GaN HEMTs employing energy-band modulation technology

Solid-State Electronics 80 (2013) 76–80

Contents lists available at SciVerse ScienceDirect

Solid-State Electronics journal homepage: www.elsevier.com/locate/sse

Analysis and reduction of the gate forward leakage current in AlGaN/GaN HEMTs employing energy-band modulation technology Wanjun Chen ⇑, Jing Zhang, Bo Zhang, Zhaoji Li The State Key Laboratory of Electronic Thin Films and Integrated Devices, University of Electronics Science and Technology of China (UESTC), Chengdu 610054, China

a r t i c l e

i n f o

Article history: Received 19 March 2012 Received in revised form 29 July 2012 Accepted 5 October 2012 Available online 21 December 2012 The review of this paper was arranged by Prof. E. Calleja Keywords: AlGaN/GaN HEMT Gate forward leakage current

a b s t r a c t The gate forward leakage current in AlGaN/GaN High Electron Mobility Transistors (HEMTs) is investigated. It is known that the gate forward leakage current reduces as effective electron barrier height (q/b) grows. Therefore, an energy-band modulation (EBM) technology using fluorine-plasma treatment or P-type doping is presented to introduce the negative fixed charges into AlGaN layer. The introduced negative fixed charges can modulate the conduction-band profile in AlGaN layer, resulting in higher effective electron barrier. An analytical model is proposed to illustrate the conduction-band profile. It is suggested that as introduced negative fixed charge concentration exceeds a critical value, an additional electron barrier (qD/b) is achieved, contributing to reducing the gate forward leakage current. Based on this theory, the fluorine-plasma treatment is implemented to carry out EBM technology in this work. Experimental results confirm that the additional electron barrier qD/b of 0.3 eV is obtained for the fluorine-plasma treatment condition of 60 W and 120 s. Thus, the gate forward leakage current is decreased by one order magnitude lower than that of HEMT device without fluorine-plasma treatment. The analytical results are in good agreement with numerical simulation and experiment results. Ó 2012 Elsevier Ltd. All rights reserved.

1. Introduction High Electron Mobility Transistors (HEMTs) based on the AlGaN/GaN material system have recently been a subject of intense investigation as attractive candidates for high temperature, highvoltage, and high-power operation [1–3]. Although the AlGaN/ GaN HEMTs have been made remarkable progress in recent years, resulting in unprecedented high performances, there are still several problems to be solved. One key problem is the gate leakage current. Recently, the mechanisms of the gate reverse leakage current have been intensively investigated and some methods have been proposed to suppress the gate reverse leakage current [4– 7]. However, there is little report on the mechanisms of gate forward leakage current and its solutions. In fact, large gate forward leakage current through the Schottky-gate is becoming another emerging obstacle for the developments and applications of the AlGaN/GaN HEMTs. Since, the device with this gate forward leakage current may suffer from high power dissipation, extra noise and reliability problems like device with gate reverse leakage current. What is more important is that the large gate forward leakage current may cause the wrong work operation with a negative drain current, though the device is biased in the forward conduction operation. It is especially serious for most of the devices operated ⇑ Corresponding author. Tel.: +86 28 83201693; fax: +86 28 83207120. E-mail address: [email protected] (W. Chen). 0038-1101/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.sse.2012.10.014

at the linear region in the application of the analog circuit and power switching. The gate forward leakage current is originated from the forward biased Schottky-gate, the transport mechanism of which may be the thermionic emission and tunneling that the electrons emit or tunnel from semiconductor to Schottky-metal, respectively [8– 10]. It is accepted that both transport mechanisms are positive exponential to the difference of applied voltage (V) and electron barrier height (q/b), i.e., I / exp [q(V  /b)] [11]. Consequently, the increase of q/b is one of the effective methods to suppress the gate forward leakage current in AlGaN/GaN HEMTs. In this paper, we focus on discussing the gate forward leakage current in AlGaN/GaN HEMT and presenting an energy-band modulation (EBM) technology to suppress it. According to EBM technology, the negative charges are introduced into AlGaN layer to modulate the conduction-band profile, resulting in higher effective electron barrier and lower gate forward leakage current. The EBM technology is thoroughly investigated by using a combination of theoretical analyses and numerical simulation, and the conclusions are supported by the experimental results.

