Design optimization of high breakdown voltage vertical GaN junction barrier Schottky diode with high-K/low-K compound dielectric structure

Kuiyuan Tian(田魁元), Yong Liu(刘勇), Jiangfeng Du(杜江锋), and Qi Yu(于奇)

State Key Laboratory of Electronic Thin Films and Integrated Devices,University of Electronic Science and Technology of China,Chengdu 610054,China

Keywords: GaN junction barrier Schottky diode,compound dielectric,breakdown voltage,turn-on voltage

1. Introduction

Gallium nitride (GaN) materials are very suitable for fabricating high-power devices due to wide band gap, high saturation electron mobility, high critical breakdown field,good thermal conductivity, and superior Baliga’s figure-ofmerit than SiC and Si.[1–4]Recently, the demonstration of low leakage current and high breakdown voltage (BV) vertical GaN diodes has become a major focus in the field of power electronics.[5–8]However,the high turn-on voltage(Von＞3 V)will cause a high conduction loss in power switching applications in p–n junction diodes.[9–11]And for vertical GaN Schottky barrier diodes(SBDs),there is an electric field peak at the Schottky contact,which can lead to a high tunneling current of the Schottky barrier.[12,13]In the past few years,vertical GaN JBS diodes have been fabricated by forming p–n junction on the basis of vertical GaN SBDs.[14,15]The JBS can reduce the leakage current of high voltage operation without sacrificing the positive characteristics, because a thick barrier produced by the p–n junction is only added to the high electric field region, and the tunneling current can be reduced significantly.However, the electric field of the p–n junction is not uniform enough in GaN JBS and causes premature breakdown, thus hindering the improvement ofBVof JBS with low leakage current.

Some literature has reported the structure of a high-Kdielectric layer as a passivation layer.[16–18]The structure can improve the reverse breakdown voltage of the device effectively by utilizing the characteristic that the electric field distribution in the high-Kdielectric layer tends to be uniform.When the high-Kdielectric layer is used as a passivation layer of a semiconductor device,the electric field distribution inside the device is modulated and tends to be uniform,so theBVof the device is increased. However, the use of only high-Kdielectric layers is far from the theoretical limit of GaN reverse characteristics. By utilizing a composite dielectric structure composed of a high-Kdielectric layer and a low-Kdielectric layer,we have reported a GaN p–n junction diode having high reverse characteristics.[19]

In this paper, a vertical GaN-based JBS with a high-K/low-Kcompound dielectric structure (GaN VCD-JBS) is proposed to increase the breakdown voltage of conventional JBS and reduce the effects of electric field crowding at the tip of the p–n junction without sacrificing forward characteristic.The breakdown voltage of the proposed GaN VCD-JBS is improved by optimizing the parameters of the device. Then,the highest figure-of-merit of 8.6 GW/cm2is obtained in comparison with the papers recently reported.

2. Device structure

The device structure of the proposed GaN VCD-JBS is shown in the following Fig. 1. The simulated epitaxial GaN structure consists of a 15-μm-thick GaN drift region doped with 1×1016cm-3, which acts as a current path and pressure-resistant structure. Then two p-type regions doped with 1×1018cm-3are formed. The depth and length of the p-type region are 1 μm and 2 μm, respectively. Two Si3N4high-Kdielectric layers are added to both sides of GaN JBS while two SiO2low-Kdielectric layers are placed at the bottom of the high-Klayers mentioned above. The device has a length (LJBS) of 5.5 μm and a thickness (TJBS) of 15 μm,respectively. For convenience, the width of Schottky contact,the distance from the interface of the p–n junction in JBS to the SiO2low-Kdielectric layer, and the length of the dielectric layer are labeled asWn,T,andLrespectively.

Fig.1. Schematic cross-section of GaN VCD-JBS.

