Parasitic source resistance at different temperatures for AlGaN/AlN/GaN heterostructure field-effect transistors?

Yan Liu(劉艷),Zhao-Jun Lin(林兆軍),?,Yuan-Jie Lv(呂元杰),Peng Cui(崔鵬), Chen Fu(付晨),Ruilong Han(韓瑞龍),Yu Huo(霍宇),and Ming Yang(楊銘)

1 School of Microelectronics,Shandong University,Jinan 250100,China

2 National Key Laboratory of Application Specific Integrated Circuit(ASIC), Hebei Semiconductor Research Institute,Shijiazhuang 050051,China

Parasitic source resistance at different temperatures for AlGaN/AlN/GaN heterostructure field-effect transistors?

Yan Liu(劉艷)1,Zhao-Jun Lin(林兆軍)1,?,Yuan-Jie Lv(呂元杰)2,Peng Cui(崔鵬)1, Chen Fu(付晨)1,Ruilong Han(韓瑞龍)1,Yu Huo(霍宇)1,and Ming Yang(楊銘)1

1 School of Microelectronics,Shandong University,Jinan 250100,China

2 National Key Laboratory of Application Specific Integrated Circuit(ASIC), Hebei Semiconductor Research Institute,Shijiazhuang 050051,China

The parasitic source resistance(RS)of AlGaN/AlN/GaN heterostructure field-effect transistors(HFETs)is studied in the temperature range 300–500 K.By using the measured RSand both capacitance–voltage(C–V)and current–voltage(I–V)characteristics for the fabricated device at 300,350,400,450,and 500 K,it is found that the polarization Coulomb field (PCF)scattering exhibits a significant impact on RSat the above-mentioned different temperatures.Furthermore,in the AlGaN/AlN/GaN HFETs,the interaction between the additional positive polarization charges underneath the gate contact and the additional negative polarization charges near the source Ohmic contact,which is related to the PCF scattering,is verified during the variable-temperature study of RS.

AlGaN/AlN/GaN heterostructure field-effect transistors(HFETs),parasitic source resistance,polarization Coulomb field scattering

1.Introduction

GaN-based heterostructure field-effect transistors (HFETs)are considered to be suitable for high temperature, high frequency,and high power applications.In GaN-based HFETs,the parasitic source resistance(RS)is an important parameter for the device performance.[1,2]Here,RSincludes the gate-to-source access resistance(RGS)and the Ohmic contact resistance(ROhmic),and it leads to some undesirable effects. For example,RSreduces the effective transconductance of the device and the current gain cutoff frequency.It is necessary to analyze the origin and properties of RSat different temperatures for enhancing the device performance in various environments.As previously known,RSis related to the scattering mechanisms for the electrons in the gate–source channel.Both theoretical and experimental results reveal that the polarization Coulomb field(PCF)scattering is an important scattering mechanism in AlGaN/AlN/GaN HFETs.[3–5]Recently,the effect of the PCF scattering on RSwas verified at room temperature.[6]However,the influence of PCF scattering on RSis unknown at elevated temperatures.Therefore,for the AlGaN/AlN/GaN HFETs,it is important to study the relationship between PCF scattering and RSat elevated temperatures. In this study,we investigate the influence of PCF scattering on RSin AlGaN/AlN/GaN HFETs in the temperature range 300–500 K using the measured temperature-dependent RSand both capacitance–voltage(C–V)and current–voltage(I–V) characteristics of the prepared device.

2.Experiments

The undoped Al0.28Ga0.72N/AlN/GaN heterostructure layers used in this study were grown on a(0001)sapphire substrate by molecular beam epitaxy(MBE).Moreover,the components of its active structure were exhibited in detail in Fig.1.The Al0.28Ga0.72N/AlN/GaN HFET was manufactured according to the standard process.[7]Its source and drain Ohmic metal stack comprised Ti/Al/Ni/Au.The source and drain regions were rectangular with a length and width of 50μm and 100μm,respectively.The drain-to-source distance was 100μm.Ni/Au was deposited to form the gate Schottky contact.The size of the gate Schottky contact was 20μm/100μm(length/width),and the gate was 20μm away from the source.The respective C–V measurements for theprepared Al0.28Ga0.72N/AlN/GaN HFET at 300,350,400, 450,and 500 K were performed using Agilent B1520A at 1 MHz.Moreover,the respective I–V and RSmeasurements at the above different temperatures were performed using the Agilent B1500A semiconductor parameter analyzer.