2. Theoretical analysis In the metal–semiconductor (MS) contact with compound semiconductor of unintended doping or light doping, the MS barrier peak originates at the MS interface with an approximate fixed

77

W. Chen et al. / Solid-State Electronics 80 (2013) 76–80

barrier height due to surface pinning effect [11]. However, if some negative charges are introduced into the semiconductor, a shift of barrier peak location and a change of its peak values in the semiconductor will take place, rendering the conduction-band distribution fundamentally modified. In that sense, an analytical model of the conduction-band in the AlGaN layer is needed to determine the barrier peak location and value. In order to simplify the analyses and highlight the keypoints, the following assumptions are made: [12] (1) all the donor impurities are ionized in the depletion region; (2) the depletion approximation is valid, i.e., the mobile-carrier densities are negligible in the depletion region and charged region; (3) the band discontinuities are independent of the doping densities; (4) the unintended doped (UID) GaN bulk is P type and light doped AlGaN barrier layer is N type; (5) effective GaN buffer trap charge is neglected; (6) the fixed charge concentration in the AlGaN barrier layer is uniform. With these assumptions, the calculations are easy to perform and can readily be utilized. Fig. 1 presents the conduction-band diagram at the equilibrium state. For the conventional device with UID or light N type doping AlGaN layer, the MS barrier peak located at the MS interface as shown in Fig. 1 (solid line). However, taking into consideration of the introduced negative fixed charges in the AlGaN layer, the conduction-band profile in the AlGaN layer is modified and a potential maximum electron barrier height maybe obtained in the AlGaN layer (dash line). Under the assumption of depletion approximation, we can get that

dFðxÞ NF ¼ q  dx e

for 0 6 x 6 d

ð1Þ

will be compensated. The concentration (Nd) of UID AlGaN layer (1016 cm3) is more than two orders of magnitude smaller than introduced negative charge concentration (1018 cm3). Therefore, the approximation, NF  Nd  NF, is taken into account in Eqs. (1) and (2). The electric field F(x) and potential V(x) are then

FðxÞ ¼ q  VðxÞ ¼ q 

NF

e

x þ Fð0þ Þ

NF 2 x þ Fð0þ Þx þ Vð0þ Þ 2e

ð3Þ

ð4Þ

where F(0+) is the electric field at the AlGaN side of MS interface, V(0+) is the potential at the AlGaN side of MS interface. Thus, the expression of conduction-band energy in the AlGaN layer is as follows

EðxÞ ¼ qVðxÞ ¼ q2 

NF 2 x  q  Fð0þ Þx þ Eð0þ Þ 2e

ð5Þ

Due to the surface pinning effect dominated by the III–V semiconductor surface defect, the barrier height at the MS interface E(0+) is weakly influenced by the changes of introduced fixed charge density [13]. As a consequence, they share the same conduction band energy at the AlGaN surface, which is given by

Eð0þ Þ ¼ q/b0

ð6Þ

From the band diagram shown in Fig. 1, the conduction-band energy at the AlGaN side of heterojunction interface can be expressed based on electrostatic equilibrium 

Eðd Þ ¼ q/h0 ¼ DEc  jEFi j

ð7Þ



2

d VðxÞ 2

dx

¼q

NF

e

for 0 6 x 6 d

ð2Þ

where F(x) is the electric field, q is the electron charge, NF is the introduced negative charge concentration, e is the dielectric constant of AlGaN, d is thickness of AlGaN layer, V(x) is potential, x is the location for conduction-band calculation in the coordinate system with origin point at MS interface, as shown in Fig. 1. The positive direction for the integration is from metal to semiconductor. 0 6 x 6 d is the place where AlGaN layer exists. Due to the co-existence of the negative charge and donor doping in the AlGaN layer, the positive charges of the ionized donors