According to Gauss’s law and Poisson’s equation, when the electric fluxes are equal at the interface of two materials with different dielectric constants, the electric field strength along the direction of the electric displacement vector inside the two materials satisfies the following relationship:

Since the dielectric constant of Si3N4is 7.5 and the dielectric constant of SiO2is 3.9,it can be seen from the formula that the electric field at the interface of the Si3N4material layer and the SiO2material layer is discontinuous. The electric field in the vertical direction has a lift from the Si3N4material layer to the SiO2material layer. Based on this principle, we designed a GaN VCD-JBS with a high breakdown voltage. JBS depends on the p–n junction formed by the p-type doping region and the n-type drift region which generates a depletion region in n-type GaN to withstand voltage without increasing turn-on voltage. However,the electric field away from the p–n junction interface in the n-type GaN is drastically reduced,which leads to a reduction in breakdown voltage. In this work,we use the discontinuous electric field in the dielectric layer to affect the electric field in the n-type GaN.By increasing the electric field in the n-type GaN away from the p–n junction interface,the electric field in the n-type drift region becomes uniform,so that the breakdown voltage is increased without sacrificing forward characteristics such as specific on-resistance,saturation current, and turn-on voltage. Then, by optimizing the vertical high-K/low-Kcompound dielectric structure and balancing the size of the Schottky contact area, the electric field away from the p–n junction is more uniform keeping the doping concentration constant.

3. Simulation models and calibration

3.1. Simulation models

In the process of simulation, low-field mobility model,high-field mobility model, Shockley–Read–Hall (SRH)model, Fermi–Dirac model, Auger model, bandgap narrowing model, phonon-assisted tunneling model for GaN SBD,and the Selberherr’s impact ionization model are adopted. The reverseI–Vcharacteristics of GaN diodes can be well explained by the phonon-assisted tunneling model.[20]The tunneling current is generated by the emission of electrons from the energy level of the metal–semiconductor interface. The density of the tunneling current is

In whichqdescribes the electron charge,Nsdescribes the occupied density of states at the interface, andWis the phonon-assisted tunneling rate as a function of temperatureTand electric field strengthE:

whereadescribes the electron–phonon interaction constant,¯hωdescribes the phonon energy, andεTdescribes the trap depth.

In this paper, low-field mobility and high-field mobility models are used as carrier mobility models. The Albrecht model is selected as low-field mobility for simulation,and the low-field mobility of carriers is described as

whereμ(N,T)is the low-field carrier mobility as a function of doping concentrationNand lattice temperatureT,wherea,b,c,T0,N0,andT1are obtained from Monte Carlo fitting results,The results are 2.61×10-4V·s·cm-2, 2.9×10-4V·s·cm-2,170.0×10-4V·s·cm-2, 300 K, 1.0×10-17cm-3, 1065 K,respectively.[21]

The high field mobility of charge carriers is described as

whereμ(E)is the high-field carrier mobility as a function of the electric field strengthE,Vsat,EC,α,n1, andn2are obtained from the Monte Carlo fitting results,and the results are 1.9064×107cm/s, 220.8936 kV/cm, 6.1973, 7.2044, 0.7857,respectively.[22]

In this paper, the impact ionization model is selected as Selberherr’s impact ionization model, and the collision ionization rate of electrons (αn) and holes (αp) are respectively described as

whereEis the local electric field,an,bn,ap,bp, andmare obtained from Monte Carlo simulation results, which are 2.52×108cm-1, 3.41×107V/cm, 5.37×106cm-1,1.96×107V/cm,1,respectively.[23]

3.2. Simulation calibration

The GaN vertical p–n diode reported by Esmat Farzana has been used to calibrate the simulation model.[24]The reported diode is grown on a free-standing substrate with an epitaxial dislocation density of 106cm-2. The epitaxial structure consists of a 0.25-μm layer of n+GaN ([Si]: 1×1019cm-3),followed by a 4-μm layer of undoped GaN as the n-drift layer,a 0.4-μm layer of p+GaN([Mg]: 3×1019cm-3)layers,and a 10-nm p++GaN([Mg]: 3×1020cm-3)layer to make ohmic contacts. The GaN p–n diode was simulated by the Silvaco ATLAS tool and the above model was used to calibrate the simulation model parameters. The simulated GaN p–n diode has a turn-on voltage of 3 V and a breakdown voltage of 1100 V,which is similar to the experimental results. The forward characteristics and the electric field characteristics of the reported GaN vertical diodes and the simulation results are shown in Fig.2.The simulation results show great fitting accuracy,proving the credibility of our model for the GaN vertical diode.

Fig.2. (a) Comparison of the simulated and reported forward characteristics. (b) Comparison of simulated electric field and reported electric field in the diode drift region.

