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SEMICONDUCTOR POWER DEVICES PROJECT - K. MATOCHA 1

AlGaN/GaN HFET Design for SwitchingApplications

K. Matocha

Semiconductor Power Devices Project

Rensselaer Polytechnic Institute, Troy, NY USA

I. INTRODUCTION

GALLIUM NITRIDE based heterojunction field-effect tran-sistors (HFETs) show great promise for high-frequency,high-power, and high-temperature applications. Many re-

searchers have fabricated AlGaN/GaN HFETs with very impres-

sive results, including a device with a current handling capa-

bility of 1.25 A/mm on SiC substrates[2]. Assuming a sheet

charge density of 1.41013 cm2, and a saturation velocity of1107 cm/s, the maximum possible current is 2.25 A/mm. The

large current handling capability of AlGaN/GaN HFETS is a re-sult of the sheet charge densities, an order of magnitude larger

than in AlGaAs/GaAs HEMTs.

The origin of this charge has been attributed to polarization

charge effects, even described as piezoelectric doping[3].

This report examines the effects of this polarization charge,

particularly how the polarization charge can affect the break-

down voltage of high-power AlGaN/GaN HEMTs. Novel Al-

GaN/GaN HFET structures for switching applications are de-

scribed and simulated.

I I . POLARIZATION CHARGE

Gallium nitride and aluminum nitride both possess the

wurtzite crystal structure also known as the hexagonal close-packed or 2H structure. The crystal consists of alternating Ga

and N atomic layers, thus the surface of an epitaxially grown

layer in the [0001] direction can terminate with either Ga or N

atoms, depending on the initial growth conditions.

Because of symmetry considerations, diamond cubic and

zinc-blende structures cannot possess polarization charges.

However, the wurtzite structure can be polarized, having a

dipole across the crystal in the [0001] direction. The sponta-

neous polarization of several wurtzite materials is shown in Ta-

ble I[1]. This spontaneous polarization varies with temperature

and is known as pyroelectric polarization. Similarly, the po-

larization charge changes with lattice strain and is described as

piezoelectric polarization. Typical epitaxial growth conditionsof GaN result in Ga-face material. Ga-face material has a dipole

with the electric field in the [0001] direction (positive charge on

the N-face, negative charge on the Ga-face).

To understand the effects of the polarization charge, one must

consider the charge components in the HFET structure (Fig-

ure 1). Solving for charge neutrality,

QsQpol1 +Qdoping +Qpol1qnsQpol2 +Qpol2 = 0 (1)

where Qs is the surface trapped charge, Qdoping is the ionizedimpurities in the AlGaN barrier layer, Qpol1 is the AlGaN po-larization charge and Qpol2 is the GaN polarization charge, and

TABLE I

SPONTANEOUS POLARIZATION CHARGE DENSITY OF SEVERAL WURTZITE

MATERIALS.

Material Polarization charge density (q/cm2)

AlN -5.11013

GaN -1.81013

InN -2.01013

ZnO -3.61013

BeO -2.81013

2D-EG

Qs

-Qpol1+Qpol1

Qdoping

q*ns

-Qpol2 +Qpol2

Fig. 1. Charge components in AlGaN/GaN heterojunction FETs.

ns is the two-dimensional electron-gas (2DEG) charge density.The polarization charge components provide no net charge, thus

simplifying,

qns = Qs + Qdoping

it it seen that the solely the ionized surface states and the AlGaN

barrier doping control the 2DEG sheet density. The notion of

piezoelectric doping is not justified.

The polarization charge can create a high electric field in the

AlGaN layer. For example, in GaN, the polarization-induced

field is given as,

E=Q

ks0=

q 1.8 1013

9.5 0= 3.4 MV/cm

This value is approximately the same as the critical field in

GaN. In actual samples, this field is reduced by surface trapped

charges and ionized impurity charges.

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2 SEMICONDUCTOR POWER DEVICES PROJECT - K. MATOCHA

0 20 40 60 80 1000

1

2

3

4

5

6x 10

13

Piezoelectric

Spontaneous

Total

% Al in AlGaN (%)

Polarizationcharg

e(Q/cm

2)(estimated))

Fig. 2. Estimated spontaneous and piezoelectric polarization charges in theAlGaN layer of AlGaN/GaN heterojunction FETs.