In the Eq. (7), E(d ) is energy at AlGaN side of heterojunction interface, DEc is the conduction-band offset at the AlGaN/GaN heterojunction, EFi is energy of the conduction-band edge with respect to the Fermi level in the GaN bulk (EF) at the GaN side of the heterojunction. The discussion about EFi is depicted in the Appendix. Due to the polarization effect in AlGaN layer, equivalent polarization charges are formed at both sides. In the absence of external electric fields, the total polarization of AlGaN/GaN heterostructure is the sum of the spontaneous polarization and the piezoelectric polarization. The total amount of polarization induced sheet charge density for AlGaN/GaN heterostructure is given as [14]

jrP ðmÞj ¼ jPsp ðAlm Ga1m NÞ  P sp ðGaNÞ þ Ppe ðAlm Ga1m NÞj

ð8Þ

where m is the aluminum mole fraction, Psp(AlmGa1mN), Psp(GaN) are the spontaneous polarization in the AlGaN and GaN layers respectively, Ppe(AlmGa1mN) is the piezoelectric polarization in AlGaN layer. As the negative fixed charges with the dose of NFd is implanted into the AlGaN layer, it is safe to assume that the compensate charges density at both boundaries is NFd/2. Consequently, the sheet charge density at two interfaces are modified into rp + NFd/2 and rp + NFd/2, respectively. Sheet charges exist at the MS interfaces and thus using Gauss’ law with electric field to be continuous. We obtain

Fð0þ Þ ¼ q  ðrp þ NF d=2Þ=e

ð9Þ

Substitute the boundary condition depicted in Eqs. (6) and (9) into Eq. (5). We obtain

EðxÞ ¼ q2

NF 2 rp þ NF d=2  x þ q2  x þ q/b0 2e e

ð10Þ

The introduced negative charges are potentially to introduce an additional electron barrier energy (qD/b) in the AlGaN layer. In that sense, the maximum conduction-band energy is Fig. 1. Conduction-band diagram for the AlGaN/GaN heterojunction without/with introduced negative fixed charges.

Emax ¼ q/b ¼ qð/b0 þ D/b Þ

ð11Þ

78

W. Chen et al. / Solid-State Electronics 80 (2013) 76–80

Fig. 2. Analytical and simulated results of the conduction-band diagram within AlGaN layer.

As shown in Eq. (10), the conduction-band energy approximates a quadratic function in respect to x with a maximum value point at x0 = d/2  rp/NF. The NF determines whether x0 is within the AlGaN layer. Thus, different additional electron barrier height (qD/b) and their positions can be approached in the following two scenarios:



D/b ¼

0 ðNF 6 2rp =dÞ Eðx0 Þ  Eð0þ Þ ðNF > 2rp =dÞ

ð12Þ

When the introduced charge concentration is below the critical value, i.e., N F 6 2rp =d, then x0 6 0. In this situation, the location of the conduction-band energy analytical maximum point x0 can be interpreted to be out of the AlGaN layer and falls into the metal side. The potential maximum energy value within AlGaN layer falls at the metal and AlGaN interface. The additional barrier energy is 0, rendering barrier peak equals to the barrier height of the MS contact. However, when the introduced charge concentration exceeds the critical value, i.e., NF > 2rp/d, then 2/d > x0 > 0. The maximum energy can be acquired at x0. Thus, the additional electron barrier energy is E(x0)  E(0+), enhancing the effective MS contact barrier height. 3. Simulation results The operating principle of EBM technology is illustrated by the conduction-band profiles simulated by the Sentaurus TCAD simulator and shown in Fig. 2. In this simulation, the negative fixed charges are implemented into AlGaN layer to modulate the conduction-band. The result shows that when the NF of 1  1018 cm3 is below the critical value (in this case, 1.8  1018 cm3), the barrier peak is at MS interface with value of