Fig.1.(color online)Schematic cross section of fabricated Al-GaN/AlN/GaN HFET.

3.Results and discussion

Figure 2(a)shows the measured C–V curves of the prepared Al0.28Ga0.72N/AlN/GaN HFET at different temperatures.The two-dimensional electron gas(2DEG)electron density(n2D)can be obtained by the C–V curve integration,and the values are shown in Fig.2(b).[8]Table 1 lists the values of n2Dcorresponding to zero gate bias(n2D0)at each testing temperature.Figure 3 displays the output characteristics at different temperatures for the prepared sample.The drain-current, IDS,with a source–drain voltage of 125 mV at zero gate bias can be obtained from the I–V characteristics,and the values at different temperatures are shown in Table 1.

Fig.2.(color online)(a)Measured C–V curves for Al0.28Ga0.72N/AlN/GaN HFET at different temperatures.(b)Obtained 2DEG electron sheet density n2D from C–V measurements for the sample at different temperatures.

Fig.3.Measured output characteristics of the sample at(a)300 K,(b)350 K,(c)400 K,(d)450 K,and(e)500 K.

The 2DEG electron mobility corresponding to zero gate bias(μn0)at 300,350,400,450,and 500 K for the prepared Al0.28Ga0.72N/AlN/GaN HFET can be obtained using the same method,[4]and the calculated results are shown in Table 1.Moreover,according to Ref.[4],the gate-to-source access resistance corresponding to zero gate bias(RGS0)can be calculated by

where LGSis the gate-to-source length,e is the electron charge, n2D0andμn0are defined earlier,and W is the gate width. Here,it is noted that the Ohmic contact resistance,ROhmic,is not included in the calculated RGS0.Using the transfer-length method(TLM),[9]the specific resistivity of the Ohmic contact is estimated to be 7.98×10?5?·cm2at 300 K,and the value of ROhmicis calculated to be 26.09 ?.Therefore,RS0(RS0is defined as the parasitic source resistance corresponding to zero gate bias),which is the sum of RGS0and ROhmic,can be obtained.In addition,it is suggested that the Ohmic contact re-sistance for the AlGaN/GaN heterojunction exhibits a weaker dependence on temperature.[10]Moreover,in this study,the values of ROhmicat different temperatures can be obtained using the TLM,as shown in Fig.4.Thus,RS0at different temperatures can be determined,and the results are shown in Table 1.

Table 1.Parameters of the prepared AlGaN/AlN/GaN HFET at different temperatures.V GS is the gate–source bias,V DS is the drain–source bias,I DS is the channel current,n2D0 is the 2DEG electron density at zero gate bias,μn0 is the 2DEG electron mobility with zero gate bias,and R S0 is the parasitic source resistance corresponding to zero gate bias,which is the sum of the source access resistance and the Ohmic contact resistance.

Fig.4.Plot of R Ohmic versus T for the sample.

RScan be measured using the gate probe method,as described in detail in Ref.[11].The gate probe method shows that an accurate RScan be extracted from the plot of the gate–source bias(VGS)versus IDSunder the conditions of IGS?IDS, the relatively low drain-to-source bias(VDS),and the constant forward gate–source current IGS.Figure 5 is the test configuration for measuring RSusing the gate probe.As shown in Fig.5, the gate was forward driven with a constant current IGSand the drain was driven with a range of currents,while the source was grounded.As IGSis fixed,the voltage drop across the gate Schottky barrier Vdropis constant.Therefore,VGSis the sum of the channel voltage at the source edge of the gate(Vb) and Vdrop.Owing to Vb=IDS×(ROhmic+RGS),Vbincreases with the increase of IDSwhen VDSincreases.Here,RGSis the gate-to-source access resistance.As VGS=Vb+Vdrop=IDS× (ROhmic+RGS)+Vdrop,the derivative of VGSwith respect to I DS is the value of R Ohmic+R GS,that is,R S.[1,6,11]

Fig.5.Test configuration for measuring R S using gate probe,I GS=constant.

