (~ V pinchoff ) V breakdown (V) 2 1 (over 2 reported for small L gd ) I max (A/mm) (over 2 reported) P out, max (W/mm) (32 highest reported)

3 Sample power calculations Let V knee be 4 V, and V bd be 12 V, and I swing be 12 ma for a 1 micron gate width device. Calculate the maximum output power in dbm and in W/mm Solution: Total maximum output power = 1/8 (12 4) 12 mw = 174 mw. So output power in dbm = 1 log174 = dbm. Output power density is 174 mw/1 micron = 174 mw/mm = 17.4 W/mm. If the gain is 15 db, what is the input power? Solution: 1 log (P out /P in ) = 15 P out = P in x = P in x Pin = 17.4/31.62 =.552 W/mm. If the dc input power is also given then the Power-Added Efficiency (PAE) can be calculated as (P out P in )/P dc Slide # 3

4 Performance criteria for microwave transistors Output Power: Total microwave power available (W/mm) Gain: G = P out /P in, log G = Log P out Log P in (Gain usually measured in db, but P in and P out are in dbm) F t : Maximum frequency of oscillation or the frequency at which the short circuit current gain is 1 F max : The frequency at which the power gain is 1 for a perfectly matched load Power added efficiency (P.A.E): (P out P in )/P dc, P in = input microwave power, P dc = total dc power in at the gate and drain terminals. Linearity: The measure of gain against input signal level. High linearity means lower harmonic content in the output signal Noise Figure: SNR in /SNR out (usually expressed in dbm by taking the log) Stability: long term and short term operational stability Slide # 4

5 AlGaN/GaN HEMT: wish list High V Br Minimize Buffer leakage: GaN:Fe Gate leakage: Insulated-gate Other device structures to improve V Br High power efficiency When efficiency is low Power dissipation at semiconductor devices R on efficiency How to maximize efficiency Eliminate surface traps (passivation/epitaxial solutions) Eliminate bulk traps (growth condition tuning) Decrease leakage (low dislocation density/insulators?) Slide # 5

6 Growth Challenge I: heteroepitaxy Tiny changes in growth conditions have strong effect on GaN properties (T,d, V/III) + very sensitive coalescence process = process much less robust than homo-epitaxy Lattice mismatch time High dislocation density in epitaxial layers and at the interface of the heteroepitaxial layers. Slide # 6

7 Growth Challenges II: alloy epitaxy GaN technology still less mature than GaAs and InP technology Crystal growth is dominantly heteroepitaxial Alloys: today s high efficiency devices Al x Ga 1-x N x Al <.4 In x Ga 1-x N x In <.4 AlN InN High Al (x=.5 ~ 1) is currently under intense research (UV LEDs and detectors etc.) GaN Alloys with high Al and/or In compositions difficulties related to interplay of Material properties and Epitaxy process Stacia Keller et al. UCSB Slide # 7

8 AlGaN/GaN high electron mobility transistor: basics Unlike AlGaAs/GaAs HEMT requiring intentional doping to form charge, 2DEG in AlGaN/GaN HEMT are polarization-induced. No intentionally doping is needed. Polarization charge Donor-like surface traps (empty) Gate Source Ē Al x Ga 1-x N P _ + _ + _ + _ + _ + + _ + _ + _ + _ + + _ + _ + _ + _ + _ + + _ + _ + _ + _ + + _ + _ + _ Drain Electrons come from surface states. Channel 2-DEG GaN + + 2DEG + UID - Surface + AlGaN DEG states - Donors AlGaAs/GaAs HEMT Polarization charge AlGaN/GaN HEMT Slide # 8

9 AlGaN/GaN HEMTs: Formation of the 2DEG Layer structure Schematic band diagram 2-3 nm Al.3 Ga.7 N Φ B AlGaN GaN E c 2DEG E c GaN buffer(1-2 µm) d E F Nucleation layer (~ 2 nm) +ve σ comp σ B Sapphire/SiC substrate 2 DEG σ surf + σ B εε n s = [ Φ B + EF ( ns ) Ec ] 2 e de The 2DEG is an explicit function of the surface barrier, AlGaN thickness, and the bound positive charge at the interface Slide # 9