1 eV. As NF surpass that value, i.e., 3  1018 cm3 and 5  1018 cm3, the barrier peak can be achieved at NF = 2rp/d away from the AlGaN surface with an increased value, 1.1 eV and 1.4 eV, respectively. It indicates that the electron barrier height will increase with the increasing of NF; that is, the electrons must overcome more potential to reach 2DEG channel with the increasing of NF. Consequently, the gate forward leakage current is restrained. The analytical results are also shown in Fig. 2. It is shown that the simulation results are in good agreement with analytical models at different cases of NF. The negative fixed charges mentioned above can be carried out using fluorine-plasma treatment [15–17]. In fact, P-type doping or other technique is also available for the EBM technology which will potentially introduce negative fixed charges in the AlGaN layer. Fig. 3a and b shows conduction-band diagrams for EBM using the fluorine-plasma treatment and P-type doping, respectively. It is seen that both techniques can modulate the conduction-band profile, resulting in higher effective electron barrier. However, there still exists some tiny difference between these two techniques. The fluorine-plasma treatment is more effective in depleting electrons in the 2DEG and uplifts the electron barrier more significantly in comparison with P-type doping. This could be attributed to the incomplete ionization procedure of P type doping in AlGaN at room temperature. By comparison, fluorine-plasma treatment can be regarded as a stable atom embedded in the crystal inter-space, the stability of which is not interfered by the ultra electric field due to the strong electro-negativity of F [13,15].

4. Experimental results In our previous works or others, [15–18] the fluorine-plasma treatment has been used to carry out enhancement mode devices. In this work, the EBM technology is experimentally implemented by using the fluorine-plasma treatment to introduce the negative charges of F into AlGaN layer. The sample used in this work is a commercial Al0.26Ga0.74N/GaN HEMT wafer grown by metal organic chemical vapor deposition on (1 1 1) silicon substrate. The fabrication process is similar to those reported in Ref. [16] with the exception of different fluorine-plasma treatment process for this work. The specific Ohmic contact resistance for both drain and source contacts is 0.7 X mm. In this work, all the fabricated devices have LGS of 1.5 um, LGD of 1.5 um and LG of 1.5 um. Fig. 4a and b shows the I–V output characteristics for the fabricated HEMT device without/with fluorine-plasma treatment, respectively. In Fig. 4a, it is found that there is a negative drain current of about 200 mA/mm which is close to 25% of the maximum saturation drain current IDmax in control device (no treatment). Fig. 4b shows the I–V output characteristics of HEMT

Fig. 3. Conduction-band distributions for (a) fluorine-plasma treatment and (b) P-type doping.

79

W. Chen et al. / Solid-State Electronics 80 (2013) 76–80

Fig. 4. Measured I–V output characteristics for the fabricated HEMT devices, (a) without fluorine-plasma surface treatment and (b) with fluoride-plasma treatment.

with fluorine-plasma surface treatment process of 60 W and 120 s prior to gate metallization. As is concluded in the theoretical analysis, the negative drain current is sharply decreased to 20 mA/ mm, which is one order of magnitude lower than that of control device. The reason for the gate forward leakage current suppression can be partly attributed to the fluoride-plasma treatment. According to the thermionic emission theory for Schottky contact, the ideality factor should be around 1–2 [19]. In this case, however, the ideality factor of 5 at a temperature of 300 K exceeds its upper limitation, owing to the tunneling mechanism. Dislocation model based on tunneling is possible when the concentration of the trapping levels is sufficiently high. It assumes that a staircase path that consists of a series of tunneling transitions between trapping levels in the AlGaN layer [20]. The I–V relation for a Schottky contact based on the tunneling is given by

  q/b þ kT lnðNC =ND Þ I ¼ qSDmD  exp E0     qðV  IRÞ 1  exp E0

ð13Þ

where S is the Schottky contact area, D is the dislocation density, mD is the Debey frequency for the AlGaN layer, T is the absolute temperature, NC is the effective density of states in the conduction band, ND is the concentration of the ionized donors in the AlGaN  layer. E0 is the tunneling parameter defined as E0 ¼ E00 coth EkT00 , where E00 is the characteristic tunneling energy that is related to the tunnel effect transmission probability. V is the applied voltage, R is the series resistance, n is the ideality factor, k is Boltzmann constant. From Eq. (13), it is clear that the increase of the barrier height q/b is one of the effective methods to reduce the tunneling electron, further, to

suppress the gate forward current in AlGaN/GaN HEMT.  leakage When q(V  IR)  E0, exp qðVIRÞ  1. So, Eq. (13) can be rewritten E0 as

ln

-1

IG (mA/mm)

 q kT lnðNC =ND Þ V  IR  /b þ E0 q

VS=VD=0V -3

Δφb=0.3V

-4

10

slope=q/E0

-5

10

0.2

0.4 0.6 VG ( V )

