4H- and 6H- Silicon Carbide in Power MOSFET Design. Md Hasanuzzaman

Transcription

1 4H- and 6H- ilicon Carbide in Power MOFET esign By Md Hasanuzzaman epartment of Electrical & Computer Engineering The University of Tennessee, Knoxville April 7,

2 Agenda Overview of silicon carbide Comparison of i and ic MOFET Vertical power MOFET model A proposed device structure MOFET temperature model Results Conclusions 2

3 Why ic? Large band gap and high melting point enable high temperature device operations Large break down field enable the fabrication of smaller and compact high voltage power devices Requires smaller cooling system due to High thermal conductivity Extremely radiation hard hows excellent reverse recovery characteristics Excellent mechanical properties make ic ideal for MEM applications High breakdown electric field Wide bandgap High power high temperature device Radiation hard High thermal conductivity High current density 3

4 Why Not ilicon? maller band gap and low melting point enable medium range of temperature device operations ue to lower thermal conductivity, requires larger cooling system maller electric breakdown field limit the device size and voltage hows high leakage current ic i Breakdown Voltage Electric Field Thermal Conductivity On-Resistance Fig: Comparison of i and ic properties 4

5 Physical & Electrical Properties of ic Properties i 6H-iC 4H-iC Bandgap(eV ) ielectric const Breakdown field (V/cm) 6x x x10 6 aturated drift velocity (cm/sec) 1x10 7 2x10 7 2x10 7 Electron mobility (in bulk) (cm 2 /V-sec) a 650 c Hole mobility (in bulk) (cm 2 /V-sec) Thermal conductivity (W/cm- o K) Melting point ( o C) Hardness (kg/mm 2) a=along a-axis, c=along c-axis 5

6 Comparison of Wide Bandgap Materials ic is unique because of its native oxide is io 2, therefore easy to fabricate field effect device Though iamond has the best material properties, it is not suitable for conventional electronic device fabrication. It is very difficult to form oxide in GaN, it is suitable for opto-electronics devices 6 Properties 4H-iC GaN iamond Lattice constant (Å) Thermal expansion (x10-6 ) o C a c ensity (g/cm 3 ) Melting point ( o C) Bandgap (ev) aturated electron velocity (x10 7 cm/s) Mobility (cm 2 /V-s) Electron Hole Breakdown Voltage (x10 5 V/cm) > ielectric constant Resistivity (Ω-cm) Thermal conductivity (W/cm.K) Hardness (kg/mm 2 ) 2130 a

7 Power MOFET design considerations pecific on-resistance has to be kept as low as possible Breakdown voltage has to increase as design requirements Low specific on-resistance reduces power losses and increases efficiency pecific on-resistance increases with the increase of the breakdown voltage A design trade-off has to be made between the specific on-resistance and the voltage rating V R Br = on sp ε. E 2 c 2. q. N = 4V d 2 Br 3 c ε. E.µ R CH n G Oxide Channel R R CH 4H-iC n- drift region 7

8 Power MOFET design considerations (contd.) ic devices demonstrate onehundredth times lower specific onresistance and ten times higher breakdown voltage than the silicon devices for the same device dimensions rift region thickness is ten times lower in silicon carbide compared to silicon device for same voltage rating Mass and volume of heat sink is 15-20% smaller in case of silicon carbide W drift 2V E Br c R CH G Oxide Channel R R CH 4H-iC n- drift region 8

9 Comparison of i & ic Power MOFETs G Oxide R CH Channel R CH R CH G Oxide Channel R CH 4H-iC n- drift region R R n- drift region i-mofet ic-mofet & Heat sink Heat sink for i devices ilicon ilicon-carbide On-Resistance 100 m.ω/cm 2 1 m.ω/cm 2 rift Region Thickness 100 µm 10 µm Operating Temperature 150 o C 500 o C Heat ink Volume & Mass 100% 80-85% evice size 100% 50% 9

