From MOSFETs to MISHEMTs -an evolution

Amitava DasGupta

Dept. of Electrical Engg.

IIT Madras

E-mail: adg@ee.iitm.ac.in

Organization

• How do we get high speed in circuits?

• MOSFETs– How do we get higher speed?

– Limitations

• GaAs MESFETs – Advantages and limitations

• HEMTs– Why HEMTs?

– AlGaAs/GaAs vs pseudomorphic vs GaN

– Technology

– Modeling

2

How do we get high speed?

• To decrease delay

– Decrease C reduce device dimensions

– Decrease V reduce power supply voltage

– Increase I For a smaller V, we expect higher I increase device conductance

GND

VDD

V VV CV

ICircuitBlock

CircuitBlock

Delay

3

MOSFETs – how do we get high speed?

• To reduce td

– Reduce L Use the smallest gate length possible

– Increase n Use a material with high carrier mobility• Not completely true: n degrades at high lateral electric field

• Also need high vsat

– Increase Higher power dissipation• Not completely true: n degrades at high

• W and tox has no effect on td !! 4

MOSFETs – for analog applications

• We need high transconductance (gm)

• For high gm

– Cox should be high tox should be low

• Gate should be as close as possible to the channel

• More charges for same change in input voltage

– n (and vsat for small geometry devices) should be high

• Charges travel faster for same drain field resulting in higher current 5

MOSFETs –present generation• For any technology node

– Channel length is as small as possible– Power supply voltage and oxide thickness dictated by scaling laws– Channel width is adjustable– IDmax/W is an important figure of merit (captures mobility degradation

at high fields)

• 14 nm node– FinFET technology– Gate length = 20 nm – Equivalent oxide thickness (EOT) = 0.9 nm

• Almost reached its limit

– VDD = 0.7 V– IDmax/W 1mA/m = 1A/mm

• To improve performance, need to go for other materials with higher carrier mobility– Germanium?– GaAs?

6

GaAs vs Silicon

• Advantages– Electron mobility is more than 5 times than in silicon

• Saturation velocity almost same as silicon

– Higher bandgap (1.42 eV compared to 1.1 eV)• Higher temperature of operation

• Possibility of semi-insulating substrate

• Disadvantages– No native oxide

– High interface states• Cannot make MOSFET

• Suitable for MESFET (Metal-Semiconductor FET)– Use a Schottky metal-semiconductor junction as gate

7

MESFET

• Normally ON device

• Uses a Schottky barrier gate to control current– conducts when gate voltage is positive

• As VGS is made more negative, depletion layer width in n-GaAs increases to switch off device

8

Schottky Metal-Semiconductor Junction

• Rectifying in nature• p-n junction like I-V characteristics• The depletion region in n-region

increases with increasing reverse bias

m > s

9

Ohmic Metal-Semiconductor Junction

• No potential barrier• Conducts almost equally for both forward and reverse bias• Depending on the work-function of the metal, the metal-

semiconductor junction will be either ohmic or Schottky type• A junction between metal and heavily doped semiconductor

is usually ohmic

EFm

n-type semiconductor

Metal

Metal

EV

EC

q(s-m)q(s-m)EFs

EFm

Vacuum level

EV

EFs

EC

qm

qs

qs

n-type semiconductor

m < s

V

I

10

MESFET characteristics

• For high drain current/transconductance– Electron concentration in channel must be

high• ND must be high

– Electron mobility (μn) must be high• μn degrades at high ND due to ionized impurity

scattering limits maximum possible ND

• Physical separation of electrons and ionized donors preferred– Can be achieved with heterojunction

11

Band diagram for AlGaAs/GaAsheterojunctions

12

HEMT

• A heterojunction (AlGaAs/GaAs) is the heart of the device• The n+ AlGaAs layer is depleted• The channel is created in a thin layer at the undoped GaAs surface

– The high concentration of electrons in the thin layer is referred to as 2-Dimensional Electron Gas (2-DEG)