(a)

Control device 60W 120s

400

10

0.0

¼

500

Control device 60W 120s

0.8

IG (mA/mm)

-2



300

VS=VD=0V

2.3V

200

1.4V

100 0

1.0

ð14Þ

Under the intermediate voltage region (0.1 V < V < 1 V) with gate bias smaller than MS contact barrier height (1 eV), the Schottky current is small. So, the term of IR can be neglected [9,21] and the logarithm value of I is proportional to applied voltage V with a fixed slope of q/E0. As can be seen from Fig. 5a, the slope is fixed in the intermediate voltage region, with tunneling parameter nearly unchanged, i.e., 0.13 eV for control device and 0.14 eV for fluoride-plasma treatment device, respectively. Meanwhile, a voltage intercept of /b  (kT/q) ln (NC/ND) is in positive linear with term /b. Thus, for device with fluorine-plasma treatment with 60 W and 120 s, IG is directly shifted in positive direction by 0.3 V compared with the control device, indicating that an extra electron barrier height qD/b of 0.3 eV is acquired due to the fact that the introduced F ions raise the conduction-band of AlGaN layer. According to Eq. (14), the higher q/b will suppress the electrons transported by mechanisms of tunneling, which results in lower gate forward leakage current. Fig. 5b shows the relationship between the gate current and gate voltage. As might be expected, the knee voltage (Vk) is increased from 1.4 V of control device to 2.3 V of the device with plasma treatment, which should be attributed to both the barrier height uplift and series resistance enhancement in the AlGaN layer after F treatment. So, the maximum IG is decreased by about one order of magnitude lower than that of control device. However, the saturation drain current (IDmax) for the device with treatment is decreased by 34% (from 850 mA/mm to 560 mA/mm).

10 10

I qSDmD

-3

-2

-1

0 VG ( V )

1

2

3

(b)

Fig. 5. Measured IG–VG relationship for the fabricated HEMT devices, (a) logarithm coordinate and (b) linear coordinate.

80

W. Chen et al. / Solid-State Electronics 80 (2013) 76–80

900

240

Gm (mS/mm)

ID (mA/mm)

180

600 450 300 150 0 -3

Control device 60W 120s

210

Control device 60W 120s

750

150 120 90 60 30

-2

-1 0 VGS ( V )

1

2

3

0 -3

-2

-1

(a)

0 1 VGS ( V )

2

3

(b)

Fig. 6. Measured transconductance for the fabricated HEMT devices.

This is due to the fact that the introduced F ions raise the conduction-band of AlGaN layer and partly deplete the 2DEG which results in that the threshold voltage is changed from 2 V for control device to 0.5 V for device with plasma treatment, as shown in Fig. 6a. However, transconductance characteristics for different devices, as shown in Fig. 6b, shows that the maximum transconductance (Gm) increased after plasma treatment. The higher Gm for the device with plasma treatment is attributed to the reduction in the gate forward leakage current. 5. Conclusion We focus on discussing the gate forward leakage current in AlGaN/GaN HEMT and presenting an energy-band modulation (EBM) technology to suppress it. The introduced negative fixed charges can cause an upward bending of the conduction-band in AlGaN layer which results in higher effective barrier. An analytical model of the effect barrier is also proposed to explain the effects of the EBM technology and this goes in accordance with the simulation result. Experimental results confirm that the maximum gate forward leakage current is decreased from 450 mA/mm for control device to 40 mA/mm for HEMT device with plasma treatment. This is due to the fact that an additional electron barrier height of 0.3 eV is achieved by the fluoride plasma treatment. The Experimental results are in good agreement with analytical results and numerical simulations. Acknowledgements The authors would like to thank Prof. Kevin J. Chen at Hong Kong University of Science and Technology (HKUST) for his supports and the fabrication of the this work. This work was supported by the National Natural Science Foundation of China (No. 60906037), the Fundamental Research Funds for the Central Universities (ZYGX2009J027) and the Research Fund for the Doctoral Program of Higher Education of China (No. 20090185120021). Appendix A EFi is a function of free electron concentration (N) and fixed charge density NF. Prior to the introduction of the negative charges, the equilibrium state of EFi is negative, featuring as deep degenerate state. As a consequence, 2DEG forms with a sheet density of