For the HEMT structure in the off-state, Equation 1 simplifies

to:

0 = Qs + Qdoping

where Qdoping is always positive, so Qs must be modified to be-come a negative charge. In the off-state, the top AlGaN surface

contains a negative charge equal to Qs + Qpol1 , creating a veryhigh polarization-induced field in the AlGaN barrier layer.

Estimates of the the polarization charges (both spontaneous

and piezoelectric) assume a linear dependence upon the Al mole

fraction (x) of the AlxGa1xN layer[4] as shown in Figure 2.The polarization-induced field is in the [0001] or vertical

direction of the HFET structure. For power HFETs, the voltage

is blocked in the lateral direction. The superposition of the two

fields can be performed by the Pythagorean theorem. Solvingfor the lateral field,

Ey,max =

E2c

Qpol1

kAlGaN0

2(2)

where Ec is the AlxGa1xN critical electric field and Ey,max isthe maximum lateral electric field. The critical electric field is

estimated as a power law function of the bandgap energy

EcAlxGa1xN = EcSi

EGAlxGa1xN

EGSi

2

and is shown in Figure 3. The maximum lateral field is reducedfrom the ideal critical field of the semiconductor by the pre-

sense of the polarization charge, with a maximum of 3 MV/cm

at about 10% Al mole fraction (Figure 4.

III. SHEET CHARGE DENSITY

For AlGaN/GaN HFETs, very high sheet charge densities

(ns = 1.4 1013 cm2) have been achieved[2]. These sheet

charge densities can be achieved because of the large conduc-

tion band offset between AlGaN and GaN. For power devices,

a normally-off device is desirable, that is there is no conduc-

tion between source and drain with the gate terminal grounded.

0 20 40 60 80 1002

3

4

5

6

7

8

9

10x 10

6

% Al in AlGaN (%)CriticalBreakdownField

(MV/cm)(estimatedfromEG

)

Fig. 3. Estimated critical electric field of AlxGa1xN.

0 20 40 60 80 1000

2

4

6

8

10

12

SchottkyUF

HFETUCSB

HFETUCSB Ey,max= sqrt(EC2

(Qpol

/epsAlGaN

)2)

(Qpol

/epsAlGaN

)

EC

% Al in AlGaN (%)

ElectricField(MV/cm)

Fig. 4. Maximum lateral electric field (solid), critical electric field (dashed),and polarization field (dotted) as a function of Al percent in AlxGa1xN

along with experimental AlGaN device characteristics.

The AlGaN barrier layer thickness can be adjusted to control the

sheet carrier density. The HFET band-diagram is useful for un-

derstanding the relationship of the thickness of the barrier layer

to the surface sheet charge concentration (Figure 5).

Assuming an undoped AlGaN barrier, the electric field in the

AlGaN layer is constant, caused by the polarization charges. As

shown in Figure 5, the energy band at the Schottky barrier is

pinned by the metal-semiconductor work-function difference, so

the thin barrier has no sheet charge in the 2DEG in equilibrium.However as the barrier layer thickness is increased, the conduc-

tion band dips below the Fermi level, forming a 2DEG at the

AlGaN/GaN interface. Similarly, in the drift region, the surface

states pin the surface potential with a thick barrier, but remain

un-ionized with the thin barrier.

Considering the undoped barrier case, the critical thickness,

tcrit, for zero 2DEG charge is given by[5]

tcrit = (ED EC)AlGaNqQpol1

(3)

where ED is equal to MS under the gate and EC Etrap un-

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2D-EG

2D-EG

Thin AlGaN Thick AlGaN

Gate region

Drift region

Thick AlGaN Thin AlGaN

-m s -m s

Fig. 5. Band structure of AlGaN/GaN barrier with varying barrier thickness.

0 20 40 60 80 1000

50

100

150

200

250

300

350

400

450

500

% Al in AlGaN (%)

Criticalthickness

(A)

Fig. 6. Barrier critical thickness for zero 2DEG charge as a function of Alpercentage in AlGaN barrier layer (ED=1.6 eV).

der the drift region, and EC is the conduction band offset atthe AlGaN/GaN interface. The critical thickness with at 1.6 V

barrier height is shown in Figure 6. Thin barriers (< 10 nm) arerequired for normally off devices.