10 Future power electronics will be ic! Today s Technology Advantages of ilicon Carbide Payoff Large energy losses Reduce loss by 10x Large improvement in efficiency Limited voltage and power level Low operating temperature (< 150ºC) Large heat sinks & filters Increase power 10 3 x Increase range to 500ºC Reduce size/weight by 3x implify use in electrical grid New applications; eliminate massive heat sinks Lightweight, compact systems 10

11 Potential applications of ic electronics Automotive Utility pace-craft Mining Nuclear power 11 Aircraft

12 Limitations of ic Higher defect densities due to micropipes and dislocations in the crystal orientations Material processing and device fabrication require high temperature process ( o C) evice quality ic wafer is very costly High interface state densities Low inversion layer mobility Power MOFETs in ic are not commercially available rain Current (µa) Micropipes defects 2.5 V =50 mv 2 Wet + NO ry + Wet Gate Voltage (V) Mobility (cm 2 /Vs) 12

13 Atomic structure ic are made by arrangement of covalently bonded tetrahedral i and C atoms 50% carbon atoms and 50 % silicon atoms ic has more than 100 known polytypes Polytype refers to a family of material which has common stoichiometric 13 C i Tetragonal bonding between Locations of i (light) and C carbon and silicon atoms atoms (dark) in ic B A C ite locations for C atoms in (0001) plan

14 Atomic structure (contd.) The arrangement of ic bi-layers is same in 2- but differs in by stacking sequence in 3-3 commonly used polytypes: 3C-iC: Cubic structure, Zincblend, ABCABC. 4H-iC: Hexagonal close packed, ABCBABCB A C A B C A A B C A B C A C 6H-iC: Hexagonal close packed, ABCACBABCAC B A B A B A B A 3C 2H 4H 6H tacking sequences for four different ic polytypes 14

15 evice fabrication:the pathway to success Material Processing Epitaxial Growth opant Activation Interface Engineering evice Fabrication Applications Circuit esign ystem Integration Material Processing evice Fabrication Potential Applications e s i g n F l o w 15

16 rawback of lateral structure for power application Power MOFETs have to handle high voltages and high currents Lateral structure is not suitable for power application Current flows laterally through the channel below the gate oxide No drift region available to hold large voltage No option is available for forward blocking capability Required large W to get large W/L ratio to increase the current capacity G OXIE Channel p-type substrate B Lateral MOFET tructure 16

17 Vertical MOFET for high voltage and high power ource and drain are made on top and bottom of the wafer respectively Current flows vertically from drain to source through drift and channel region rift region supports most of the applied drain voltage Channel region provides the current gain p-n junction between and n- drift region provides forward blocking capability G Oxide Channel 4H-iC n- drift region Vertical MOFET tructure 17

18 Vertical MOFET for high voltage and high power P-bodies can be made either round or hexagonal shaped to get large W Length L can be made very small For same die area, W/L ratio is higher compared to lateral MOFET G Oxide Channel rift region doping can be adjusted to get high breakdown voltage with low onresistance 4H-iC n- drift region 4H-iC is suitable for vertical structure due to higher mobility in vertical direction 18

19 Modeling of vertical IMO structure For analysis the structure is divided into two regions: (i) the channel region, and (ii) the drift region Channel forms in the below the gate oxide where gate voltage controls the channel currents rain current is divided into two channel currents in two p-bodies on both sides rift region begins at the drain and ends at the channel near the oxide layer L G Oxide Region A L s Region B Region C L p W d Cross-section details of VIMO structure used in the model α W j W t M Hasanuzzaman,.K. Islam, L. Tolbert, and Burak Ozpineci, Model imulation and Verification of a Vertical ouble Implanted (IMO) Transistor in 4H-iC, proceedings of the IATE international conference, February 24-26, 2003, Palm pring, CA. 19

20 Modeling of vertical IMO structure (contd.) rift region is divided into three regions Region A: an accumulation region between the p-bodies with small cross-section Region B: a drift region with varying cross-section Region C: a drift region with constant cross-section Current voltage relations for channel region and three drift regions have been developed L G Oxide Region A L s Region B Region C L p W d α W j W t 20