4. Results and discussion

Figure 3 shows the forward and reverse characteristics of the conventional GaN SBD, the conventional GaN JBS,the GaN JBS with SiO2dielectric layer, and the GaN VCDJBS designed in the work. As shown in Fig. 3. The breakdown voltage of the conventional SBD is 320 V, while the breakdown voltage of the conventional JBS shown in Fig.3 is 2262 V.The reason is that the p-type doping region implanted inside the JBS forms a p–n junction with the n-type drift region. In the case of withstanding voltage, the high electric field which should have been on the Schottky contact surface is transferred to the p–n junction inside the device. At the same time, the p–n junction forms a higher barrier than the Schottky barrier and can withstand higher voltages. For GaN VCD-JBS,theBVis 3165 V when the length of the dielectric layerL=4 μm and the distance from the p–n junction interface to the low-Kdielectric layerT=5 μm,while the breakdown voltage of the GaN JBS with only SiO2dielectric layer is 2585 V.The results confirmed that the layer of highK/lowKcompound dielectric can effectively modulate the electric field and effectively improve the withstand voltage.

The inset of Fig.3 shows a comparison of the output characteristics of conventional GaN SBD,conventional GaN JBS,GaN JBS with SiO2dielectric layer,and GaN VCD-JBS.Simulated results show that the turn-on voltage of GaN VCD-JBS is 0.6 V,which is comparable to conventional GaN-based SBD and conventional GaN-based JBS and much less than the turnon voltage of the p–n diode(＞3 V).As can be seen from the inset of Fig.3,the forward current density of GaN VCD-JBS,GaN JBS with SiO2dielectric layer and conventional GaN JBS is slightly less than the forward current density of GaN SBD,because the Schottky contact area is reduced due to the presence of a p–n junction in the JBS. However, compared with p–n junction diodes of the same size,the forward characteristics are significantly improved.

Figure 4 shows the equipotential contours of the conventional GaN JBS,GaN JBS with SiO2dielectric layer,and designed GaN VCD-JBS.As shown in Fig.4,the depletion depth of the drift region in conventional JBS is shorter than the depletion depth of the drift region in VCD GaN JBS.Meanwhile,the potential in the dielectric layers on both sides can also help to withstand voltage. Simulation results show that the n-type drift region can be increased significantly and a higher breakdown voltage can be achieved by optimizing the high-K/low-Kdielectric layer.

Fig.3. Reverse I–V characteristics of the conventional GaN SBD,the conventional GaN JBS,the GaN JBS with SiO2 dielectric layer,and the GaN VCD-JBS.The inset shows the forward I–V characteristics of the conventional GaN SBD,the conventional GaN JBS,the GaN JBS with SiO2 dielectric layer,and the GaN VCD-JBS.

Fig.4. Simulated equipotential contours when breakdown for(a)the conventional GaN JBS,(b)the GaN JBS with SiO2 dielectric layer,and(c)the GaN VCD-JBS.

Figure 5 shows the electric field distribution of the conventional GaN JBS, the GaN JBS with SiO2dielectric layer,and GaN VCD-JBS when the dimension of the JBS is 15 μm and the electric field curve distribution of the two devices whenx=0.1 μm.As shown in Figs.5(a)and 5(b),the electric field is concentrated on the p–n junction of JBS and decreases away from the interface of the p–n junction in the n-type drift region. As shown in Fig.5(c),there is a discontinuity of electric field at the interface of Si3N4and SiO2due to the different dielectric constants of Si3N4and SiO2. The electric field of Si3N4at the interface is suppressed and the electric field of SiO2is enhanced.

As shown in Fig.5(d), the areas enclosed by the electric field curves of conventional GaN JBS and GaN JBS with SiO2dielectric layer are approximately triangular, and the electric field gradually decreases as the distance from the p–n junction increases.As we all know,the integral of the electric field over the depletion region can represent the withstand voltage of the region. For GaN VCD-JBS, by introducing a high-K/low-Kdielectric layer,the electric field in the drift region away from the p–n junction is increased, and the electric field distribution tends to be uniform. However, due to the electric field crowding effect, the electric field is concentrated at the sharp spike of the p–n junction interface. The peak electric field of the sharp spike is relatively high, so the electric field at other areas of the p–n junction interface is lower than the critical breakdown electric field when the device is breaking down.As shown in the inset of Fig.5(d),the addition of a high-K/low-Kdielectric layer is equivalent to the introduction of a high electric field at the contact surface of the dielectric layer and GaN JBS,which improves the electric field away from the spikes of the p–n junction interface, further increasing the breakdown voltage. Compared to a conventional junction termination,the proposed structure raises the electric fields in the n-type region away from the p–n junction interface.