Fig.6.(color online)Plot of V GS versus I DS at different temperatures with I GS=25μA for the sample.

According to the gate probe method,the value of RSis equal to the slope in the VGSversus IDSplot,and the prominent linear relationship of VGSversus IDSsuggests that an accurate extraction of RScan be obtained.In order to study the effect of PCF scattering on RSin the temperature range 300–500 K,the measurements of RSwere conducted at the same value of IGSunder the above different temperatures.Figure 6 shows the measured curves,that is,the plot of VGSversus IDSat different temperatures with forward gate current of 25μA (IGS=25μA).Here,the drain-to-source bias is maintained in the range from 0 V to 3 V.From Fig.6,the value of RScorresponding to IGS=25μA at each testing temperature,RS25,can be extracted.The black trace in Fig.7 corresponds to the extracted RS25.Moreover,the red trace in Fig.7 corresponds to RS0(the value of RS0at each testing temperature is also listed in Table 1).

Fig.7.(color online)Variations of R S25(black trace)and R S0(red trace) with temperature.R S25 represents the value of R S at I GS=25μA,and R S0 represents the value of R S at V GS=0 V.

As observed from Fig.7,RS25is larger than RS0at each corresponding testing temperature,and the resistance difference between RS25and RS0(ΔRS=RS25?RS0)decreases with increasing temperature.RSis related to the scattering mechanisms and n2Dfor the electrons in the gate–source channel. As n2Din the gate–source channel is not modulated by the gate–source bias VGS,[4]the value of n2Din the gate–source channel is assumed to remain unchanged as VGSor IGSvaries. It indicates that the value of n2Din the gate–source channel corresponding to IGS=25μA is equal to that corresponding to IGS=0μA at the same temperature.Therefore,RS25and RS0correspond to the same value of n2Dat the same temperature.Hence,the resistance difference,ΔRS,does not result from n2Dof the gate–source channel.

In undoped AlGaN/AlN/GaN HFETs,the longitudinal optical(LO)phonon scattering,the interface roughness(IFR) scattering,and the PCF scattering are primarily the three types of important scattering mechanisms.[3,4]For LO phonon scattering,it is primarily related to the average phonon number and n2D.For IFR scattering,it is mostly determined by the average distance of the 2DEG electrons from the AlN/GaN interface.The average phonon number is primarily determined by temperature,and the average distance of the 2DEG electrons from the AlN/GaN interface is considerably impacted by n2D.As mentioned earlier,n2Din the gate–source channel does not vary with IGS.Therefore,for both LO phonon and IFR scatterings,they are IGS-independent at the same temperature.Hence,both LO phonon and IFR scatterings cannot lead to the difference between RS25and RS0at the same temperature.

Fig.8.(color online)Schematic of polarization charges distribution.(a) Distribution of polarization charges with the Ohmic and Schottky contact metals.(b)Distribution of polarization charges without the contact metals.(c)Distribution of the additional polarization charge density. +:Positive polarization charges;—:Negative polarization charges;°–: Two-dimensional electron gas electrons.

For PCF scattering as one of the primary types of important scattering mechanisms in AlGaN/AlN/GaN HFETs as mentioned earlier,it is closely related to the distribution of the polarization charges along the AlGaN/AlN/GaN heterostructure interface.The detailed illustration is provided in the following.The distribution of the polarization charges at the AlGaN/AlN/GaN heterostructure interface using the device processing mechanism and both the gate–source and drain–source biases is not uniform(see Fig.8(a)).Without the deposition of the contact metals,the distribution of the polarization charges at the AlGaN/AlN/GaN heterostructure material interface is uniform(see Fig.8(b)).The difference between the nonuniform polarization(Fig.8(a))and the uniform polarization(Fig.8(b))is considered as the additional polarization charges.Figure 8(c)provides the distribution of the additional polarization charge density.In Fig.8(c),Δσ1is the additional negative polarization charge density near the Ohmic contact metals,which is generated by the Ohmic-contact processing owing to the diffusion of the Ohmic contact metal atoms.l is the diffusion length of the Ohmic contact metal atoms.[4]Δσ1andlare related to the Ohmic contact processing only and not modulated by the gate bias and temperature.Δσ2is the additional polarization charge density under the ungated region except the range of l.Neither the Ohmic-contact processing nor the bias voltage influence Δσ2.Hence,the value of Δσ2is considered to be 0.[3,4]Δσ3is the positive additional polarization charge density underneath the gate,which is induced by the forward gate–source bias owing to the converse piezoelectric effect.

