Exploring SiC Planar IGBTs towards Enhanced Conductivity Modulation Comparable to SiC Trench IGBTs

Abstract

The state-of-the-art silicon insulated-gate bipolar transistor (IGBT) features a trench gate, since it enhances the conductivity modulation. The SiC trench IGBT, however, faces the critical challenge of a high electric field in the gate oxide, which is a crucial threat to the device’s reliability. In this work, we explore the possibility of using a SiC planar IGBT structure to approach high performance to the level of a SiC trench IGBT, without suffering the high gate oxide field. The proposed SiC planar IGBT features buried p-layers directly under the p-bodies, and thus can be formed using the same mask set. The region between the buried p-layer and the p-body is heavily doped with n-type dopants so that the conductivity modulation is improved. Comprehensive TCAD simulations have been carried out to verify this concept, and the simulation results show the new SiC planar IGBT exhibits a high performance comparable to the trench IGBT, and also exhibits a low gate oxide field.

1. Introduction

Due to the excellent material properties (wide bandgap, a high critical electric field, a high temperature endurance, etc.), silicon carbide (SiC) is attractive for power devices such as the Schottky barrier diode, the metal–oxide–semiconductor field effect transistor (MOSFET), and the insulated-gate bipolar transistor (IGBT) [1,2,3,4].

SiC IGBT is welcomed in ultrahigh voltage power applications because the conductivity modulation in IGBTs can effectively reduce conduction loss [5,6,7,8]. For traditional silicon IGBTs, the trench-gate structure is the state-of-the-art technology, which takes advantage of the injection enhancement effect that further improves the conductivity modulation at the top side of the device [9,10]. However, SiC trench-gate devices face the issue of a high gate oxide field in the off-state [11].

In a conventional SiC planar IGBT, the gate oxide field is often low enough. The grounded p-body in the conventional SiC planar IGBT helps to protect the planar-gate. The drawback of the SiC planar IGBT is the compromised conductivity modulation, which is caused by the grounded p-bodies that extract minority carriers [9,12]. This compromised conductivity modulation will cause considerable increase of the on-state voltage (V_ON) and conduction power loss.

In this paper, we explore the possibility of using a SiC planar IGBT structure to approach a high performance comparable to the SiC trench IGBT, and at the same time, to maintain a low gate oxide field. A new SiC planar IGBT (BP-IGBT) with buried p-layers under the p-body is proposed. The buried p-layers are aligned with p-bodies, and thus they can share the same mask set. Numerical simulation results show the BP-IGBT obtains a low V_ON close to the SiC trench IGBT. The breakdown voltage (BV) of the BP-IGBT is kept nearly the same as the SiC trench IGBT. The maximum gate oxide field in the BP-IGBT is dramatically lower than that in the SiC trench IGBT.

Sentaurus TCAD tools are used for the simulations [13]. Sentaurus Structure Editor is used for structure and mesh construction. The mesh of the device consists of a coarse mesh definition and several refined mesh definitions. The coarse mesh is placed for the whole device with lateral/vertical mesh sizes of 0.4/0.6 µm. The top 10-µm region of the device is fitted with a refined mesh with lateral/vertical mesh sizes of 0.1/0.2 µm. The PN junctions are further refined with a mesh size of ~0.02 µm along the direction perpendicular to the junction. At the oxide/semiconductor interfaces, the mesh size along the direction perpendicular to the interface is refined to 0.002 µm. The Sentaurus Device is used for device simulations and mixed-mode circuit simulations. In device simulations, the Poisson equation and electron/hole continuity equations are solved self-consistently. In mixed-mode circuit simulations, the circuit equations are also solved in the same manners as traditional SPICE tools. The physics models, including the Auger and Shockley–Reed–Hall combinations; impact ionization (Lackner model); incomplete ionization; the high-field saturation effect; doping dependent transport; band narrowing; and the anisotropic mobility model are all considered.

2. Device Structure and Proposed Process Flow

Figure 1 shows the cross-sectional schematic views of the studied conventional SiC planar IGBT (P-IGBT), SiC trench IGBT (T-IGBT), and the proposed buried p-layer IGBT (BP-IGBT). A heavily doped n-type region is located between the buried p-layer and the p-body in the BP-IGBT. The devices are designed for a 20-kV voltage level. The trench gate in the T-IGBT has a width of 1.5 μm and rounded bottom gate corners with a radius of 0.2 μm. The buried p-layer in the BP-IGBT has a doping density of 1 × 10¹⁸ cm⁻³ to avoid being fully depleted in the off-state. The distance between neighboring p-bodies or neighboring buried p-layers is 2 μm. The channel mobility is set to be 30 cm²/V-s. The channel length is 1 μm. The devices are designed for 20-kV voltage rating. The device parameters are carefully chosen based on the state-of-the-art SiC IGBTs and MOSFETs [5,6,7,8,9,10,14,15].