• The electrons in 2-DEG have very high mobility as ionized impurity scattering is eliminated

• Hence these transistors are referred to as High Electron Mobility Transistor (HEMT)

13

MOSFET -> MESFET -> HEMT

p

n+

n+

Drain

Source

Gat

e

EC

EVEF

p

n+

n+

Drain

Source

Gat

e

VG

EC

EV

EF

qVG

MOSFET

14

MOSFET -> MESFET -> HEMT

Depletion region

SI-S

ub

stra

te

n

n+

n+

Drain

Source

Gat

e

EC

EF

n+

n+

Drain

Source

Gat

e

SI-S

ub

stra

te

VG

EC

EF

qVG

MESFET

15

MOSFET -> MESFET -> HEMT

n+

Drain

Source

Gat

e

Un

-do

ped

ECEF

2DEG

Depletion region

Drain

Source

Gat

e

Un

-do

ped

VG

EC

EF

qVG

HEMT

16

Metal Oxide Semiconductor

Similarities between HEMT and MOSFET

• Charges formed in a quantum well in conduction band at interface between narrow bandgap and wide bandgap material

17

-qVBS

qVGB >0

EC

Ei

EFp

EV

EFn

qB

qB

EFm

AlxGa1-xAs/GaAs heterojunction

• Lattice matched for all values of Aluminium mole fraction (x)– Usually limited to x < 0.3 due to introduction of

defects resulting in DX centres in the bandgap

• Bandgap of AlxGa1-xAs: 1.424+1.247x eV• Type 1 band alignment with EC = 0.79x eV

18

Variation of sheet charge concentration with gate bias

• The maximum value of ns for Al0.3Ga0.7As/GaAs– limited to 1.4x1012/cm2

– depends on EC

• Why not use other heterojunctions to get higher EC ?

19

Al0.3Ga0.7As/GaAs

Pseudomorphic HEMT• It is possible to grow good

quality heterostructures from materials with different lattice constants, provided the thickness of the grown layer does not exceed a certain critical value tc.

• If the grown layer is thinner than tc, its crystalline structure accommodates to that of the substrate material. a lattice deformation in the grown layer (pseudomorphic layer)

• PseudomorphicAlGaAs/InGaAs/GaAsheterostructures with In contents in the range of 15-25% were successfully grown on GaAs substrates and used in pseudomorphic HEMTs.

20

AlGaAs/InGaAs/GaAs Pseudomorphic HEMT

• Compared to AlGaAs/GaAs HEMT

– Larger EC larger ns larger gm

– Larger n larger gm

21

Properties of different heterojunctions

22

Advantages of GaN over GaAs

• Higher Eg operation at higher temperature• Higher Ebr Higher breakdown voltage• Higher vsat Higher saturation currents• AlGaN/GaN system attractive for HEMT devices• For Al0.3Ga0.7N/GaN

– EG 0.84 eV compared to 0.37 eV for Al0.3Ga0.7As/GaAs– EC 0.42 eV compared to 0.22 eV for Al0.3Ga0.7As/GaAs

23

Si GaAs 4H-SiC GaN

Eg(eV) 1.1 1.42 3.26 3.4

Ebr (MV/cm) 0.3 0.4 3.0 3.3

µn (cm2/V s) 1350 8500 700 2000

Vsat (107 cm/s) 1.0 1.0 2.0 2.5

EBR

TMAX

fMAX

NFMINIMAX

GaN

GaAsSi

24Adapted from Ph.D thesis of Behat Tridel , Technischen Universität Berlin, 2012.

High Power

Operation

High Frequency

Operation

GaN based devices! Why?

GaN based devices! Why?

25Adapted from Dr. Palacios , Associate Professor, Dept. of Electrical Engg. and Comp. Sci., MIT .