10 Comparison with GaAs HEMT Physics Schematic band diagram AlGaAs/GaAs HEMT Φ B AlGaN GaN E c E c d E F +ve σ comp σ B AlGaAs donor layer GaAs buffer 2 DEG σ surf AlGaAs spacer No doping is required for the 2DEG to be present at the interface. Higher sheet charge and higher conduction band discontinuity for AlGaN/GaN heterostructure Slide # 1

11 Heart of HEMT: 2DEG for high power, high frequency HEMTs: high x Al, coherently strained, trap free AlGaN/GaN heterojuction, (abrupt + smooth on an atomic level) carrier confinement, high breakdown voltage, high currents AlGaN u.i.d. AlGaN:Si? GaN S.I. Al 2 O 3 /SiC 2DEG (density and mobility) Determined by - x Al - interface roughness - alloy scattering - dislocation, etc. Ambacher et al, JAP 87(1) 2 Slide # 11

12 Properties of the 2DEG 2DEG Mobility vs. density Spacer layer thickness vs. 2DEG density and mobility d spacer depends on intended application For AlGaAs/GaAs heterostructures, the spacer layer thickness is important for 2DEG mobility and density The 2DEG does not freeze out at very low temperature unlike the 3D doping The 2DEG mobility does not decrease with decrease in temperature unlike the 3D case The 2DEG mobility can increase with increase in 2DEG density due to increased screening unlike the 3D doping Slide # 12

13 2DEG Influence of the Al-composition µ 3Κ [cm 2 / Vs] MOCVD x Al >.2: µ 3K ~ 1/x Al x Al : - interface problems - strain induced defects - higher impurity incorporation - alloy ordering/clustering n s ~ x Al n s [1 13 cm -2 ] - charge increases due to spontaneous polarization and piezoelectric effects x Al <.2: µ 3K ~ x Al x Al - better confinement of the 2DEG at higher x Al relaxed - low x Al = low n s : less efficient screening of defects Slide # 13

14 Temperature dependence of v-f curve Electron Velocity (1 7 cm/s) GaN 3 K 5 K 7 K Electric Field (kv/cm) Usually the regions are separated into regions of constant and zero mobility A velocity overshoot is expected for GaN similar to GaAs case, but usually not seen, possibly due to high background doping At higher temperature, the degradation of v-f curve for GaN is much smaller than GaAs Electron Velocity (1 7 cm/s) GaAs 3 K 5 K 7 K Electric Field (kv/cm) Slide # 14

15 Temperature dependent mobility Increasing alloy composition in barrier Debdeep Jena Ph.D dissertation Slide # 15

16 Electron transport Phonon scattering: ---most important at room temperature Alloy disorder scattering ---potential disorder from ternary alloy ---important at low and room temperature Surface roughness scattering ---important at low temperature Ionized impurities scattering Dislocation scattering Dipole scattering Mattheissen rule for total mobility: Alloy disorder scattering is the limiting factor at low temperature. Alloy disorder scattering also plays an important role at room temperature when carrier concentration is high. It is due to the ternary nature of AlGaN. Debdeep Jena Ph.D dissertation 1 1 = µ net µ i i where i refers to the mobility corresponding to different sources Slide # 16

17 Methods for reducing scattering Controllable scattering mechanisms Background impurity scattering: By growing the material purer Alloy scattering: By putting a thin binary alloy (AlN) at the interface Dislocation scattering: By growing on lattice and thermally matched substrate Interface roughness scattering: By growing very smooth interfacial layers Rest of the scattering processes are usually physics limited Slide # 17

18 2DEG High-mobility AlN interlayers AlGaN 1 nm AlN S.I. GaN sapphire d AlN = 1 nm N (1 12 cm -2 ) S 3 25 AlGaN/GaN AlGaN/AlN/GaN T = 17 K Al mole fraction x Mobility µ (1 4, cm 2 /Vs) 2.5 AlGaN/GaN 2 AlGaN/AlN/GaN T = 17 K Alloy composition x by MBE, I.P. Smorchkova et al., J. Appl. Phys. 9 (21) 5196 Similar results obtained by MOCVD no alloy scattering Slide # 18