Table 1. Key structural parameters of the studied SiC IGBTs.

Parameter	P-IGBT	T-IGBT	BP-IGBT	Unit
n-drift thickness	180	180	180	μm
n-drift doping	2.5 × 1014	2.5 × 1014	2.5 × 1014	cm−3
gate oxide thickness	50	50	50	nm
n+-emitter thickness	0.2	0.2	0.2	μm
n+-emitter doping	5 × 1019	5 × 1019	5 × 1019	cm−3
p-body thickness	1.0	1.2	1.0	μm
p-body doping	1 × 1017	1 × 1017	1 × 1017	cm−3
cell pitch	14	3.5	14	μm
carrier lifetime	2.0	2.0	2.0	μs
n+-region doping	2 × 1016	2 × 1016	2 × 1016	cm−3
thickness of buried p-layer	n/a	n/a	1	μm
buried p-layer doping	n/a	n/a	1 × 1018	cm−3
n-buffer thickness	10	10	10	μm
n-buffer doping	4 × 1017	4 × 1017	4 × 1017	cm−3
p-collector thickness	5	5	5	μm
p-collector doping	1 × 1019	1 × 1019	1 × 1019	cm−3

lists the key device parameters used in this work.

The fabrication method of the proposed BP-IGBT is similar to the P-IGBT, except for the buried p-layers. A proposed fabrication process flow is shown in Figure 2. After growth of the drift layer and n+ layer, the buried p-layers can be formed by ion-implantation. Then, a second n+ layer is regrown. The surface structure can be formed in the same way as a traditional SiC IGBT, which includes implantation for the p-body and n+-emitter region, formation of the MOS-structure, contact holes for the emitter, and metallization. For SiC technology, formation of buried p-layers is a practically viable process, and has been reported in literature [16,17]. Therefore, the proposed structure in this paper is a technically feasible solution for the SiC IGBT.

3. Device Characteristics

The vertical distance (D_pp) between the p-body and the buried p-layer in the BP-IGBT is critical to the device’s performances. Figure 3 shows the influence of D_pp on V_ON and BV for the BP-IGBT. Here, V_ON is defined as the collector voltage with I_C = 50 A/cm² and V_GE = 15 V. BV is defined as the collector voltage with I_C = 1 A/cm² and V_GE = 0 V. V_ON decreases with larger D_pp, because the heavily doped n+-region is beneficial for increasing conductivity modulation in the IGBT. As for the BV, when D_pp is smaller than 6.5 μm, a stably higher BV (>25 kV) is obtained. Then, if the buried p-layer is much deeper, BV degrades quickly as premature breakdown happens at the p-body/n+-region junction. Thus, in the following study, D_pp = 5 μm is adopted to leave enough of a margin for a stably high BV.

Figure 4 plots the forward I_C–V_CE characteristics of the SiC IGBTs. The V_ON of the proposed BP-IGBT is only 3.6 V, close to the 3.4 V of the T-IGBT. This is because the BP-IGBT obtains an enhanced conductivity modulation induced by the heavily doped n+-region at the top side. The P-IGBT suffers a much higher V_ON of 5.2 V as the grounded p-body extracts holes around it.

The enhanced conductivity modulation in the BP-IGBT and T-IGBT can be evidenced by the hole density, as is displayed in Figure 5. At the lower side of the IGBTs, all of them exhibit a similar hole density distribution. At the topside, the BP-IGBT obtains a high hole density to the same level of the T-IGB, while the P-IGBT shows an obviously decreased hole density. The grounded p-body in the P-IGBT presents a low barrier for holes, and thus quickly extracts holes and causes a low hole density. For the T-IGBT, the protruded trench gate creates an energy barrier for the holes and screens the p-body from extracting holes, which leads to a high density of holes at the topside of the device. For this purpose, a small cell size of the T-IGBT is preferable since the p-body is better screened by the gate [9,18,19]. For the proposed BP-IGBT, the n+-region under the p-body creates an energy barrier for holes, and also results in a high hole density.

The BV capabilities under V_GE = 0 V are plotted in Figure 6a. All the studied SiC IGBTs exhibit a nearly same BV of ~26.5 kV. The BV is mainly determined by the thickness and doping of the n-drift region. The difference in the top structure just slightly changes the BV. In the P-IGBT and T-IGBT, the BV is supported by the PN junction below the p-body. While in the proposed BP-IGBT, the BV is mainly sustained by the p-shield/n-drift junction, as the n+-layer below the p-body is depleted under a small voltage. For the SiC planar IGBT with only the n+-region but no buried p-layer, the BV is less than 2 kV as the n+-region tends to increase the electric field at the p-body/n junction. The buried p-layer can overcome the problem caused by the n+-region, so a low V_ON and a high BV can be simultaneously obtained in the BP-IGBT.