GaN based HEMTs: status

26

• AlGaN/GaN based HEMTs replacing Si-based devices for poweramplifier and power switching applications- wide signal bandwidth, multiband power amplifier, higher efficiency,

higher breakdown voltage, low Ron, reduced size and weight

• GaN-based multiband power amplifiers in production for bothcommercial and defense purposeLeading players : Cree, Analog devices, Infineon, RFMD

• High efficiency GaN-based power devices (600 V) are alreadycommercially availableLeading players : Panasonic, Infineon, Transphorm

• Besides AlGaN/GaN, AlInN/GaN and AlN/GaN based HEMTsare also emerging- thinner barrier layer, higher 2DEG, stable contacts, higher reliability

GaNPsp

GaNPsp

AlGaNPsp PPE

NGa

N

N

N

Psp

Ideal Tetrahedron Tetrahedron in GaN Wurzite structure

27

Polarization effect in GaN

GaNPsp

AlGaNPsp PPE ( ) ( )SP PE AlGaN SP GaNq P P P

13 21 1.5 10 cm

12 27 8 10sn cm

( )sq nE

GaNPsp

AlGaNPsp PPE

ECEF

2DEG(ns)

E 1 MV/cm at VG = 0 V

E 2.5 MV/cm at VG = VTh28

Polarization effect in AlGaN/GaN HEMT

HEMT operation

GaN

AlGaN

2DEG(Channel)

S DG

VTh can be extracted from (ID)0.5

vs. VGS curve

29

AlGaN/GaN HEMT devices

30

High Polarization induced 2DEG – High Current Density

High Breakdown Electric Field – High Drain Bias

High Peak Electron Velocity – High Speed/Frequency Operation

Output Power Density Pout > 10W/mm (in C – Band)*

Current Gain Cutoff Frequency, ft > 100 GHz#

*Y. F. Wu, et al, IEEE Trans. on Electron Devices, vol. 28, pp.586-590, 2001.

#V. Kumar, et al, IEEE Electron Device Lett., vol 23., pp 455-457, 2002.

AlGaN/GaN HEMTs have proven its high frequency and high power operation capability

Inverse piezoelectric effect

31

AlGaN

GaN

GateSource Drain

Electric

Field

StrainCharge

Separation

Piezoelectric

Inverse

Piezoelectric

Negative voltage applied

to turn-off the device

increases the electric field

across the barrier

Increased electric field

increases the tensile strain in

the barrier structural

breakdown

del Alamo and Joh, Microelectronics Reliability, 49, pp1200 – 1206, 2009.

Al0.83In0.17N/GaN HEMT devices

32

Al0.83In0.17N is lattice matched to GaN – Stress free barrier layer

Band gap about 4.6 eV for 83% Al mole fraction

Difference in spontaneous polarization is close to 2-3 times

higher than that for AlGaN with 30% Al mole fraction

Higher conduction band offset (1 eV)*F. Medjdoub et al., The open electrical and electronic journal., vol. 2, pp. 1–7, 2008.

Comparison with AlGaN/GaN HEMTs

Al0.83In0.17N/GaN HEMT devices

33

Parameter

(barrier thickness)

Al0.3Ga0.7N/GaN

(30nm)

Al0.83In0.17N/GaN

(10 nm)

2DEG concentration (cm-2) ~ 1.5 x 1013 2.5 x 1013

2DEG mobility (cm2V-1s-1 ) ~ 2000 ~1200

Conduction band offset (eV) 0.42 1

Barrier band-gap (eV) 4.03 4.6

Surface potential (eV) ~ 1.5 ~ 0.5

Lower surface potential Thinner barrier Higher gm

(Little) Additional tensile strain

Enhanced carrier concentration

Demonstrated excellent thermal stability

Al0.83In0.17N/GaN AlN/GaN: Why?