19 AlN as a barrier layer Probability AlGaN/GaN interface Al.22 Ga.78 N/GaN AlN/GaN Distance (nm) 2DEG density (1 13 cm -2 ) Simulations AlN barrier thickness (nm) Alloy disorder scattering: ---Wavefunction penetration ---Ternary material: AlGaN Reduce alloy scattering: ---Increase Al composition ---Binary material: AlN Use AlN as barrier material ---No alloy disorder scattering: higher mobility ---Higher polarization charge density: higher carrier concentration However, after gate metal deposition, it was found to be almost ohmic due to tunneling! Slide # 19

20 AlGaN/AlN/GaN Heterostructure 25 nm Al.3 GaN.7-1 nm AlN UID GaN SiC Substrate Incorporation of a thin AlN (<1nm) into a standard AlGaN/GaN HEMT The thickness of AlN interfacial layer is below critical thickness for formation of 2DEG. The main purpose is to improve mobility. Thin AlN layer forms a larger effective E c, which affects both mobility and carrier concentration. Slide # 2

21 Charge and mobility vs. AlN thickness AlGaN/AlN/GaN HEMT 2DEG Density (1 13 cm -2 ) optimum thickness Charge(Simulation) Charge(Experiment) Mobility(Experiment) Mobility (cm 2 V -1 s -1 ) Thickness of AlN (nm) 6 Theory predicts that n s increases with AlN thickness In real growth, thick AlN suffers by the relaxation. Above.5nm, charge saturates and mobility drops Slide # 21

22 Band Diagram 25 nm Al.33 Ga.67 N/ 1 nm AlN/GaN HEMT 25 nm Al.33 Ga.67 N/GaN HEMT 3 3 Thin AlN Energy (ev) Effective E C Energy (ev) 2 1 AlGaN GaN E C Thickness (nm) Thickness (nm) n s σ = t εε εε φ + E q q t + t + d ' AlGaN AlGaN B 2 c, eff AlGaN AlN n s σ = εε εε t φ + E q q t + d AlGaN AlGaN B 2 C, AlGaN AlGaN q E = E + t 2 ' c, eff C, AlGaN σ AlN AlN εε Slide # 22

23 Hall data and DC I-V Hall Data: Conventional undoped AlGaN/GaN n s = cm -2 µ = 12 cm 2 /V s Undoped AlGaN/AlN/GaN: n s = cm -2 µ = 152 cm 2 /V s I D (ma/mm) V G = 2 V V G = 1 V g m = 2 ms/mm Si-doped AlGaN /AlN/GaN: n s = cm -2 µ = 15 cm 2 /V s V DS (V) Mobility was improved with a slight increase of 2DEG Si doping increased 2DEG density while retaining high mobility Slide # 23

24 Power Performance Pout (dbm), Gain (db) Undoped AlGaN Pout Gain PAE 8.1 W/mm P in (dbm) PAE (%) Pout (dbm), Gain (db) Si-doped AlGaN Pout Gain PAE 8.47 W/mm P in (dbm) PAE (%) On SiC substrate. SiN passivated. 8.1W/mm with a peak PAE of 23% was obtained at 8GHz at V D =5V, I D =13mA/mm from an undoped AlGaN barrier HEMT. 8.47W/mm with a PAE of 41% was obtained at 1GHz at 8GHz at V D =45V, I D =16mA/mm from a Si-doped barrier HEMT. Slide # 24

25 Effect of Si doping density Energy (ev) N d /Polarization=1.2 N d /Polarization=.8 N d /Polarization=.5 parallel conduction Thickness (nm) Thickness (nm) holes Thickness (nm) n s = cm -2 n s = cm -2 n s = cm -2 n para = cm -2 p s = cm -2 Too much Si doping results in free electrons in graded layer, leading to parallel conduction Too little Si doping is not enough to remove holes ~8% compensation puts fermi level in the middle of bandgap Slide # 25 Electron, Hole Concentration (1 18 cm -3 )