21 Modeling of vertical IMO structure (contd.) Channel Current I ch = 2L Wµ [ 1+ ( µ / 2v L) V ] neff neff sat ch V [ 2C ( V V ) ( C + C ) V ] Where C ox +C do is the total capacitance of the oxide and the body depletion ch Current increases with the decrease of channel length and increase in channel width and voltage ox G T ox do ch L G Oxide Region A L s Region B Region C L p W d α W j W t Gate voltage controls the channel current At saturation, channel current is given by, Ich( sat ) = WvsatCox ( VG VT ) 21

22 Modeling of vertical IMO structure (contd.) rift Region Voltage Total drift region voltage is the sum of the voltages of the three regions, Where, V drift = V A + V B + V C L G Oxide Region A L s Region B L p W d α W j W t V A = W j + W d I ( W j + W d ) E y dy = W ( L s qn d µ n ) I 0 / E C Region C V V B C I WqNd µ n( Ls + 2LP ) I = log WqNd µ n cot α WqNd Ls µ n I / E I = WqN ( W W W L tanα ) t d n j µ ( L d + 2L P P ) I E c C / E C 22

23 Results from the analytical model Voltage and current equations are solved numerically because of the implicit nature of the two sets of equations µ n- ic = 40 cm2 /V.s, t ox = 500 Å ε ox =3.9x8.854x10-12 F/cm The parameter values of 4H-iC are used to evaluate the drain currents A number of output characteristics are obtained for different gate voltages L = 1 µm, W t = 25 µm L p = 15 µm, W j = 5 µm L s = 18 µm V T = 1V 23 Output Characteristics of vertical IMO with 4H-iC parameters

24 Results of analytical model(contd.) Effects of impurity concentrations of drift region and narrow spacing regions have been observed Gate loses its control over the drain current if velocity saturation occurs before the drain pinches off Larger separation reduces the quasi-saturation effect Lower impurity concentrations increase the quasi-saturation effect Transfer characteristics show a clear crossover with temperature variation at a gate voltage of 15V 24 rain currents (Amp) Transfer characteristic of Vertical MO for different temperature T=300 o K Crossover Gate voltage (volts) T=500 o K T=400 o K Transfer Characteristics of VIMO with 4H-iC parameters

25 Model verification using ATLA simulations Vertical MOFET structure in 4H- ic is proposed to verify the analytical model n-rift is doped lightly (4.2x10 15 cm - 3 ) to hold the desired breakdown voltage n+ regions are doped (1.5x10 20 cm -3 ) with Nitrogen is formed by Aluminum implantations (4x10 17 cm -3 ) p-bodies are separated by 20µm Oxide thickness is 500Å rift region thickness is 20µm G Oxide Channel 4H-iC n- drift region Vertical MOFET tructure in 4H-iC 25

26 evice dimensions for the proposed 4H-iC IMO evice dimensions oping Region oping level Impurity Channel width 400 µm n-drift 4x10 15 cm -3 Nitrogen Channel length 1 µm p-bodies 4x10 17 cm -3 Aluminum Oxide thickness 500 Å n+ region (1.5x10 20 cm -3 ) Nitrogen p-bodies separation 20 µm Epilayer thickness 25 µm 26

27 imulation results using ATLA device simulator Analytical model ATLA simulations rain currents 220 ma at V G =10V 232 ma at V G =10V pecific on-resistance 54 mω.cm 2 60 mω.cm 2 Breakdown voltage 2.6kV 2.3kV Output characteristics from ATLA simulator 27 Breakdown voltage variation with gate length

28 imulation results using ATLA device simulator (contd.) Optimum gate width of 20 µm is obtained for minimum specific on-resistance A good agreement between the theoretical and the simulated values of breakdown voltage pecific on-resistance values are in the same magnitude compared to the recently published values Clear quasi-saturation effect has been observed in ATLA simulation Good agreement between the analytical model and the proposed device structure Comparison of theoretical and simulated values of breakdown voltages with drift layer thickness as a parameter 28 Variation of specific on-resistance with gate width