34

a) Al0.83In0.17N is lattice matched to GaN stress free barrier layer

No/less inverse piezoelectric effect

b) Stable ohmic contact up to 900oC higher temperature operation

AlInN/GaN and AlN/GaN also enjoys:

c) Higher spontaneous polarization than conventional AlGaN higher 2DEG density (>1013 cm-3)

d) Thinner barrier layer better gate control

higher Id, gm, fT

e) Higher conduction band offset better 2DEG confinement

AlGaN

GaN

GateSource Drain

For AlInN/GaN

Al.83In.17N/GaN AlN/GaN HEMTs: Gate leakage issues

35

Schottky barrier gate in HEMTs High gate leakage

increased off-power dissipation

limited positive gate voltage swing

low current drive

35

AlGaN/GaN HEMT

AlInN/GaN HEMT

These issues are more severe inAlInN and AlN compared to AlGaN

o due to higher polarization charge

hence, higher electric field

o higher gate leakage current due toFN tunneling

S. Turuvekere et al., IEEE Trans. Electron Devices,

vol. 60, p. 3157, 2013

Solution: MIS-HEMTs

GaN based MIS-HEMTs

36

Gate dielectrics tried : Al2O3, HfO2, TiO2, SiO2, SiNx

Al2O3 found to be very promising- techniques: ALD, MOCVD, thermal oxidation of Al

MIS-HEMT characteristics:

significant decrease in gate leakage

higher gate over-drive

negative shift in VTh compared to HEMT

This negative shift is not only due to reduced gate capacitance,but due to large positive charge density (>1013 cm-2) atoxide/AlGaN interface*

Ga-O or Al-O bond at the interface is reported to beresponsible for this positive interface charge*

* S. Ganguly et al., Appl. Phys. Lett., V 99, p. 193504, 2011

Enhancement mode devices – Why?

37

simplifies circuit design - only positive power supply required

no/insignificant off-state leakage current reduced power loss increased efficiency smaller heat sink

(compact size, reduced cost)

fail safe operation (device remains in OFF state, even if gate supply fails)

complementary FET based logic applications

Earlier attempts

F-ion implantation, gate recessing, p-(Al)GaN cap layer

have several reliability and process related issues:

stability of VTh (over temperature, time, VG swing)

control of positive interface charge or donor states – more negative VTh

limited positive VTh

MIS-HEMTs using RIS-Al2O3

38

Reactive-Ion-Sputtered (RIS) Al2O3 has been reported to havestable negative fixed oxide charge in itmay be useful for reducing interfacial positive charge

• Room temperature deposited oxide is self-aligned with gate metal

• Ohmic contact resistance 1.1 Ω. mm (Ti/Al/Ni/Au 780o C) *Feb. 2014

• Lg = 4 µm, Lsd = 30 µm, Lgd = 20 µm (Gen-1 devices) *Feb. 2014

MIS-HEMTs using RIS-Al2O3

39

Deposited RIS-Al2O3

XPS analysis confirms theformation of Al2O3

slow deposition rate (~1nm/min) better tox control

room temp. deposition1. self-alignment of insulator and gate metal

2. more useful for recessed-gate MIS-HEMT

Comparison of gate leakage:

Significant reduction of leakagecurrent with 7 nm of Al2O3

G. Dutta et al., IEEE Electron Device Letters, V 35 (11), p 1085, 2014

Transfer characteristics

40

• HEMT VTh = – 7 VMIS-HEMT VTh = – 5.5 V- positive shift in VTh

• same ID for same (VGS-VTh)

• low ID-VGS hysteresis – less trapping

• Ion/Ioff ratio improves by ~ 102 times

• increase in gm for MIS-HEMT– due to increase in channel mobility

G. Dutta et al., IEEE Electron Device Letters, vol. 35 (11), p 1085, 2014

Reason for positive shift in VTh

41

1st level modelling of VTh:

Qf < Qsp2 will result positive shift in VTh

Present case, Qf = 1.2 × 1013 cm-2 and Qsp2 = 2.1 × 1013 cm-2

- net negative interface charge (~ 9 × 1012 cm-2) results inpositive shift in VTh

1

_

Qp

Th HEMT b

AlInN

VC

_

2 1Q Q Qf sp p

Th b

ox AlIn

M

N

IS HEMTVC C

2 -sp f

x

h

o

TVQ Q

C

Prospect for Enhancement mode device

42

GaNAlI

nNAl2O3Metal

Thicker oxide Thinner barrier

Threshold voltage can be adjusted by controlling oxide and barrier layer thicknesses