Fig.5. The distribution of electric field for(a)the conventional GaN JBS,(b)the GaN JBS with SiO2 dielectric layer,and(c)the GaN VCD-JBS.(d)The electric field profile at x=0.1 μm. The inset of(d)shows the electric field profile at y=1 μm.

We investigate the relationship betweenBVand the distance from the p–n junction interface to low-Kdielectric layerTand the length of the dielectric layerL. For convenience of further investigation, GaN VCD-JBS with one block of low-Kand one block of high-Kon both sides is optimized firstly by Silvaco simulation. The distance between p–n junction and low-Kdielectric layer varied from 1 μm to 11 μm, and the length of the dielectric layer was changed from 1 μm to 12 μm.When the leakage current is detected to be 50 mA/cm2in the Silvaco simulation, the voltage is defined as the breakdown voltage.

Figure 6 shows the relationship between the breakdown voltage and the length of the dielectric layerL. The inset of Fig.6 shows the relationship between the breakdown voltage and the distance from the interface of p–n junction to the lowKdielectric layerT. As the length of the dielectric layerLincreases,the breakdown voltage gradually increases until saturation. When the length of the dielectric layer reaches 10 μm,the breakdown voltage of GaN VCD-JBS reaches the maximum value. However,longer dielectric layers result in a waste of conductive area in the device. The length of the dielectric layerL=4 μm has been chosen, and the device has a large breakdown voltage with a small size. With the increase ofT,the breakdown voltage increases firstly and then decreases. If the distance between the p–n junction interface and the low-Kdielectric layer is undersized,a high electric field peak close to the p–n junction in JBS causes a premature breakdown. But if the distance between the p–n junction interface and the low-Kdielectric layer is too large, the new electric field peak introduced in the n-type drift region is relatively low and causes a low breakdown voltage. The distance between the p–n junction interface and the low-Kdielectric layerT=5 μm has been chosen. WhenT=5 μm,the breakdown voltage of GaN VCD-JBS reaches the maximum value.

Fig.6. Relationship between BV and L when T =5 μm. The inset shows the relationship between BV and T when L=4 μm.

Fig.7. Electric field distributions for various T at x=0.1 μm when L=4 μm.

Figure 7 shows that the electric field distribution inside the device varies with the variety of the distance between p–n junction interface and low-Kdielectric layerTwhen it breaks down. The breakdown voltages of the device can be observed by the different electric field distributions inside the devices.With the observation of Fig.7,the breakdown voltage of conventional GaN JBS is 2262 V.By optimizing the distance between interface of the p–n junction and the low-Kdielectric layerT,the electric field in JBS becomes more uniform.WhenTis optimized to be 5 μm, the GaN VCD-JBS achieves aBV=3165 V,which is larger by 38%than conventional GaN JBS.

For GaN JBS diodes,the Schottky contact area is important for the forward and reverse characteristics of the diode.As the width of the Schottky contact region increases,the specific on-resistance of the JBS (Ron,sp) will decrease. However, the wide Schottky contact increases the reverse leakage current of the diode and reduces the breakdown voltage.

Fig.8. BV and Ron as functions of the Schottky contact width of GaN conventional JBS Wn. The inset shows the figures-of-merit (FOM) of GaN JBS.

Figure 8 shows the relationship between theBVandRonand the Schottky contact width of GaN conventional JBSWnobtained by simulation. As the Schottky contact width increases,the device’s resistance gradually decreases,but theBVof the JBS diode also decreases. From the inset of Fig.8,we can conclude that the figure-of-merit (FOM) of JBS increases firstly and then decreases withWnincreasing. In the range ofWnbetween 1.3 μm and 1.6 μm, theFOMof JBS reaches 3 GW/cm2.Therefore,we choose 1.5 μm as the Schottky contact width of the device.

