Modelling of Temperature Dependence on Current Collapse Phenomenon in AlGaN/GaN HEMT Devices

Ganesh S. Samudra, member IEEE, Yung C. Liang*, senior member IEEE, Yuling Li and Yee-Chia Yeo Department of Electrical and Computer Engineering, National University of Singapore,

Kent Ridge Crescent, Singapore 119260 *Contact author e-mail: chii@nus.edu.sg

Abstract— This paper reports the studies of the temperature dependence on the current collapse behaviours of AlGaN/GaN high electron mobility transistors (HEMTs). A physical-based model is proposed to analyse the trapping and de-trapping process along the surface with the effect of temperature included for the first time. The temperature-dependent gate leakage current is treated as the source for electron trapping and it can be predicted by the proposed model quantitatively. Then the relationship of the capture cross section of the surface trap on the electric field is investigated with respect to temperature variations. By applying the Poole-Frenkel emission mechanism, the dynamics of the trapped electrons at different temperatures are described in this model. The analytical results on current recovery time-constant are then verified by comparing with the laboratory measurement as well as the numerical results obtained from Sentaurus TCAD simulations.

I. INTRODUCTION There is presently a strong research interest in the area of

AlGaN/GaN high electron mobility transistors (HEMTs) and their applications in power electronics. This heterostructure HEMT system offers tremendous advantages due to wide band gap of GaN and formation of high density 2D electron gas (2DEG) at AlGaN/GaN interface. The specific advantages include high mobility leading to low on-state resistance, a large breakdown voltage due to high critical field in GaN and high temperature operation [1,2]. However there are several challenges that need to be addressed including those related to performance and reliability issues before these devices can be used in wider power electronic and high-frequency applications. One such phenomenon, known as current collapse, is of the drop in the HEMT current when it is turned-on after it has been turned off for some time with a large reverse voltage, which recovers very slowly [3]. Current collapse phenomenon causes detrimental effects on device transient performance, involving drain current reduction, transconductance-frequency dispersion and limited microwave output power. Some previous studies through experimental approaches showed that current collapse phenomenon has temperature dependence [4]. However, little device modelling work has been done on this aspect. In this paper, an analytical

model is developed to provide a clear perspective in understanding the temperature dependence of current collapse phenomenon in AlGaN/GaN HEMT power devices. The predictions by the model are compared to the measured data for validation and establishment of accuracy. With this model at hand, current collapse at various temperatures can be easily predicted and it can guide to resolve related issues through technology evolution and innovation.

II. MODELLING PROCESS AND LABORATORY VERIFICATION

Fig.1 shows AlGaN/GaN HEMT on sapphire substrate used in this modelling work as well as for the laboratory measurement. 2DEG is formed on the GaN side of AlGaN/GaN interface. The gate contact is on top of 10nm aluminium oxide insulator in the MIS gate structure. When a large negative gate voltage and reverse drain bias voltage are applied at the HEMT device, the electrons are injected into AlGaN surface layer through the gate leakage. They get captured at the AlGaN surface traps when hopping under the high electric field stress at off-state. The injected electron density n can be modelled by (1) where C is the fitting constant, Ey is the vertical electric field within the gate insulator, ϕt is the potential barrier for electron emission referenced to the metal Fermi level [5,6]. This equation is based on the standard theory of thermionic injection over the barrier. The electric field values were obtained by Sentaurus simulations for a standard GaN power HEMT with MIS gate. The injected gate current can be obtained by I = nqSv(T) where S is the gate effective area and v(T) is the temperature-dependent electron velocity. Fig. 2 shows the comparison of the leakage current obtained from the model and measurement. To describe the dynamics of the trapped charges, the rate equation (2) is applied, where NT is the density of surface trap occupied with electrons, NDT

+(0) is the total ionised surface trap density. Considering the Poole-Frenkel assisted emission mechanism [7], the capture time constant and emission time constant can be described as in (3) and (4), where A is trap cross section and kTEq //3 επα = . The enhanced cross section σenh is affected by both electric field and temperature ((5) and Fig. 3). Thus, the spatial distribution of the trapped electrons under steady state can be obtained using the relationship of (6) with results shown in We acknowledge the funding support from A*STAR Singapore under

the GaN-on-Si TSRP Grant number 1021690127 for this work.

