Microelectronics Reliability xxx (2013) xxx–xxx

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Microelectronics Reliability

journal homepage: www.elsevier .com/locate /microrel

Two-dimensional transient simulations of the self-heating effects in GaN-basedHEMTs

Yamin Zhang, Shiwei Feng ⇑, Hui Zhu, Jianwei Zhang, Bing DengSemiconductor Device Reliability Institute, College of Electronic Information and Control Engineering, Beijing University of Technology, Beijing 100124, China

a r t i c l e i n f o

Article history:Received 27 October 2012Received in revised form 17 January 2013Accepted 7 February 2013Available online xxxx

0026-2714/$ - see front matter � 2013 Elsevier Ltd. Ahttp://dx.doi.org/10.1016/j.microrel.2013.02.004

⇑ Corresponding author. Tel./fax: +86 010 6739653E-mail address: shwfeng@bjut.edu.cn (Shiwei Feng

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a b s t r a c t

The characteristics of transient temperature under the pulse on and cycle pulse mode are studied forGaN-based HEMT. The increase of channel transient temperature under pulse operation for different ris-ing time, duty cycle and frequency has been determined using physical-based simulations, respectively.The results show that the peak temperature in pulse mode with the same operating frequency increaseswith the duty cycle under quasi-steady-state, but the changing rate decreases as the temperature goesup. The maximum of peak temperature change in one cycle is reached at 50% duty cycle. For the pulsewith the same duty cycle, the peak temperature decreases as the frequency increases under quasi-steady-state, while the changing rate slows down. The results can be used to improve the lifetime andperformance reliability.

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1. Introduction

GaN-based HEMTs have shown exceptional promise for highfrequency, high-voltage, and high-power applications [1–4]. How-ever, the high power operation for GaN-based HEMTs can result insubstantial self-heating effect [5–7], which will reduce the perfor-mance reliability because the channel temperature can have badeffect on the electrical properties, such as bandgap, electron mobil-ity, electron saturation velocity, pinch-off voltage, breakdown volt-age, transconductance, and noise performance [8–13]. Moreover,the channel temperature is correlated with the device lifetimeaccording to the Arrhenius equation [14].

The channel temperature can be determined through either thetheoretical model [15–17] or experimental method [18–21]. Thephysical-based numerical simulation is important for the charac-terization and development of HEMT devices [22–26]. Earlynumerical simulation of AlGaN/GaN HEMT which employed thethermal characteristics could load the power on the device pre-cisely to solve heat diffusion equations. However, the method justconsidered the process of heat conduction, not considering electro-thermal coupling [15,23]. Later, Braga et al. [24], Turin et al. [25]and Hu et al. [26] simulated the self-heating effect of GaN-basedHEMTs, which combined the heat diffusion equation with theequation of electrical properties, taking into consideration theinfluence of interface charge, polarization charge, and hot electron.Based on the method, more accurate simulation of the self-heatingeffect for devices in DC operation can be derived. However, most ofthe GaN-based HEMTs in practical application are operated under

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9.).

l. Two-dimensional transient srel.2013.02.004

pulse mode. It is desired to understand the change of transienttemperature in high-frequency and high-power operation whichshould have a significant impact on the device performance andreliability.

In this paper, the characteristics of transient temperature underthe direct current and cycle pulse mode are investigated. The effectof the pulse on the channel temperature, and hence the device reli-ability is studied using the physical-based simulation.

2. Device description and simulation model

The schematic of the device structure for the simulation is givenin Fig. 1a. Device parameters from Binari et al. [27] are used for thesimulation. For the device, the thickness of undoped GaN and Al0.3-

Ga0.7N layer are of 2 lm and 25 nm, respectively. The gate length is1 lm, and the spacing of gate-drain and gate-source are 2.4 and1.5 lm, respectively. The thickness of SiC substrate is 100 lm.The ohmic contacts are used for the source and drain.