Figure 9 shows the electric field distribution of the two cells’ combination of GaN VCD-JBS at the breakdown voltage. The optimized structure above is just one cell of a complete GaN VCD-JBS. The complete GaN VCD-JBS consists of many identical cells in parallel. A cell and the adjacent cell share a common dielectric layer. Therefore, it is necessary to verify whether the GaN VCD-JBS composed of two or more cells can achieve the effect. It has been verified that the breakdown voltage of GaN VCD-JBS composed of two cells is 3050 V.Compared with the single cell GaN VCD-JBS,the breakdown voltage is only reduced by 3%, which is negligible. Since each cell shares an identical dielectric layer with an adjacent cell, half of the dielectric layer should be calculated when calculating the on-resistance of the structure.

Fig.9. The distribution of electric field for two cells combination of GaN VCD-JBS.

The above optimization simulations are performed with only one layer of low dielectric constant material. Even if the optimum value is reached,the electric field away from the p–n junction is still low and not uniform enough. In order to make the electric field distributions more uniform and realize a higher breakdown voltage of GaN VCD-JBS,it is necessary to add the multi-layer of the low-Kdielectric. The multi-layer of the low-Kdielectric can introduce a plurality of electric field peaks to increase the electric field away from the p–n junction surface, resulting in an enhancedBVof the GaN VCD-JBS.Parameters of an optimized structure with three blocks of low-Klayers are listed in Table 1.

Table 1. GaN VCD-JBS with three blocks of low-K dielectric layers specifications.

Fig.10. The electric field distributions corresponding to conventional GaNbased JBS and optimized GaN VCD-JBS at x=0.1 μm.

Figure 10 shows the electric field distribution curve of a conventional JBS and the electric field distribution curve of a GaN VCD-JBS with an optimized multi-layer of the low-Kdielectric atx=0.1 μm. Its parameters are shown in Table 1.The concentration of the n-type drift region is 1.5×1016cm-3in the device. The electric field distribution of the optimized GaN VCD-JBS becomes more uniform. The area enclosed by the electric field curve is increased by an enhanced electric field away from the p–n junction. TheBVof GaN VCDJBS reaches 4171 V. Compared with the traditional JBS ofBV=1560 V,an increase of 167%is obtained. The forward characteristics of GaN VCD-JBS using multi-layer of low-Kdielectric are also excellent. It has a very low on-resistance of 2.07 mΩ·cm2, and a very high Baliga’s figure-of-merit of 8.6 GW/cm2is obtained in the optimized GaN VCD-JBS.Therefore, a reasonable optimization of multi-layer of low-Kdielectric and high-Kdielectric structure can make the electric field distribution more uniform, and thus achieve greater breakdown voltage.

Figure 11 shows the comparison of GaN VCD-JBS in this work with other reported diodes. The high-performance GaN VCD-JBS makes it an advanced high-voltage power device, with the highestBVof 4171 V, an extremely lowRonof 2.07 mΩ·cm2, and an excellentFOMof 8.6 GW/cm2. The slash lines in Fig.11 represent the theoretical limits of Si,SiC,and GaN,respectively. The optimized structure is far beyond the theoretical limit of SiC and close to the theoretical limit of GaN.The work represents great potential for GaN VCD-JBS performance.

Fig.11. Baliga’s figure-of-merit is shown for state-of-art devices and for our proposed devices from this paper,the 3491 V devices with Ron values of 2.2 mΩ·cm2.

5. Conclusion

In this work, a vertical GaN-based JBS with a high-K/low-Kcompound dielectric structure (GaN VCD-JBS) is proposed. The electric field peaks introduced by high-K/low-Kdielectric layers make the electric field away from the p–n junction rise and the distribution of the electric field becomes more uniform. The width of Schottky contactWn,the distance from the interface of p–n junction in JBS to low-Kdielectric layerT, and the length of the dielectric layerLof the highK/lowKdielectric composite layer are optimized respectively in subsequent work,resulting in superior forward and reverse characteristics.The simulation results show that when the drift region thickness of GaN VCD-JBS is 15 μm and the drift region concentration is 1.5×1016cm-3, a highBVof 4171 V and lowRon,spof 2.07 mΩ·cm2for GaN VCD-JBS with three lowKdielectric composite layers can be achieved, and the Baliga’s figure-of-merit is 8.6 GW/cm2. This structure shows the potential of GaN JBS as a power diode and provides new ideas for designing high withstand voltage devices.

Acknowledgements

Project supported by the National Natural Science Foundation of China(Grant No.61376078)and the Natural Science Foundation of Sichuan Province, China (Grant No. 2022NSFSC0515).