Integrating a Soft Body Diode in the Super-Junction MOSFET by Using an n⁻/n⁺-Buffer Layer

Abstract

In this paper, a novel silicon super-junction (SJ) MOSFET embedded with a soft reverse recovery body diode is proposed and studied by numerical simulation. The device introduces an n⁺-buffer layer between the n⁻-buffer layer and the n⁺-substrate to improve the reverse recovery behaviour of its body diode. The n⁺-buffer layer provides residual carriers during the reverse recovery process, reduces the overshoot voltage, and suppresses oscillation. Simulated results demonstrate that the increment of the on-resistance and the drain-to-source overshoot voltage can be respectively kept below 5% and 20 V, if a 10 μm n⁺-buffer layer whose impurity concentration ranges from 4 × 10¹⁵ cm⁻³ to 6 × 10¹⁶ cm⁻³ is used. In addition, the fabrication process is the same as that of the conventional SJ-MOSFET. These features make the proposed SJ-MOSFET suitable for inverter applications.

1. Introduction

The performance of the traditional silicon-based power MOSFET is limited by the contradictory relationship between its specific on-resistance and breakdown voltage, which is known as the silicon limit [1]. In order to overcome this limitation, new semiconductor materials are adopted and novel device structures are developed. Power MOSFETs made from the third-generation semiconductors, such as the SiC MOSEFT and GaN HEMT, have demonstrated their superior performance over their silicon-based counterparts [2,3]. On the other hand, the super-junction (SJ) MOSFET breaks the silicon limit by using alternative highly doped n-pillars and p-pillars in its drift region [1,4,5,6]. The most attractive advantage is that its specific on-resistance is less than one fifth of that of the conventional power MOSFET with the same rated voltage. As a result, the device area and parasitic capacitors are reduced. These features make it suitable for high-frequency circuits. Therefore, it is now widely used in modem power converters [7]. One of its disadvantages is the poor reverse recovery behaviour of its body diode, which can be used as the free-wheeling diode in power inverters [8]. In inverter circuits with an inductance load, free-wheeling diodes are necessary to conduct load currents when power devices are switched off. There are two main problems with its body diodes: the reverse recovery charge is large, and the reverse recovery current is snappy [9]. The former causes high energy loss and reduces power efficiency. The latter causes overshoot voltage and oscillation. The reverse recovery problem of the body diode is a key scientific problem toward super-junction MOSFETs. It limits their applications in power inverters. Many solutions have been proposed to reduce the reverse recovery charge, such as lifetime killing techniques using irradiation by electrons, protons, helium ion and heavy metal doping by gold, platinum [10,11]. Some researchers suggest integrated unipolar diodes [12,13]. Little attention has been paid to the snappy reverse recovery current. Modern SJ-MOSFETs usually use a lightly doped n⁻-buffer layer to improve the softness of the reverse recovery current [14]. The thicker the n⁻-buffer layer, the softer the reverse recovery current [15]. However, a thick n⁻-buffer layer increases the on-resistance of the SJ-MOSFET.

In this study, we propose a practical solution to integrate a soft body diode in the SJ-MOSFET without compromising its on-resistance. It uses an additional n⁺-buffer layer to reduce the overshoot voltage and suppress oscillation during the reverse recovery process of the body diode. The device structure and mechanism are shown in Section 2. Section 3 summarizes the optimised results. The conclusion is in Section 4.

2. Device Structure and Mechanism

Figure 1 shows the schematic cross-sections of the conventional SJ-MOSFET and the proposed SJ-MOSFET, which both have a symmetrical interdigitated layout. The difference lies in the buffer region. As shown in Figure 1b, an n⁺-buffer layer is added between the n⁻-buffer layer and the n⁺-substrate in the proposed structure. Its impurity concentration is much lower than that of the n⁺-substrate but much higher than that of the n⁻-buffer layer. It is used to improve the softness of the body diode. As explained before [9], the reverse recovery current of the conventional SJ-MOSEFT is snappy because there are not enough carriers when it reaches its negative peak value. Since the n⁺-substrate is heavily doped and the net recombination rate in the semiconductor is proportional to the carrier density [16], the holes’ density in the n⁺-substrate is tiny. Once the pillars are depleted, only a few carriers exist in the n⁻-buffer layer. Then, the reverse recovery current drops sharply. In the proposed SJ-MOSEFT, since the n⁺-buffer layer has a much lower impurity concentration than the n⁺-substrate, some nonequilibrium carriers accumulate in this region when the body diode is forward biased. Then, during the reverse recovery process, these carriers keep the current continuous after the pillars are depleted. On the other hand, the n⁺-buffer layer introduces the series resistance. To reduce its impact on the on-resistance and the breakdown voltage, its impurity concentration should be higher than those of pillars and the n⁻-buffer layer.