The physical-based two-dimensional numerical simulation isperformed by the Sentaurus Device of the Sentaurus TCAD soft-ware which is developed by Synopsys Inc., whose validity has beenconfirmed in Refs. [25,26]. Mixed-mode simulations are selectedbecause the device is operated under the pulse mode. The diagramof the circuit connection is shown in Fig. 1b. The drain and sourceare applied with the periodic pulse and DC voltage, respectively.The gate is grounded. The electric potential between gate andsource (Vgs) is 0 V. The potential switches between 10 and 0 V be-tween the drain and source (Vds) and lasts for a period as referredto the heating and cooling time, respectively. A thermal electrodeis attached to the bottom of the substrate, whose temperature

imulations of the self-heating effects in GaN-based HEMTs. Microelectron

Fig. 1. The schematic of the device structure. (a) The structure parameters of deviceis from Ref. [27]. (b) The diagram of the circuit connection. It is used in simulationswith a source–drain periodic pulse voltage to obtain the current vs time. The drainand source are applied with the periodic pulse and DC voltage, respectively. Theelectric potential between gate and source is Vgs. The potential of drain–sourceswitches between Vdd and 0 V.

Table 1The physical parameters of the material.

Material property Units GaN AlN SiC

Electron mobility cm2/(V s) 1000 300 xShockley–Read–Hall lifetime ns 1 1 1Bandgap eV 3.47 6.2 xRelative permittivity 1 9.5 8.5 10Electron saturation velocity cm/s 1.5 � 107 1.5 � 107 2 � 107

Lattice thermal conductivity W/(K cm) 1.3 2.85 3.7Lattice heat capacity J/(K cm3) 3.0 1.94 2.2Thermal diffusion factor cm2/s 0.43 1.47 1.67

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and thermal resistance are set as the room temperature of 300 Kand 1 � 10�5 K cm2/W in all simulations, respectively.

The thermodynamic model (also known as non-isothermalmodel) is applied to calculate the self-heating effect. The thermo-dynamic model is based on drift–diffusion model and the effectof temperature gradient on the carrier transport is taken into ac-count, which can be used to calibrate the uneven heat distributionfrom the self-heating effect. The equation of lattice heat flow is in-cluded besides the basic Poisson equation and the continuity equa-tion. The current of the device end and lattice temperature can beobtained from the self-consistent calculation of above equations.

The traps of GaN buffer layer and AlGaN barrier layer are usedin the simulation because they play an important role in chargetransfer. The traps density of GaN and AlGaN is NGaN = 1 � 1017/cm3 with a capture cross section of rGaN = 1 � 10�15/cm2, andNAlGaN = 1e17/cm3 with a capture cross section of rAlGaN = 1� 10�15/cm2, respectively. The trap band is 2.2 eV below the con-duction band.

According to the relationship between the mobility and electricfield, the drift speed of carriers increases proportionally to the elec-tric field at the beginning state, but tends to a saturation value. Tocalculate the saturation velocity, the field-dependent drift velocitymodel is used in the simulation.

The electric field induced by the high piezoelectric effect andspontaneous polarization can lead to a significant increase ofthe sheet carrier concentration and a more narrow confinementof the 2-dimensional electron gas [2DEG] at the AlGaN/GaN het-erointerface without doping in wurtzite group-III nitrides HEM-Ts.[28,29]. In the absence of external fields, the polarizationresults in accumulation of sheet interface charge on the end ofthe crystals. These interface charges are, of course, equal in mag-nitude and opposite in sign to maintain overall charge neutralityand will induce free carriers to compensate themselves. The inter-face charge induced by the polarization at AlGaN/GaN is includedin our simulation as 1.0 � 1013/cm2. The interface charge at Al-GaN/passivation layer is 3.5 � 1013/cm2. The calculation methodsof polarization charge was described in Refs. [30,31]. Other re-lated models, such as the mobility model caused by surface scat-tering, the SRH (Shockley’s–Read–Hall) composite model. are alsoused.

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The material parameters [23–26,32–34] for the simulation oftransient temperature are listed in Table 1.