For amplifying devices, high-sheet charge densities are desir-

able, so thick barriers are used. As seen in Figure 7, the sheet

charge density increases with Al percent in the AlGaN barrier

layer.

Normally off AlGaN/GaN HFETs have been fabricated with

a single thin AlGaN layer, but these devices suffer from a large

series resistance between the gate-drain and gate-source[7] sinceno 2DEG exists in those regions. This problem is alleviated by

a recessed gate structure. In this structure (Figure 8), the gate

recess provides for normally-off operation, yet the thick AlGaN

between the gate-drain and gate-source develop a 2DEG under

these regions, reducing the parasitic resistance.

Another HFET structure that is promising is the MOSHFET

structure which uses an oxide layer on top of the gate region to

reduce the gate leakage currents[6]. For normally-off operation,

a similar gate-recessed structure can be fabricated (Figure 9).

The gate oxide can be used as a mask during selective epitaxy

of the thick AlGaN regions.

0 200 400 600 800 10000

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

2x 10

13

Al0.15

Ga0.85

N

Al0.25

Ga0.75

N

Al0.35

Ga0.65

N

AlGaN thickness (Angstroms)

Sheetchargedensity(q/cm

2)

Fig. 7. Sheet charge density as a function of barrier thickness for several Al

mole fractions in the AlGaN barrier layer (ED=1.6 eV).

Al Ga NG

S D

n-GaN2D-EG2D-EGx1-x

Sapphire

Fig. 8. The recessed-gate AlGaN/GaN HFET structure provides reduced Gate-

Drain and Gate-Source resistances while maintaining normally-off opera-tion.

IV. DEVICE SIMUL ATIONS

The two recessed gate structures described in the previous

section were simulated using a two-dimensional finite elementsolver (MEDICI by Avant!). The gate length is 1 m, with 1 mgate-source and gate-drain spacing. The Al mole fraction of

the AlGaN barrier layer is 34%. The AlGaN layer thickness is

1 nm. The surface trap density is 11013 cm2 located 1.67 eVbelow the conduction band. For the recessed-gate MOSHFET

simulated, the oxide thickness is 4 nm.

The drain characteristics of the recessed gate HFET are shown

in Figure 10. The device is normally-off, and the threshold volt-

age is 0.15 V. The gate swing is limited by the gate conduction,

whose characteristics are shown in Figure 11.

The MOSHFET does not allow significant gate current be-

cause of the insulating oxide layer. The MOSHFET structure

Al Ga N

S D

n-GaN2D-EG2D-EG

G insulator

x1-x

Sapphire

Fig. 9. The recessed-gate AlGaN/GaN MOSHFET structure provides reducedGate-Drain and Gate-Source resistances and reduced gate leakage current

while maintaining normally-off operation .

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4 SEMICONDUCTOR POWER DEVICES PROJECT - K. MATOCHA

GaN HFET - Recessed Gate - Drain char.

0.00 0.10 0.20 0.30 0.40 0.50 0.60 0.70 0.80 0.90 1.00

v(drain)(Volts)

0.

00

2.

00

4.

00

6.

00

8.

00

i(dra

in)

(Amps

/um

)

*10^-

5

Vg=0.0,0.1

Vg=0.2

Vg=0.3

Vg=0.4

Fig. 10. Drain characteristics of 1m gate length AlGaN/GaN HFET.

GaN HFET - Recessed Gate - Gate current

0.00 0.10 0.20 0.30 0.40 0.50 0.60 0.70 0.80 0.90 1.00

v(gate)(Volts)

0.

00

0.

25

0.

50

0.

75

1.

00

1

.25

1.

50

i(gate

)(Amps

/um

)

*10^-

7

Fig. 11. Turn-on characteristics of Schottky gate diode of recessed-gate Al-GaN/GaN HFET.

provides for an increased gate swing to provide a higher current

handling capability, with a corresponding reduction in transcon-

ductance (Figure 12). The recessed-gate MOSHFET is normally

off with a threshold voltage of 0.25 V.