In the following simulations, 650 V-rated silicon SJ-MOSFEFTs with a half cell pitch of b = 6 μm are used. The pillar thickness T_SJ is 40 μm. For simplicity, SJ pillars are uniformly doped. The p-pillar concentration N_p and the n-pillar concentration N_n are both set to 4 × 10¹⁵ cm⁻³. The thickness of the n⁻-buffer layer T_Bn- is 10 μm. Its impurity concentration T_Bn- is 1 × 10¹⁵ cm⁻³. The thickness and the impurity concentration of the n⁺-buffer layer, namely T_Bn+ and N_Bn+, are respectively chosen as 10 μm and 4 × 10¹⁶ cm⁻³, unless otherwise specified. Each device has an area of 10 mm². The Sentaurus Device simulator [17] is used to analyse the electrical performances of devices. Employed mobility models include the DopingDenpendnce model, the HighFieldSturation model, and the Enormal model. The Shockley–Read–Hall recombination model and the Auger recombination model are also used. The Okuto avalanche model is used in blocking characteristics’ analysis. The electron lifetime and the hole lifetime are, respectively, 3 μs and 1 μs.

To explore the reverse recovery behaviour of body diodes, the double pulse test is carried out in mix-mode simulations. It uses the circuit shown in Figure 2. The Device Under Test (DUT) and the switching transistor M0 are both constructed with finite element models. The DUT is marked in red. To close the channel, its gate electrode and source electrode are connected. Then, its body diode serves as the free-wheeling diode. Other components are constructed with compact models. L_D and L_S are the load inductor and the parasitic inductor, respectively. R_G is the gate resistor. It controls the commutation velocity di/dt during the reverse recovery process. V_D is bus voltage source. V_G drives M0 with a double pulse signal.

3. Results and Discussion

Figure 3a compares the simulated blocking characteristic of the proposed SJ-MOSFET with that of the conventional SJ-MOSFET. Figure 3b,c show respective equipotential lines at V_DS = 650 V. The extracted breakdown voltage of the conventional SJ-MOSFET is 979.5 V, while that of the proposed device is 979.6 V. Their blocking characteristics are almost the same, because their voltage-sustaining layers, namely the pillars and the n⁻-buffer layer, are the same. The n⁺-buffer layer is hardly contributed to the breakdown voltage due to its high impurity concentration. As shown in Figure 3c, the depletion region in the n⁺-buffer layer is negligible.

Figure 4 compares output characteristics of the two SJ-MOSFETs. It is demonstrated that the on-state current of the proposed SJ-MOSFET is a little smaller than that of the conventional device. The on-resistance, R_ON, of the proposed SJ-MOSFET and conventional SJ-MOSFET, extracted at V_GS = 10 V, are 207.7 mΩ and 209.3 mΩ, respectively. The increment of on-resistance, ΔR_ON, is 1%. ΔR_ON is defined as the percentage increase in on-resistance with respect to the conventional device

$$\Delta R_{\mathrm{ON}}=\frac{R_{\mathrm{ON}, \mathrm{proposal}}-R_{\mathrm{ON}, \mathrm{convention}}}{R_{\mathrm{ON}, \mathrm{convention}}}\times 100\%$$

(1)

where R_{ON, proposal} and R_{ON, convention} are on-resistances of the proposed SJ-MOSEFT and the conventional SJ-MOSEFT, respectively. This increment comes from the series resistance of the n⁺-buffer layer R_Bn+, as illustrated in Figure 1b. Since the impurity concentration of the n⁺-buffer layer is lower than that of the n⁺-substrate, its series resistance is also larger. R_Bn+ can be simply calculated as

$$R_{\mathrm{Bn}+}=\frac{1}{q{\mu }_{\mathrm{n}}A}\frac{T_{\mathrm{Bn}+}}{N_{\mathrm{Bn}+}}$$

(2)

where q is electron charge, μ_n is the electron mobility, and A is the area of the device. Hence, R_Bn+, as well as ΔR_ON, increases with T_Bn+ and decreases with N_Bn+.