The lattice heat capacity of AlGaN layer is used as same as GaNbecause it has less effect on the heat dissipation. The change ofmaterial thermal conductivity with respect to temperature canbe treated as constant for its small value. There exists a thermalboundary resistance between the GaN and the SiC [35], becausethe real interface between the epitaxial film and the growth sub-strate usually contains nucleation layers with a high concentrationof defects and impurities. Here, we use a rather low thermal con-ductivity of 3.7 W/(K cm) to take into account of the presence ofthermal boundary resistance effectively [36]. The device usuallyhas a top passivation layer with good thermal insulation so thatthe heat dissipation from the top surface can be ignored. Such achoice of boundary condition has been validated by Turin et al.[25]. The total thermal resistance of the simulated single-gate fin-ger transistor on SiC substrate is approximately 20–50 K/W, whichis in agreement with the value obtained by Kuball et al. [19] andZhang et al. [37]. The thermal resistance of the electrode withthe length of 150 lm has been estimated by Wang et al. [34] tobe 2.5 � 105 K/W, which is much larger than the total thermalresistance so that the heat transfer along the electrode can be ig-nored. Therefore, the heat is considered to diffuse only throughthe bottom of substrate. Default values are used for other materialparameters in software.

3. Simulation results and discussion

3.1. Transient temperature under DC operation

The temperature distribution and rise of transient temperatureof AlGaN/GaN HEMT on SiC substrate are shown in Fig. 2. When thegate voltage is 0 V, the electric potential between drain and sourceis a step voltage from 0 to 10 V with the rising time of 500 ns. Thedevice power density is equal to 5.8 W/mm. It is shown in the insetof Fig. 2 that the peak temperature of the channel is 533 K. The heatis generated in the gate close to the drain contact where most ofthe potential drop occurs in HEMTs [38,39]. Therefore, the heat dis-sipates away from the gate close to the drain contact to the sub-strate by thermal diffusion. The peak temperature of channelincreases rapidly from 0.1 to �100 ls to a stable level of around533 K as it is shown in Fig. 2. All the analysis is consistent withthe experimental results, confirming the accuracy of our simula-tion [40,41].

3.2. Effect of pulse rising time

It is noteworthy that there is a knee point at 500 ns on the curveof transient peak temperature. Different rising time of pulse,including 0.125, 0.25, 0.5, 1.0, and 2 ls, are simulated for calibra-tion, respectively. It is found that the appearance of knee point isdelayed and becomes invisible as the increase of pulse rising time(Fig. 3), which is caused by the increase of traps occupation by

imulations of the self-heating effects in GaN-based HEMTs. Microelectron

Fig. 2. The transient peak temperature rise of AlGaN/GaN HEMT on SiC substrate under DC mode. The temperature distribution under stable state is shown in the inset.

Fig. 3. The transient temperature in different rising time of pulse, including 0.125, 0.25, 0.5, 1.0, and 2 ls, respectively. The power density vary with the rising time of pulse inthe time of rising edge arrived, as shown in the inset.

Y. Zhang et al. / Microelectronics Reliability xxx (2013) xxx–xxx 3

2DEG [42,43] and the resulting decrease of drain current [44], asshown in the inset of Fig. 3. Therefore, the rising and falling timeof pulse are selected to be 500 ns in all simulations to avoid the ef-fect of different rising time on the channel temperature.

3.3. Transient temperature rise under pulse mode

The dependence of temperature and dissipation of channelpower on time for the AlGaN/GaN HEMT device are simulated tostudy the effect of traps on drain current and transient tempera-ture of channel. The device is under the quasi-steady-state, withthe duty cycle and period of pulse as 50% and 20 ls, respectively.The peak temperature of channel changes between 487.65 and

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376.89 K (Fig. 4). It rises rapidly at the initial state, but the slopebecomes smaller as the temperature goes up. Its rising rate de-pends on the temperature gradient between the position of peaktemperature and thermal electrode. At the pulse falling edge, thepeak temperature falls rapidly and the falling rate reduces. Whenthe pulse rising edge arrives, the current and the power dissipationincreases rapidly, generating a huge amount of heat in the channel.The peak temperature changes 65 K in 1 ls. Because the GaN bufferlayer has small size (2 lm), and the heating can be seen as an iso-lated process [23,45], the channel temperature increases accord-ingly. The rate of the heat transfer increases due to the increaseof the temperature gradient. Therefore, the temperature changingrate decreases gradually during the heating time.