26 Design rules for AlGaN/GaN HEMTs: Materials perspective Thickness of the barrier layer: affects 2DEG concentration and vertical gate field (which controls gate leakage current, V D, breakdown, and can also affect device degradation) Al composition of the barrier layer: affects 2DEG concentration and E C, which confines the 2DEG Nucleation and buffer layer: affects dislocation density, and surface morphology (both affect mobility, one by charged line scattering and other by interface roughness scattering) and parasitic conduction. Substrate for epitaxial growth: affects the heat conductivity and ultimate output power performance as well as defect density, and parasitics. Slide # 26

27 Transistor fabrication layout Submicron Ni/Au mushroom gate defined by e-beam Ti/Al/Ti/Au ohmic contact annealed at 2 8 C (.3 to.6 Ω-mm) SEM image of a submicron mushroom gate 3 4 Air-bridge to connect isolated source pads 1 Cl 2 based ECR mesa isolation SEM photo showing air-bridge over the gate metal (T-layout) Slide # 27

28 Design rules for AlGaN/GaN HEMTs: Fabrication perspective 2 x 125 µm U-gate 2 x 75 µm T-gate D D S S S S G G The gate footprint and the cross-sectional area and width controls the frequency response L g lower means f T goes up Cross-section and gate width control gate resistance (this is why mushroom gates are used) The gate drain spacing as well as gate footprint determines the breakdown voltage L g lower means V BR down Gate-drain spacing up means V BR up The geometry of the device also plays a role The U-geometry device has 1 15 % lower g m, I dss due to self heating Slide # 28

29 Large periphery devices Parallel fingers or fishbone layout for 12 x 125 µm devices: Parallel fingers Fishbone Air bridges Larger periphery devices used for higher actual output power NOT power density (usually more than 1 mm gate finger width) The fabrication processes are complicated as this involves airbridging the source or the drain. Large periphery design issues: electrical and thermal Slide # 29

30 Design issues for large periphery devices Electrical issues: The voltage drop along the gate length causes lower PAE Phase difference at the gate fingers reduce overall PAE Finite R on reduces PAE. This becomes severe in presence of trapping as R on increases Thermal issues: Device heating is a problem at higher output power, since power wasted is also larger The maximum possible output power depends on the conductivity of the substrates. SiC substrates are commonly used. Thinned sapphire substrates have also been used. The number of gate fingers as well as the gate finger pitch determine the maximum temperature rise in a device. Slide # 3

31 DC characteristics of AlGaN/GaN HEMTs µm devices (~35% Al) The negative slope in the dc characteristics of sapphire is either due to heating or trapping The dc characteristics are better for HEMTs fabricated on SiC than on sapphire possibly because of reduced dislocation density and increased thermal conductivity The difference becomes more severe with scaling Slide # 31

32 RF performance Small signal Large signal h21, UPG (db) h21 UPG f (GHz) Pout (dbm), Gain (db) P out Gain PAE 3.4W/mm P in (dbm) PAE (%) f t of 22GHz and f max of 4GHz were obtained from a.7um-gate-length HEMT at drain bias of 1V and drain current of 24mA/mm. On sapphire substrate. No SiN passivation. 3.4W/mm with peak PAE 32% was obtained at 1GHz when V D =15V and I D =23mA/mm. Slide # 32

33 RF performance Small signal Large signal h21, UPG (db) h21 UPG Frequency (GHz) Pout (dbm), Gain (db) P out Gain PAE 12W/mm 44% P in (dbm) PAE (%) f t of 21GHz and f max of 39GHz were obtained from a.7um-gate-length HEMT at drain bias of 15V and drain current of 28mA/mm. On SiC substrate 12W/mm with a peak PAE of 44% was obtained at 4GHz at VD=5V, ID=27mA/mm Slide # 33