Figure 5a shows the simulated forward I-V curves of the two body diodes. At I_S = 20 A, the extracted voltage drops are 0.82 V and 0.83 V, respectively. Again, the n⁺-buffer layer introduces the series resistance and increases the voltage drop. The corresponding hole density distributions along the n-pillar centre are shown in Figure 5b. For the conventional device, since both the SJ pillars and the n⁻-buffer layer are lightly doped, a large amount of non-equilibrium carriers accumulated. The injected holes vanish quickly in the n⁺-substrate because the recombination rate is very high in this heavily doped region. For the proposed device, since the n⁺-buffer layer is not heavily doped, the recombination rate is much smaller than that in the n⁺-substrate. Thus, many non-equilibrium carriers also accumulate there. These additional carriers help to the improve the reverse recovery behaviour of its body diode.

Figure 6a shows reverse recovery waveforms with di/dt = 200 A/μs and L_S = 10 nH, at 300 K. V_D is 400 V. It is obvious that the reverse recovery current of the proposed device is softer than that of the conventional device. During the reverse recovery period, the maximum commutation velocity di/dt_max decreases by 62.9%, from 2.4 × 10¹⁰ A/s to 8.9 × 10⁹ A/s. As a result, the drain-to-source overshoot voltage ΔV_DS decreases from 220.4 V to 11.5 V. Figure 6b,c show hole density distributions at select points. Once the space charge region reaches the n⁻-buffer region at t₂(t₂′), the reverse recovery current reaches its negative peak value. After that, the reverse recovery current decreases gradually. For the conventional device, only a few holes exit in the SJ pillars and the n⁻-buffer layer. Then, its reverse recovery current decays steeply, which causes a large voltage spike. On the other hand, for the proposed device, there are still plenty of holes in the n⁻/n⁺-buffer layer. Therefore, the reverse recovery current decays slowly and the voltage spike is suppressed. Of course, it takes some time to extract these residual holes. Thus, the reverse recovery charges increase. The reverse recovery behaviour of the proposed device can be improved further by increasing the thickness, or decreasing the impurity concentration of the n⁺-buffer layer.

Table 1. Performance comparison of SJ-MOSFETs.

Device		BV (V)	RON (mΩ)	VF (V)	di/dtmax (A/s)	ΔVDS (V)	QRR (μC)
Convention	TBn− = 10 μm	979.5	207.7	0.82	2.4 × 1010	220.4	2.8
	TBn− = 15 μm	993.3	229.4	0.82	9.1 × 109	9.9	3.2
[13]		720.0	178.8	0.95	-	141.2	0.3
Proposal		979.6	209.3	0.83	8.9 × 109	11.5	3.4

summarizes performances of the above two SJ-MOSFETs. The transfer characteristics of the two SJ-MOSEFT are not exhibited. Their threshold voltages are equal because they have the same MOS channel structure. The most affected parameters are R_ON and ΔV_DS (or di/dt_max). They are contradictory. Increasing T_Bn+ or decreasing N_Bn+ introduces more non-equilibrium carriers in the n⁺-buffer layer. Then, both di/dt_max and ΔV_DS are reduced. However, R_ON also increases, as Equation (2) demonstrates. For example, ΔV_DS can be reduced to 9.9 V by increasing T_Bn- from 10 μm to 15 μm, as listed in Table 1. However, R_ON increases to 229.4 mΩ. The increment exceeds 10%. Table 1 also includes a SJ-MOSEFT with reduced Q_RR; but, its V_F and ΔV_DS increases. In the rest of this section, we discuss the influence of N_Bn+ and T_Bn+ on ΔV_DS and ΔR_ON.