Rd ns ¼ 0 2 NðxÞdx at the hetero-structure interface. d2 is the two dimensional quantum well width. After the negative charges implementation, the compensate charges at the hetero-structure interface decrease the electron total sheet density by NFd/2. ConseRd quently, ns is modified into ns ¼ 0 2 NðxÞdx  N F d=2. As the NF increases, shrink procedure of ns turns the energy state from degenerate into non-degenerate, determined by space energy state density (NC): 2

EFi ¼ E0  p  h ns =mGaN

N > NC

EFi ¼ ðkT=qÞ lnðNC =NÞ N 6 NC

ð15Þ ð16Þ

mGaN

is the effective mass of electrons in GaN, E0 is the lowest sub-band level of ffi the 2DEG, which is given by pffiffiffiffiffiffiffiffiffiffiffiffiffi E0 ¼ ð9ph  q2 =8e0 8mGaN Þ  ðns =eGaN Þ. EFi increases from a negative value towards zero as N reduces. When N is less than NC it increases from zero as N shrinks in the non-degenerate state. References [1] Dora Y, Chakraborty A, McCarthy L, Keller S, DenBaars SP, Mishra UK. IEEE Electron Dev Lett 2006;27:713. [2] Tipirneni N, Koudymov A, Adivarahan V, Yang J, Simin G, Asif Khan M. IEEE Electron Dev Lett 2006;27:716. [3] Chen Wanjun, Wong KY, Chen Kevin J. IEEE Electron Dev Lett 2009;30:430. [4] Miller EJ, Dang XZ, Yu ET. J Appl Phys 2000;88:5951. [5] Shen L, Palacios T, Poblenz C, Corrion A, et al. IEEE Electron Dev Lett 2006;27:214. [6] Karmalkar Shreepad, Sathaiya DM. Appl Phys Lett 2003;82:3976. [7] Hashizume T, Kotani J, Hasegawa H. Appl Phys Lett 2004;84:4884. [8] Hasegawa H, Inagaki T, Ootomo S, Hashizume T. J Vac Sci Technol B 2003;21:1844. [9] Saadaoui S, Salem MMB, Gassoumi M, Maaref H, Gaquie‘re C. J Appl Phys 2011;110:013701. [10] Arslan E, Altında S, Özçelik S, Ozbay Ekmel. J Appl Phys 2009;105:023705. [11] Sze SM, Ng Kwok K. Physics of Semiconductor Devices. New York: Wiley; 1981. [12] Naresh C, Hadis M. Trans. IEEE Trans Electron Dev 1985;32:1064. [13] Huang S, Chen H, Chen Kevin J. Appl Phys Lett 2010;96:233510. [14] Ambacher O et al. J Appl Phys 1999;85(6):3222–33. [15] Cai Y, Zhou Y, Chen KJ, Lau KM. IEEE Electron Dev Lett 2005;26:435. [16] Chen Wanjun, Wong KY, Huang W, Chen KJ. Appl Phys Lett 2008;92. [17] Wanjun Chen, King-Yuen Wong, Kevin J. Chen. In: International Electron Devices Meeting (IEDM 2008), vol. 141. San Francisco; 2008. [18] Cai Yong, Zhou Yugang, Lau KM, Chen KJ. IEEE Trans Electron Dev 2006;53:2207. [19] Qiao D, Yu LS, Lau SS. J Appl Phys 2000;87:1880. [20] Engin A, Semsettin A, Suleyman O, Ekmel O. Semicond Sci Technol 2009;24:075003. [21] Chand S, Bala S. Semicond Appl Surf Sci 2005;252:358.