The breakdown voltage of the recessed-gate MOSHFET is

less than 50 Volts, when the device should ideally block 300 V

with a 1 m gate-drain spacing. The electric field profile (Fig-ure 13) in the AlGaN layer (VG = 0, VDS = 50 shows a peak inthe electric field at the drain side of the gate. This device would

not be able to support 50 Volts, since the electric field is sim-

ulated to be above 10 MV/cm at 50 Volts drain bias. This is a

direct result of the polarization charge that exists in the AlGaNlayer. In the drift regions, the electric field is low, because the

2DEG is present in those regions.

V. SUMMARY

The polarization charge does play a role in the surface charge

densities found in AlGaN/GaN heterostructures. However, the

polarization charge does not act as a dopant, but only modifies

the field in the barrier layer, leading to an increase in the ion-

ization of surface states. In turn, the ionization of surface states

leads to an increase in the sheet charge densities in the 2DEG.

This polarization-induced field reduces the lateral voltage han-

GaN MOSHFET - Recessed Gate - Drain Char.

0.00 0.10 0.20 0.30 0.40 0.50 0.60 0.70 0.80 0.90 1.00

v(drain)(Volts)

0.

00

0.

50

1.

00

1.

50

2.

00

2.

50

i(dra

in)

(Amps

/um

)

*10^-

4

Vg=0

Vg=0.5

Vg=1

Vg=2

Vg=3

Vg=4

Fig. 12. Drain characteristics of 1 m gate length AlGaN/GaN MOSHFET.

Traps=1e13, V=50

1.00 1.50 2.00 2.50 3.00 3.50 4.00

Distance (Microns)

0.

00

0.

20

0.

40

0.

60

0.

80

1.

00

1.

20

Electr

ic

fie

ld

(V

/cm

)

*10^7

Fig. 13. AlGaN/GaN recessed-gate HFET electric field profile in the AlGaNbarrier layer with VG = 0 V, VDS = 50 V.

dling capabilities of the HFET structure.

Two device structures for power switching HFETs have been

proposed and simulated, a normally-off recessed gate HFET and

a normally-off recessed gate MOSHFET. The simulations con-

sider both polarization charge as well as ionized surface states.

These recessed gate devices provide a reduced series resistance

compared to planar normally-off HFETs. Similar to planar

HFETs, these structures suffer from less than ideal breakdown

voltages due to the effect of polarization charges.

V I . ACKNOWLEDGEMENTS

The author would like to thank Ken Chu for helpful dis-cussions on design tradeoffs of high-frequency AlGaN/GaN

HFETs.

REFERENCES

[1] F. Bernardini, V. Fiorentini, D. Vanderbilt, Spontaneous polarization andpiezoelecctric constatns of III-V nitrides, Phys. Rev. B 56, p 10024-7.

[2] M.A. Khan, J.W. Yang, W. Knap, E. Frayssinet, X. Hu, G. Simin, P. Prys-tawko, M. Leszczynski, I. Grzegory, S. Porowski, R. Gaska, M.S. Shur, B.Beaumont, M. Teisseire, G. Neu, GaN-AlGaN heterostructure field-effecttransistors over bulk GaN substrates, Appl. Phys. Letters 76, p. 3807-9.

[3] M.S. Shur, A.D. Bykhovski, R. Gaska, M.A. Khan, GaN-base Pyroelec-tronics and Piezeoelectronics - in press.

[4] E.T. Yu, X.Z. Dang. P.M. Asbeck, S.S. Lau, G.J. Sullivan, Spontaneous

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and piezoelectric polarization effects in III-V nitride heterostructures, J.Vac. Sci. Tech. B 17, p. 1742-9.

[5] J.P. Ibbetson, P.T. Fini, K.D. Ness, S.P. DenBaars, J.S. Speck, U.K. Mishra,Polarization effects, surface states, and the source of electrons in Al-GaN/GaN heterostructure field effect transistors, Appl. Phys. Letters 77,p. 250-3.

[6] M.A. Khan, X. Hu, A. Tarakji, G. Simin, J. Yang, R. Gaska, M.S. Shur,AlGaN/GaN metal-oxide-semiconductor heterostructure field-effect tran-sistors on SiC substrates, Appl. Phys. Letters 77, p. 1339-41.

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10.1.1.22.821[1]