The PCF scattering theory indicates that the additional polarization charges establish the elastic scattering potential that scatters 2DEG electrons,and the larger the PCF scattering potential is,the stronger the PCF scattering will be.[3–5]Here,the PCF scattering potential,V(x,y,z),can be expressed as[6]

From Eq.(2),it can be observed that the magnitude of the PCF scattering potential relates to the diffusion length of the Ohmic contact metal atoms l,the additional polarization charge density Δσ1and Δσ3,and the device structure parameters such as the gate–source length LGS,the gate length LG,the gate–drain length LGD,and the device width W.For the same device studied in this paper,the device structure parameters are the same and do not vary with the gate bias and temperature. In addition,as mentioned earlier,as Δσ1and l are related to the Ohmic contact processing only,the values of Δσ1and l for the sample remain constant at different gate biases and temperatures.For RS25and RS0in Fig.7,they correspond to IGS=25μA and IGS=0μA,respectively.At the same temperature,the voltage drop across the gate Schottky barrier, Vdrop,corresponding to IGS=25μA,is higher than that corresponding to IGS=0μA(VGS=0 V).Thus,owing to the converse piezoelectric effect,the value of Δσ3corresponding to IGS=25μA is larger than that corresponding to IGS=0μA at the same temperature,which results in a magnitude of the PCF scattering potential corresponding to IGS=25μA that is larger than that corresponding to IGS=0μA.As a result,the PCF scattering corresponding to IGS=25μA is stronger than that corresponding to IGS=0μA at the same temperature.Therefore,the resistance difference between RS25and RS0,ΔRS,results from PCF scattering.Moreover,RS25is larger than RS0at the same temperature,as shown in Fig.7.

Fig.9.(color online)Measured forward I–V characteristics of Ni/Au/AlGaN/AlN/GaN Schottky diode at different temperatures.

The polarization charges along the AlGaN/AlN/GaN interface involve both spontaneous and piezoelectric polarization.The spontaneous polarization does not vary with gate bias,[4]and seldom changes with temperature.[12,13]Hence, the variation of ΔRSwith temperature is closely related to the AlGaN barrier layer piezoelectric polarization.The piezoelectric polarization varies with z-direction electric field in the Al-GaN barrier layer owing to the converse piezoelectric effect, and it is expressed as[14]

where PPE,AlGaNis the piezoelectric polarization charge density,C33is the elastic constant of AlGaN,e33is piezoelectric constant,EPE,AlGaN=Vdrop/dAlGaNis the biased z-direction electric field in the AlGaN barrier layer,and dAlGaNis the AlGaN barrier layer thickness.As the voltage drops across both the gate–source channel and the Ohmic contact,that is, IGS×(ROhmic+RGS)are negligible compared to the voltage drop across the gate Schottky barrier Vdropfor all the testing temperatures when IGSis equal to 25μA,it is reasonable to assume that Vdropis approximately equal to VGS.VGSat different temperatures corresponding to IGS=25μA can be obtained from Fig.9,which provides the forward I–V characteristics of the gate–source Schottky diode at different temperatures.[6]Thus,the biased z-direction electric field EPE,AlGaNat different temperatures corresponding to IGS=25μA can be determined.According to Eq.(3),the piezoelectric polarization charge density at different temperatures corresponding to EPE,AlGaNcan also be obtained.Here,as C33and e33are considered temperature-independent,[15–18]the values of C33and e33for Al0.28Ga0.72N barrier layer can be used as 396 GP and 0.93 C/m2,respectively,in the above calculation.[19]Figure 10 provides these calculated results at different temperatures corresponding to IGS=25μA,the black trace and the red trace correspond to EPE,AlGaNand PPE,AlGaN,respectively.As shown in Fig.10,both the piezoelectric polarizations corresponding to IGS=25μA and the biased z-direction electric field decrease with increasing temperature.The piezoelectric polarization charge density in Eq.(3)is precisely the additional positive polarization charge density underneath the gate,that is,Δσ3in Fig.8(c),which is proportional to the PCF scattering intensity.Therefore,the PCF scattering decreases for the prepared AlGaN/AlN/GaN HFET with increasing temperature.Thus,the resistance difference,ΔRS,decreases with increasing temperature,as shown in Fig.7.

