Introduction

SBD is the so called unipolar device since its forward current is attributed to the transportation of majority carriers. It has characteristics such as fast reverse recovery time, high switch speed, and low power consumption. The reverse recovery time of an SBD can be as small as a few nanoseconds. And its on-voltage is about 0.1~0.3 V lower than that of corresponding PN junction devices. Generally, SBDs can be used in high frequency, low voltage, and large current situations^[1]. They can also be used as freewheel diodes and protected diodes. Deliberated designed SBDs are used as ultra high frequency rectifier diodes and small signal detector diodes in microwave communication^[2-3].

The basic requirements for a rectifier device are higher current capacity, lower conduction loss, higher transient speed, and higher reverse breakdown voltage. For SBD, the resistance of the drift layer is a major component of the on-resistance, and this layer mainly supports the reverse voltage. On the one hand, the drift layer should be thinner and heavily doped to make sure that the on-resistance is small enough. On the other hand, higher reverse breakdown voltage needs a thicker drift layer with lighter doping. Therefore, we must compromise the thickness and doping concentration of the drift layer of an SBD to balance the on-resistance and the reverse breakdown voltage. In this paper, the Super-Junction structure which is composed of alternate P-pillar and N-pillar^[4] was used to replace the drift layer to alleviate the contradiction between low on-resistance and high reverse breakdown voltage. The Super-Junction structure was fabricated through four times N-type epitaxy and four times P-type implant. The diode with super-junction structure is called SJ-SBD^[5-6]. The geometric parameters and process parameters of the proposed SJ-SBD were optimized through simulation under the SILVACO platform. The reverse breakdown voltage of the fabricated SJ-SBD is about 100 V higher than that of the traditional SBD.

1 Design of the SJ-SBD

A traditional SBD consists of a heavy doped substrate, a lightly doped epitaxial layer, and a metal-epitaxial layer Schottky contact. When the diode is forward biased, the resistance of the lightly doped epitaxial layer is the major component of the on-resistance. When it is reverse biased, the lightly doped epitaxial layer mainly supports the reverse voltage. The relationship between the on-resistance R_on and the reverse breakdown voltage BV_PP^[7] is:

${R_{{\rm{on}}}} = 5.53 \times {10^ - }^9B{V_{{\rm{pp}}}}^{2.5}$

(1)

This equation is the well known "silicon limit". It shows that there is contradiction between low on-resistance and high reverse breakdown voltage for SBDs.

In this paper, an SJ-SBD was proposed, in which the lightly doped epitaxial layer was replaced by the super-junction structure composed of alternate P-pillar and N-pillar. Fig.1 shows the basic structure. In this structure, all of the P-pillars and the N-pillars can be heavily doped. So, the on-resistance of the device can be reduced. When the SJ-SBD is reverse biased and close to breakdown, the fully depleted P-pillars and N-pillars can support much larger electric field. Therefore, the reverse breakdown voltage can be increased^[4]. For the SJ structure, the relationship between the on-resistance R_on and the reverse breakdown voltage BV_PP^[8] is:

${R_{{\rm{on}}}} \propto B{V_{{\rm{pp}}}}^{1.32}$

(2)

Compared with equation (1), the contradiction between low on-resistance and high reverse breakdown voltage was alleviated obviously.

The specification of an SJ-SBD is 200 V of breakdown voltage and 0.9 V of on-voltage under 10 A current. By using a quasi 2-D model, the relationship between the maximum electric field E₀ and reverse voltage V_B can be expressed as^[9-10]:

$\left\{ \begin{gathered} {E_0} = 6.18 \times {10^5}{V_{\rm{B}}}^{ - 1/6}{(1 + {f_{\rm{p}}}b/W)^{ - 1/6}}{\rm{ (point }}C{\rm{)}} \hfill \\ {E_0} = 6.18 \times {10^5}{V_{\rm{B}}}^{ - 1/6}{(1 + {f_{\rm{n}}}b/W)^{ - 1/6}}{\rm{ (point }}D{\rm{)}} \hfill \\ \end{gathered} \right.$

(3)

where b is the horizontal size of SJ cell, W is the thickness of the SJ layer, and f_p, f_n are the fit parameters for optimization for the P-pillar and N-pillar respectively.

By using V_B=220 V, W/b=2.5, and f_p=f_n=16.1, E₀=1.72×10⁵ V/cm, the thickness of SJ layer can be expressed as:

$W = {V_{\rm{B}}}/{E_0} = 12.79{\rm{ \mathsf{ μ} m}}$

(4)

For interdigitated SJ cell, the optimized design is X_N=X_P=b and N_SJ =N_a= N_d, where X_N and X_P are the width of the N-pillar and the P-pillar, respectively, N_d and N_a are the doping concentration of the N-pillar and the P-pillar, respectively, and X_N=X_P =5.116 µm is then determined.

The relationship between the electric field and doping concentration in N-pillar ^{[9, 11]} is:

${E_{1{\rm{n}}}} = {c_{\rm{n}}}q{N_{\rm{D}}}b/{\varepsilon _{\rm{s}}}$

(5)

Where ε_s is the dielectric constant of silicon, q is the electron charge, and c_n equals 0.371 for inter-digital SJ cell. By using c_n, b, E₀, ε_s, q and equation (5), the doping concentration of N-pillar is N_d=5.8×10¹⁵ cm-³. Using the same method, the doping concentration of P-pillar can be solved as N_a=5.8×10¹⁵ cm-³.