978-1-4799-1194-3/13/$31.00 ©2013 IEEE 139

Fig. 4. To verify the analytical model, transient measurements (Fig. 5) were carried out at device turn-on at different device temperatures, with emission time constant of the surface traps extracted. The measurements were performed by using Agilent power semiconductor parametric analyser B1505A probing at wafer level without packaging. Sentaurus TCAD transient simulations were also performed to get the emission rate in de-trapping process. Fig. 6 shows good consistency of the emission time constants, namely data obtained from the analytical model, from the Sentaurus TCAD simulations and from laboratory measurements. Fig.7 shows the device I-V characteristics with and without current collapse from model calculation/simulations and laboratory measurement with temperature variations. In the case without current collapse, the device was directly measured under the condition of Vg=0V and Vd=2V. When the current collapse is considered, the device was first placed in the stressed conditions, i.e. Vg=−12V and Vd=25V for 2s, followed by the measurement with Vg=0V and Vd=2V. A good agreement was obtained between the model prediction and the laboratory measurement.

)))(/((

(exp 32

kT

TqEqCn OAlyt πεϕ −

−⋅= (1)

e

T

c

TDTT NNNnt

Nττ

−−

=∂

∂ + ))0(( (2)

)(exp)0(

10 α

στ −=

kTE

NvA

DTthenh

nc (3)

)exp(21 ατ −= −−

kTETA A

e (4)

⎥⎦

⎤⎢⎣

⎡

⎭⎬⎫

⎩⎨⎧ −−++=

21)exp()1(11 2

0

ααασσ n

enhn

(5)

)0(1 ++

=

DTc

e

c

e

T

Nn

n

N

ττ

ττ

(6)

(a) (b)

Fig.1 AlGaN/GaN HEMT device structure (a) device cross section with dimensions indicated on the diagram; (b)

microscopy photo of the fabricated device.

III. CONCLUSION We have developed a useful analytical model on current collapse based on the gate electric field that encompasses transient behaviours under current collapse at various device temperatures. Both the gate leakage predictions and emission time constants predicted by the model match with the measurements clearly establishing validity of the model.

Fig.2 Comparison of gate leakage current obtained from the

model and measurement shows good agreement. The measurement conditions: Vg=−10V and Vd=20V at the

temperature range from 300K to 450K.

Fig.3 The relationship between capture cross section and

electric field at different temperatures

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Fig.4 The spatial distribution of trapped electron density from

the gate edge at the drain side at different temperatures.

Fig.5 Laboratory measurements under stressed conditions at

different device temperatures

Fig.6 Comparison on the emission time constants obtained

from the analytical model, Sentaurus simulations and laboratory measurement at different temperature.

(a)

(b)

141

(c)

(d)

Fig.7 The comparison between the model prediction (lines) and the laboratory measurement data (solid rounded and

triangular dots) on the Id-Vd vharacteristics without current collapse (black) and with current collapse (red) at various

temperatures between 300K and 450K.

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[3] H. Hasegawa, T. Inagaki, S. Ootomo, and T. Hashizume, “Mechnisms of current collapse and gate leakage currents in AlGaN/GaN heterostructure field effect transistors,” J. Vac. Sci. Technol. B, vol. 21, no. 4, pp.1844-1855, Jul./Aug. 2003.

[4] A. G. Hemts, et al., "Slow Detrapping Transients due to Gate and Drain Bias Stress in High Breakdown Voltage," IEEE T-ED, vol. 59, pp. 2115-2122, 2012.

[5] O. Mitrofanov, "Poole-Frenkel electron emission from the traps in AlGaN/GaN transistors," J. of Applied Physics, vol. 95, pp. 6414-6414, 2004.

[6] Y. Chang, et al., "A thermal model for static current characteristics of AlGaN⁄GaN high electron mobility transistors including self-heating effect," J. of Applied Physics, vol. 99, pp. 044501-044501, 2006.

[7] O. Mitrofanov and M. Manfra, "Mechanisms of gate lag in GaN/AlGaN/GaN high electron mobility transistors," Superlattices and Microstructures, vol. 34, pp. 33-53, 2003.

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