Possibility of achieving normally-off (E-mode) devices using thicker oxide / thinner barrier layer

AlGaN/GaN and AlInN/GaN MIS-HEMTs

43

AlInN/GaN AlGaN/GaN

Device dimensions: Lg = 1.5 µm, Lsd = 6 µm and Lgd = 3 µm (Gen-2)

Contact resistance: AlInN/GaN 0.4 - 0.5 Ω. mm AlGaN/GaN 0.5 – 0.6 Ω. mm

Non-recessed MIS-HEMTs

Recessed gateMIS-HEMTs

MIS-HEMT transfer characteristics

44

Non-RecessedMIS-HEMTs

Recessed gateMIS-HEMTs

AlInN/GaN AlGaN/GaN

• AlInN/GaN MIS-HEMTs showed greater positive shift in VTh

• Quasi normally-off AlInN/GaN MIS-HEMT with RIS-Al2O3

• Possibility of E- mode with optimized layer thicknesses

MIS-HEMT output characteristics

45

AlGaN/GaN

AlInN/GaN

Maximum ID of 800 mA/mm achieved: Comparable to best in literature for devices of similar geometryG. Dutta et al., IEEE Trans. Electron Devices, vol. 63, no. 4, pp. 1480–1488, Apr. 2016

Enhancement mode MIS-HEMT realized!

46G. Dutta et al., IEEE Trans. Electron Devices, vol. 63, no. 4, pp. 1480–1488, Apr. 2016

Enhancement mode devices achieved for the first time using RIS-Al203

Evolution of models for HEMTs from MOSFET models

QM effect in MOSFETs – Why & What?

• Scaling of device dimensions– Lower oxide thickness (< 2 nm)

– Higher doping conc. (> 1018 /cm3)

– Higher electric field

• Carriers are confined in a short distance from the Si/SiO2

interface (2-Dimensional Electron Gas or 2-DEG)– Creation of sub-bands

• Classical theory no longer sufficient for modelling

48

What is the result of QM effect?

• QM effects result in

– Increased effective bandgap

– Reduced inversion layer charge density

– Modified inversion layer charge profile

– Increased effective oxide thickness

– Reduced gate capacitance

– Modified surface potential

– Increased threshold voltage

– Modified carrier mobility

– Lower drain current

– Modified gate current

G.S. Jayadeva & A. DasGupta, “Quantum mechanical effects in bulk MOSFETs from a compact modelling perspective: A review”, IETE Technical Review, vol. 29, no.1, pp. 3 – 28, Jan-Feb 2012

49

Inversion charge concentration profile

• Due to QM effects, peak carrier concentration occurs away from Si/SiO2 interface– The depth is more for holes than electrons

a: classicalb: quantum mechanical

A.P. Gnädinger and H.E. Talley, “Quantum mechanical calculation of the carrier distribution and thickness of inversion layer of a MOS field-effect transistor”, Solid-State Electron, vol. 13, pp. 1301-1309, 1970

C.Y. Hu, S. Banerjee, K. Sadra, B.G. Streetman and R. Sivan, “Quantization effects in inversion layers of PMOSFET’s on Si (100) substrates”, IEEE Electron Dev. Letter, Vol. 17, pp. 276-278, 199650

Calculation of inversion charge

• Consider formation of sub-bands

• Solve Schrödinger and Poisson equations self-consistently– computationally very

intensive

– not suitable for compact models

– benchmark for simpler models

F. Stern and W.E. Howard, “Properties of semiconductor surface inversion layer in the electric quantum limit”, Phys. Rev., vol. 163, pp. 816-835, 1967. 51

Self Consistent Poisson-Schrödinger equation solution

52

• Poisson equation ( polysilicon, oxide and silicon region )

– Boundary condition : at the gate contact and

at the bulk

• Schrodinger equation ( dielectric and silicon regions )