29 Temperature modeling for lateral MOFET I = MOFET channel current, at room temperature I R = Body leakage current I TH = Current change due to the threshold voltage change I R = Current change due to the change of drain and source contact region resistance I is kept constant over the temperature variation The compensating currents incorporate the change in the MOFET's current I total I G I R I TH I R MOFET model with temperature compensation *M. Hasanuzzaman,.K. Islam and L.M. Tolbert, Effects of temperature variation (300 o -600 o K) in MOFET modeling in 6H silicon carbide, olid tate Electronics, Vol 48/1 pp ,

30 Temperature modeling for lateral MOFET(contd.) Total rain Current I = I + I + I + I total R R Where, W I = µ n C ox ( V G V TH ) V / 2 V L I I I R TH R TH [ ] 2 E = nni 3 g qa αat exp Ln N A kt W = µ n C ox L 1 1 = β ' R R [ V TH V TH ] V V I total I G I R I TH I R MOFET model with temperature compensation 30

31 Results of temperature model in 6H-iC The model simulations match perfectly with the measured data at 300 o K The simulations at 600 o K match reasonably with the measured data with minor discrepancies At 300 o K, the drain current is 15µA, whereas it rises to 235µA at 600 o K for a gate voltage V G = 6V rain Currents ( µa) simulated o+x* measured V gs =6V V gs =5V 2 V gs =4V V 0 gs =3V rain-ource Voltage (volts) Output characteristics for simulated and measured data at 300 o K 31 rain Currents ( µa) simulated o+x* measured V gs =6V V gs =5V V gs =4V V gs =3V rain-ource Voltage (volts) Output characteristics for simulated and measured data at 600 o K

32 Results of temperature model in 6H-iC (contd.) Contribution of the compensating currents to the total current change Temperature ( o K) Total Current change (µa) ue to I TH ue to I R ue to I R % 2% 8% % 6% 9% % 15% 7% simulated at V ds =5v --- simulated at V ds =5v 200 rain Currents (µa) 10 5 rain Currents (µa) Gate to ource Voltage (volts) Gate to ource Voltage (volts) Transfer characteristics simulated at drain-source voltage V =5V, temperature 300 o K 32 Transfer characteristics simulated at drain-source voltage V =5V, temperature 600 o K

33 Results of temperature model in 4H- & 6H-iC rain currents of 6H-iC MOFET are greater than their 4H-iC counterparts by a factor of approximately 2.5 Higher mobility of 6H-iC ( 100 cm 2 /V.sec) results in higher drain current of 6H-iC MOFET compared to 4H-iC MOFETs imulation results show a better performance for 6H-iC MOFETs otted line: 4H-iC olid line : 6H-iC otted line: 4H-iC olid line : 6H-iC rain Currents ( µa) V gs =6V V gs =5V rain Currents (µa) o K V gs =4V 4 6 rain-ource Voltage (volts) 8 10 Output characteristics of 4H- & 6H-iC lateral MOFET at 300 o K 300 o K Gate to ource Voltage (Volts) Transfer characteristics of 4H- & 6H-iC lateral MOFET *M. Hasanuzzaman,.K. Islam and L.M. Tolbert, Temperature ependency of MOFET evice Characteristics in 4H- and 6H-ilicon Carbide (ic), to be presented in IR

34 Results of temperature model in 4H- & 6H-iC Threshold voltage changes from 3.2V at 300 o K to 1.2V at 600 o K imulated values of the threshold voltages matches with the measured data Variation of threshold voltage for 4H-iC is predicted using the model Threshold Voltage (volts) simulated o measured Threshold Voltage (volts) H-iC 6H-iC Temperature( o K) Temperature( o K) Threshold voltages obtained from simulation and measurement for different temperature 34 Threshold voltage variation of 4H- & 6H-iC lateral MOFET with temperature

35 Conclusions ilicon carbide is better alternate material for high power and high temperature device application An analytical model for vertical IMO in 4H-iC is developed A device structure is proposed to verify the model A temperature model for lateral MOFET is developed imulation with MOFET temperature model is carried out for 4H- and 6H-iC materials 35

36 Thanks!! 36