imulations of the self-heating effects in GaN-based HEMTs. Microelectron

Fig. 4. The peak temperature and the dissipation power of channel vary with the time for the AlGaN/GaN HEMT device, when the duty cycle and the frequency of the pulseare 50% and 50 K Hz, respectively.

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3.4. Influence of pulse duty cycle on channel temperature

It is also found that the drain current decreases with time whenthe drain voltage is kept as 10 V, which can be attributed to thecharges trapped in the GaN buffer layer [22]. When the tempera-ture increases, the electrons of channel can gain enough energy.The one near the gate are captured by the trapped charges in theGaN buffer layer, significantly reducing the concentration of2-dimensional electron gas [44].

By comparing Figs. 3 and 4, one can see that the maximum ofpeak temperature Tmax in pulse operation is lower than that inDC operation, indicating that the duty cycle plays an important rolein affecting the channel temperature. The changing trend of Tmax

Fig. 5. The peak temperature of channel vary with the time under pulse mode in the frequ25%, 40%, 50%, 60% and 75%, respectively. The peak temperature of channel vary with thwhen the duty cycle of the pulse is 25%, 40%, 50%, 60% and 75%, respectively, as shown

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under different duty cycle for 100%, 75%, 60%, 50%, 40%, and 25%are plotted in Fig. 5, respectively, showing that it increases withthe duty cycle in quasi-steady-state. There are two reasons for thisbehavior. One is that the channel temperature of GaN-based deviceincreases with the increase of power density [40] as a result of theincrease of duty cycle. The other reason is because the cooling timeis shortened with the duty cycle, thus reducing the diffusion ofheat to the thermal electrode.

The transient peak temperature in a cycle under quasi-steady-state for different duty cycle is plotted in the inset of Fig. 5. Themaximum and minimum peak temperature (Tmax and Tmin) areplotted in Fig. 6, which increases with the duty cycle.Because the temperature gradient between the position of peak

ency of 50 K Hz for the AlGaN/GaN HEMT device, when the duty cycle of the pulse ise time in one cycle with the frequency of 50 K Hz for the AlGaN/GaN HEMT device,in the inset.

imulations of the self-heating effects in GaN-based HEMTs. Microelectron

Fig. 6. The maximum and minimum of peak temperature in a cycle under quasi-station-state in pulse mode vary with the duty cycle, when the frequency of 50 K Hz. Thepeak temperature range in a cycle under quasi-station-state in pulse mode vary with the duty cycle, when the frequency of 50 K Hz, as shown in the inset.

Fig. 7. The peak temperature of channel vary with the time under pulse mode induty cycle of 25% for the AlGaN/GaN HEMT device, when the frequency of the pulseis 10, 12.5, 25, 50, 100, 125, and 200 K Hz, respectively.

Y. Zhang et al. / Microelectronics Reliability xxx (2013) xxx–xxx 5

temperature and thermal electrode becomes larger, thus increas-ing the heat diffusion. Moreover, the speed of heat diffusion de-creases gradually in cooling time. The rate of heat transfer to thethermal electrode increases as the increase of duty cycle. There-fore, the increasing trend of peak temperature is reduced. The dif-ference between Tmax and Tmin reaches the maximum value at 50%duty cycle, and then decreases, as it is shown in the inset of Fig. 6.For the minimum of peak temperature, it increases as the increaseof duty cycle due to the reduction of temperature difference. This isbecause the rate of heat diffusion increases during cooling time,but the heat of diffusion is reduced due to the cooling time isshortened.