The actual parameters should be larger than the calculated values to tolerate process variations. So, the thickness of the SJ layer was designed for 14 µm and the doping concentration of both N-pillar and P-pillar were designed for 6×10¹⁵ cm-³. In order to analyze the influence of the quantity of charge in N-pillar and P-pillar on the reverse breakdown voltage of SJ-SBD, we designed three kinds of SJ structures. The width of N-pillar and P-pillar were X_N=X_P=6 µm, X_N =6 µm and X_P=10 µm, X_N =10 µm and X_P=6 µm, respectively.

2 Simulation and Analysis

Fig.2 shows the 2-D structure of traditional SBD and the proposed SJ-SBD simulated in process simulation tool ATHENA. Both of them consist of a heavily doped substrate, a light doped drift layer, and a Schottky contact formed by depositing PtSi_x on the drift layer. The difference between these two SBDs is that the drift layer in SJ-SBD is a SJ layer which was formed through four times N-type epitaxy and four times P-type implant. After the completion of all process steps, the impurity concentration of P-pillar and N-pillar in the SJ layer is same as that of the drift layer in SBD. Moreover, the top of P-pillar is necessarily heavily doped to make sure a good ohmic contact between P-pillar and PtSi_x.

Electrical characteristics of these two diodes were simulated in the device simulation tool ATLAS. The simulated result indicated that the on-voltage is about 0.7 V. The electric field distribution in case of reverse breakdown and characteristic curve under reverse bias are shown in Fig.3. In the SJ structure, charge compensation occured between P-pillar and N-pillar interactively under reverse bias. And the whole SJ layer would be depleted completely when the device is breakdown. From Fig.3a and Fig.3b, we can see that the depletion layer in SJ-SBD is much thicker than that in SBD under reverse breakdown situation. So, the breakdown voltage of SJ-SBD must be higher than that of traditional SBD obviously. Fig.3c shows the characteristic curve under reverse bias. It indicates that the breakdown voltage of SJ-SBD is higher about 120 V than that of traditional SBD.

The SJ layer can improve the reverse breakdown voltage of the SJ-SBD, but many parasitic PN junctions will be formed in the drift layer, which is not only the reverse voltage support layer, but also the region where forward current flows through. These PN junctions increase the parasitic capacitance, and they are also forward biased when the SJ-SBD is forward biased. when the SJ-SBD turns off, the recombination of minority carriers which exist near the boundary of the space charge region of these parasitic PN junctions takes a long time. So, the high frequency response of the SJ-SBD goes bad. The simulated reverse recovery characteristic curves of traditional SBD and SJ-SBD are shown in Fig.4. From these two curves, we can see that the reverse recovery time of SJ-SBD is observably longer than that of traditional SBD when forward current of two devices is equal.

3 Fabrication and Test Results

The proposed SJ-SBD consisted of a heavily doped N-type substrate, an SJ layer, and a Schottky contact. Firstly, an N-type epitaxial layer with a thickness of about 1/4 designed SJ layer (W in Fig.1) was prepared on the start substrate. Then P type dopant was implanted in the place where to form P-pillar. This process four times was repeated and the sample in proper temperature and time was annealed. The SJ structure with N pillar and P pillar were then formed as shown in Fig.2b. Subsequently, the top of P-pillar was heavily doped. After the deposition of PtSi_x, an ohmic contact between P-pillar and metal anode and Schottky contact between N-pillar and metal anode were formed. The fabrication of SJ-SBD chip was finally completed.

A traditional SBD with the same thickness and doping concentration of drift layer was fabricated at the same time. Fig.5 shows the tested breakdown voltage curves of two SBDs. We can see that the breakdown voltage of traditional SBD is about 110 V from Fig.5a, and about 229 V of SJ-SBD from Fig.5b.

Electric characteristics of the fabricated SBDs were tested in the semiconductor testing system (351-GT/P). Tested results are listed in table.1. The tested results in table.1 indicate that breakdown voltages of SJ-SBDs are higher than that of traditional SBD. On-voltage and inrush current of these two kinds of SBDs are nearly equal. Reverse current of traditional SBD is lower than that of SJ-SBDs. The relatively higher reverse current of SJ-SBDs is caused by the parasitic PN junctions composed of alternate P-pillar and N-pillar in the SJ structure. Reverse recovery time of SJ-SBDs is obviously longer than that of the traditional SBD. The reason is that the SJ structure in SJ-SBDs makes minority carriers contribute to forward current and increase the parasitic capacitance.

High breakdown voltage of SJ structure profits from the same charge quantity in N-pillar and P-pillar. If the charge quantity in these two areas is different, the breakdown voltage of this layer will be significantly reduced. From the tested results in table.1, we can conclude that a symmetric structure of N-pillar and P-pillar will have higher breakdown voltage.

4 Summary

The reverse breakdown voltage of the fabricated SJ-SBD is about 100 V higher than that of the traditional SBD for the same thickness and doping concentration of the N-type drift layer. The results indicate that for the proposed SJ-SBD, the lightly doped epitaxial layer for reverse voltage supporting can be replaced by alternate N and P pillars (SJ-structure), where a higher dopant concentration can be adopted to reduce the on-resistance while keeping a rather high breakdown voltage. Using the SJ layer as the reverse voltage support layer can alleviate the contradiction between low on-resistance and high reverse breakdown voltage. But the PN junctions composed of alternate P-pillars and N-pillars in the SJ structure cause higher reverse current and longer reverse recovery time.

We would like to acknowledge the Dr. fund of Guizhou university [Gui da Ren Ji He Zi (2013) 20Hao], which partly supported this work.