Fig.10.(color online)Variations of electric field and piezoelectric polarization of AlGaN barrier layer corresponding to I GS=25μA at different temperatures.

In addition,figure 10 shows that the biased z-direction electric field(EPE,AlGaN=5.57×105V/cm)at 500 K is still strong.Figure 7 shows the resistance difference ΔRSat 500 K is considerably small.The reason can be explained below.

As mentioned earlier,the Ohmic-contact processing can generate the additional negative polarization charge density near the Ohmic contact metals,that is,Δσ1in Fig.8(c).The positive gate–source bias corresponding to the forward gate current of 25μA can generate the piezoelectric polarization charge density under the gate,that is,the additional positive polarization charge density underneath the gate contact,thatis, Δσ3in Fig.8(c).The scattering potential of the PCF scattering corresponding to IGS=0μA(VGS=0 V)is only constituted by Δσ1.The scattering potential of the PCF scattering corresponding to IGS=25μA is constituted by Δσ1and Δσ3.From Eq.(2),the PCF scattering potential between Δσ1and Δσ3can be eliminated.As mentioned earlier,as Δσ1do not vary with both temperature and gate bias,and Δσ3,which is the piezoelectric polarization charge density induced by Vdrop,decreases with increasing temperature.It can be inferred that the difference in the absolute value of Δσ1and Δσ3decreases with increasing temperature,and the absolute value of the above difference is approximately equal to that of Δσ1at 500 K.As a result,the scattering potential of PCF scattering corresponding to IGS=25μA is approximately equal to that corresponding to IGS=0μA(VGS=0 V).Therefore,RS25and RS0at 500 K are approximately equal.Thus,although the z-direction biased electric field EPE,AlGaNat 500 K is still strong,the resistance difference ΔRSis considerably small.Hence,all the above analysis shows that the PCF scattering exhibits an important influence on the parasitic source resistance RSin the temperature range 300–500 K.This indicates that PCF scattering should be considered at elevated temperatures.In addition,the interaction between the positive additional polarization charges underneath the gate contact and the negative additional polarization charges near the source contact,which is related to PCF scattering,was verified during the variable temperature study of RSin the temperature range 300–500 K. Moreover,the PCF scattering,as a type of Coulomb field scattering,is related to distance.The shorter the gate–source spacing of the AlGaN/AlN/GaN HFETs is,the closer is the distance between the additional polarization charges and the 2DEG in the gate–source channel,and the stronger is the PCF scattering at elevated temperatures.Further research should be performed for the dependence of RSon temperature,which is based on PCF scattering being the dominant mechanism in the AlGaN/AlN/GaN HFETs with the shorter gate–source distance.

4.Conclusions

In this study,we analyzed and studied the influence of PCF scattering on the parasitic source resistance RSusing the measured C–V and I–V characteristics and RSin the temperature range 300–500 K for the prepared Al0.28Ga0.72N/AlN/GaN HFET.The results indicate that the PCF scattering should be considered at elevated temperatures as it was found to exhibit an important influence on RSin the investigated temperature range.In addition,the interaction between the positive additional polarization charges underneath the gate contact and the negative additional polarization charges near the source contact was verified to influence the intensity of the PCF scattering.

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14 January 2017;revised manuscript

23 April 2017;published online 31 July 2017)

10.1088/1674-1056/26/9/097104

?Project supported by the National Natural Science Foundation of China(Grant Nos.11174182,11574182,and 61306113)and the Specialized Research Fund for the Doctoral Program of Higher Education of China(Grant No.20110131110005).

?Corresponding author.E-mail:linzj@sdu.edu.cn

?2017 Chinese Physical Society and IOP Publishing Ltd http://iopscience.iop.org/cpb http://cpb.iphy.ac.cn