As it is known, the lifetime of semiconductor devices reduces to50% of the original value when the temperature increases by 10 K.It is desirable to have the device channel temperature as low aspossible to improve the lifetime of the semiconductor devices. Inaddition, the device electrical properties and performance reliabil-ity also change as a result of the temperature change of channel.The change of channel temperature should be as small as possiblein pulse operation. According to our simulation results, the devicelifetime and performance reliability can be improved effectively byreducing the pulse duty cycle for GaN-based HEMT device in pulseoperation.

3.5. Influence of pulse frequency on channel temperature

Besides the duty cycle, the temperature of channel is also af-fected by the frequency for its effect on thermal impedance.Although the difference between Tmax and Tmin in the duty cycleof 0.25 is higher than in 0.7, the maximum peak temperature inthe duty cycle of 0.7 is higher than that in 0.25, which enhancesthe lifetime and the performance reliability. For this reason, theduty cycle of 25% is selected for the simulation of temperaturedependence on frequency. Other parameter of voltage is the sameas the previous simulation.

It is shown in Fig. 7 that the Tmax decreases with the increase ofpulse frequency, and the changing rate reduces. The Tmin displaysan opposite changing trend as it is shown in the inset of Fig. 8. Be-cause the average power density keeps constant with the sameduty cycle at different frequencies, it is reasonable to exclude the

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average power density from the cause of this phenomenon. How-ever, the rate of heat diffusion decreases with prolonged coolingtime in every cycle (Fig. 6). The average temperature of the devicerises as the increase of the period of power pulse and so does theheating time. Therefore, the peak temperature increases with theperiod of power pulse, as it is shown in the inset of Fig. 8. Thedependence of the temperature on frequency is also plotted inFig. 8, which displays an opposite changing trend as comparedwith that of pulse period.

The change of peak temperature which is equal to the differencebetween the Tmax and Tmin is plotted in Fig. 9, showing a decreasingtrend as the increase of frequency and a reduction of the changingslope. It is caused by the shortening of heating and cooling timeand by the reduction of the peak temperature change range inheating and cooling time. In addition, both the slope of maximumand minimum peak temperature decreases with frequency.

According to our simulation, the device lifetime can be pro-longed and the performance reliability can be enhanced effectively

imulations of the self-heating effects in GaN-based HEMTs. Microelectron

Fig. 8. The maximum and minimum of peak temperature in a cycle under quasi-station-state in pulse mode vary with the frequency, when the duty cycle of 25%. The peaktemperature of channel vary with the period in one cycle with the duty cycle of 50 K Hz for the AlGaN/GaN HEMT device, when the frequency of the pulse is 10, 12.5, 25, 50,100, 125, and 200 K Hz, respectively, as shown in the inset.

Fig. 9. The difference between the Tmax and Tmin in a cycle is a function offrequency, when the device under quasi-station-state in pulse mode with the dutycycle of 25%.

6 Y. Zhang et al. / Microelectronics Reliability xxx (2013) xxx–xxx

by increasing the frequency of power pulse for GaN-based HEMTdevice.

4. Conclusion

In summary, we have investigated the self-heating effect inGaN-based high electronic mobility transistor analytically andnumerically by using two-dimensional transient simulation. Thesimulation result of temperature distribution and transient tem-perature rise is consistent with the experimental data. The maxi-mum of peak temperature in channel under quasi-steady-stateincreases gradually with the increase of duty cycle and the chang-ing slop decreases. The variation range of peak temperature in a cy-cle increases to a maximum value at the duty cycle of 50%, andthen decreases. The maximum of peak temperature under quasi-steady-state in pulse operation decreases with the increase of thefrequency. The temperature range in one cycle decreases withthe increase of the frequency. The simulation results show thatthe device operation in high frequency and low duty cycle is an

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effective way to improve the lifetime and performance reliabilityfor AlGaN/GaN HEMT device.

Acknowledgments

This work was supported by in part by grants from the NationalNatural Science Foundation of China (61201046), Beijing NaturalScience Foundation of China (4132022 and 4122005) and theGuangdong Strategic Emerging Industry Project of China(2012A080304003).

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imulations of the self-heating effects in GaN-based HEMTs. Microelectron