Figure 7a shows reverse recovery waveforms of devices with T_Bn+ = 10 μm and N_Bn+ ranges from 1 × 10¹⁹ cm⁻³ to 1 × 10¹⁵ cm⁻³. N_Bn+ = 1 × 10¹⁹ cm⁻³ represents the conventional SJ-MOSFET. As N_Bn+ decreases, the reverse recovery current becomes smoother. Both di/dt_max and ΔV_DS decreases. Oscillation is also gradually suppressed. This is because the net recombination rate in the semiconductor is proportional to the carrier density. If N_Bn+ decreases, the net recombination rate also decreases. Then, more holes accumulate in the n⁺-buffer layer when the body diode is forward biased. During the reverse recovery process, they make the reverse recovery current smoother. Figure 7b shows reverse recovery waveforms of devices with N_Bn+ = 4 × 10¹⁶ cm⁻³ and T_Bn+ ranges from 0 μm to 40 μm. T_Bn+ = 0 μm represents the conventional SJ-MOSFET. As T_Bn+ increases, the reverse recovery current becomes smoother. Both di/dt_max and ΔV_DS decreases. Oscillation is also gradually suppressed. This is because more holes accumulate in the n⁺ buffer layer when the body diode is forward biased if T_Bn+ increases. During the reverse recovery process, they make the reverse recovery current smoother. The improvement becomes insignificant once T_Bn+ exceeds 20 μm. The reason is that holes deep into the n⁺ buffer layer recombine with electrons during the reverse recovery process. Their contribution to the reverse recovery current is negligible.

Figure 8 summarizes the influence of N_Bn+ and T_Bn+ on ΔV_DS and ΔR_ON. T_Bn+ increases from 5 μm to 40 μm. For each T_Bn+, N_Bn+ ranges from 1 × 10¹⁵ cm⁻³ to 1 × 10¹⁹ cm⁻³. If N_Bn+ = 1 × 10¹⁹ cm⁻³, the n⁺-buffer layer merges with the n⁺-substrate. Then, all samples are the same with the conventional SJ-MOSFET. If N_Bn+ = 1 × 10¹⁵ cm⁻³, the n⁺-buffer layer merges with the n⁻-buffer layer. Increasing T_Bn+ is equivalent to increasing the thickness of the n⁻-buffer layer. This is the usual method used in conventional SJ-MOSFETs. Without a doubt, ΔR_ON increases rapidly. For the proposed SJ-MOSFET with the same T_Bn+, ΔV_DS increases, and ΔR_ON decreases with N_Bn+. When N_Bn+ is below 2 × 10¹⁶ cm⁻³, ΔV_DS is ignorable but ΔR_ON is remarkable. The overshoot voltage is almost eliminated. When N_Bn+ is above 1 × 10¹⁷ cm⁻³, ΔR_ON is ignorable but ΔV_DS is remarkable. For the same N_Bn+, ΔV_DS decreases, and ΔR_ON increases with T_Bn+. Figure 8 also shows that N_Bn+ plays a more effective role in reducing ΔV_DS than T_Bn+, especially when T_Bn+ exceeds 10 μm. It means that a thick n⁺-buffer layer is not necessary. For T_Bn+ = 10 μm, the optimal range of N_Bn+ is 4 × 10¹⁵ cm⁻³ to 6 × 10¹⁶ cm⁻³. In this range, ΔR_ON is within 5% and ΔV_DS is within 20 V. Even T_Bn+ increases to 40 μm, ΔR_ON is within 20% and ΔV_DS is within 4 V.

4. Conclusions

This study demonstrates that a soft reverse recovery body diode can be integrated into the SJ-MOSFET by adding an n⁺-buffer layer between the n⁻-buffer layer and the n⁺-substrate. During the reverse recovery process, the drain-to-source overshoot voltage ΔV_DS can be greatly reduced and oscillation can be eliminated. Compared with the conventional device that uses a thick n⁻-buffer layer, the proposal solution does not compromise the on-resistance too much. According to the simulated results, the increment of R_ON and the overshoot voltage can be, respectively, kept below 5% and 20 V, if a 10 μm n⁺-buffer layer whose impurity concentration ranges from 4 × 10¹⁵ cm⁻³ to 6 × 10¹⁶ cm⁻³ is used. Moreover, the fabrication process is the same as that of the conventional SJ-MOSFET.