– Boundary condition : at the gate dielectric interface

• Electron concentration

ij 0 2

ij ij ij*di

h d 1 dV(z) (z) E (z)

2 dz dzm

2F ij*Bi di ij2

i j B

E Ek Tn(z) g m ln 1 exp

k T

D A

d d (z)(z) q N (z) N (z) n(z) p(z)

dz dz

G(z) V (z) 0

Flowchart for self-consistent solution

53

Triangular potential well approximation• Constant electric field

• Standard form of solution of Schrödinger equation involving Airy functions

• Simplifies self-consistent solution, although iteration is not avoided

• Very good match obtained with exact solution

54

T. Janik and B. Majkusiak, “Analysis of the MOS transistor based on the self-consistent solution to the Schrodinger and Poisson equations and on the local mobility model”, IEEE Trans. Electron Devices, vol. 45, pp. 1263-71, 1998

Drain current model of intrinsic HEMT

• First step– Derive expression for

electron density in channel (ns) vs Gate voltage (Vg)

– Result of self-consistent solution of Poisson and Schrodinger equations

– Expression too complicated and not integrable

– However, ns for a particular Vg can be easily calculated

55

𝑛𝑠𝑎𝑡

=𝐶𝑔𝑉𝑔𝑚1

𝑞

𝑉𝑔𝑚1 + 𝑉𝑡ℎ 1 − 𝑙𝑛 𝛽𝑉𝑔𝑜1 − 𝛼1𝛾03

𝐶𝑔𝑉𝑔𝑚1

𝑞

𝑉𝑔𝑚1 1 +𝑉𝑡ℎ𝑉𝑔𝑜2

𝛼1 +2𝛾03 𝛼1

𝐶𝑔𝑉𝑔𝑚1

𝑞

N. Karumuri, S. Turuvekere, N. DasGupta and A. DasGupta, “A continuous analytical model for 2-DEG charge density in AlGaN/GaN HEMTs valid for all bias voltages,” IEEE Trans. Electron Devices, vol. 61, no. 7, pp. 2343-2349, July 2014.

Core Drain current model of intrinsic HEMT

Based on charge linearization

56

N. Karumuri, G. Dutta, N. DasGupta and A. DasGupta, “A compact model of drain current for GaN HEMTs based on 2DEG charge linearization,” IEEE Trans. Electron Devices, vol. 63, no. 11, pp. 4226-4232, Nov. 2016.

Add secondary effects

• Field dependent mobility– Effect of both lateral and transverse field

• Velocity saturation of electrons

• Parasitic conduction in barrier layer

• Channel length modulation (CLM)

• Drain Induced barrier lowering (DIBL)

• Self-heating

• Capacitances – for small signal and transient simulations

57

Modeling Access regions• Access regions

modeled as HEMTs– Unlike resistors in most

models

• Experimental results show that access regions have HEMT like characteristics– Current saturates due

to velocity saturation of electrons

58

Model validation

59

Sapphire

GaN

Al0.83 In0.17 N

S G D

15 nm

1.5 µm1.5 µm

3 µm

Sapphire

GaN

Al0.15 Ga0.85N

S G D

25 nm

1 µm 1 µm1 µm

Model validation

60

Sapphire

GaN

Al0.5 Ga0.5N

S G D

18 nm

1 µm 0.7 µm 1 µm

SiC

GaN

Al0.15 GaN

S G D

30 nm

1 µm 0.35 µm 3 µm

Summary &Conclusions• HEMTs have evolved from MOSFETs for high frequency (HF)

operation– Pseudomorphic HEMTs provide best HF performance

• GaN based HEMTs are best suited for high frequency power amplifiers and power switches– Depletion mode operation due to Schottky gate

• MIS-HEMTs can provide– Lower gate leakage– Higher gate swing as gate voltage can be positive

• Enhancement mode AlInN/GaN MIS-HEMTs with RIS-Al2O3demonstrated

• Model for GaN based HEMT developed• Further need for model and technology development• Lots of scope for research

61

THANK YOU

62