Figure 6b further depicts the process in the BP-IGBT using potential contour. With the rise of V_CE, in the beginning, the voltage is first supported by the PN junction below the p-body, as shown in Figure 6b (V_CE = 450 V). When the n+-region is depleted, the potential of the buried p-layer is clamped, and the rest of the voltage is supported by the PN junction below buried p-layers, as shown in Figure 6b (V_CE = 800 V and V_CE = 20,000 V). This analysis also explains the dependence of BV on D_pp in Figure 3. Therefore, the blocking mechanism of the BP-IGBT is different to the conventional P-IGBT and T-IGBT. As shown in Figure 6c, in the conventional IGBTs without buried p-layers, the p-body/n junction and/or MOS junction support the whole off-state voltage. The electric field is highest near the p-body/n junction, and/or MOS junction, where the equipotential lines are most crowded.

The gate oxide field distribution in the off-state of the SiC IGBTs are displayed in Figure 7. As the gate protrudes downward, the T-IGBT suffers an extremely large oxide field (E_ox-m) of ~8 MV/cm, which presents a serious threat on the device’s long-term reliability [20]. In this study, the radius of the gate trench corner is 0.2 µm. The maximum oxide field (E_ox-m) of the T-IGBT is dependent on the shape of the trench corner. With the radius of the trench corner changing from 0.1 µm to 0.5 µm, the E_ox-m reduces from 9.76 MV/cm to 7.58 MV/cm. Therefore, a rounded trench corner helps to relieve the electric field stress in the gate oxide, but cannot overcome this problem. While in the P-IGBT and the proposed BP-IGBT, the planar-gate structures help to decrease the E_ox-m to a safe level of below 3 MV/cm.

Introducing floating p-layers in the unipolar SiC MOSFETs is found to cause a storage of negative charges and a degradation of dynamic V_ON [21,22]. But for IGBTs with bipolar conduction, the stored negative charges in the off-state can be neutralized in the following on-state, by the holes injected from p-collector. The dynamic V_ON characteristics of the BP-IGBT are also studied in this paper and are shown in Figure 8. Mixed-mode TCAD simulations are utilized. The schematic circuit used for switching characteristics is shown in Figure 8a. The supply voltage is 13 kV. The stray inductance is assumed to be 10 nH in the power loop. The chip area of the device under test (DUT) is set to be 1 cm². To turn the device on and off, the gate to emitter voltage is switched between +15 V and −5 V.

The multi-cycle switching waveforms are plotted in Figure 8b. For comparison, the dynamic V_ON characteristics of the corresponding SiC MOSFET (BP-MOSFET) are also studied. The BP-MOSFET is designed for the same voltage rating as the BP-IGBT by replacing the p-collector with the n-drain. For the BP-MOSFET, when the device experiences a high voltage in the off-state, the buried p-layers support a large portion of the voltage, and the p-layers become negatively charged. When the device returns to the on-state, the negative charged p-layers possess a negative potential, and the negative charges cannot be released through the negatively biased PN junctions. Therefore, the negative charges lead to an expansion of depletion regions, which significantly impedes the current flow and results in a V_CE as high as 5.5 kV [21,22]. The BP-IGBT is a bipolar device. For the BP-IGBT, when it returns to the on-state, the large amount of positively charged holes can quickly eliminate the negative charges in the buried p-layers. Therefore, the BP-IGBT does not suffer degraded dynamic V_ON. The V_ON at I_C = 50 A/cm² in this Figure 8b is the same as the static condition in Figure 4.

With the schematic circuit in Figure 8a, the turn-off characteristics of the studied SiC IGBTs are studied and shown in Figure 9. The load current is 50 A/cm². The turn-off loss (E_OFF) of the devices can be obtained by integrating the power generation (V_CE × I_C) from 0 to 6 μs from Figure 9. The E_OFF is normalized to the device area. E_OFF is 0.65 J/cm² for the P-IGBT, 0.65 J/cm² for the T-IGBT, and 0.62 J/cm² for the BP-IGBT. Although in the on-state, the T-IGBT and BP-IGBT feature a much higher hole density at the top side of the device compared to the P-IGBT, the E_OFF values of all the devices are nearly same. This is because at the initial stage of the turn-off transient, the holes at the topside of the IGBT are first removed. For this period, the V_CE is still very small, so it does not appreciably influence the E_OFF. The hole density at the bottom side of the device has a much higher influence on E_OFF, since when these holes are removed, V_CE is also very large. In the figure, V_CE of the P-IGBT and the proposed BP-IGBT start to rise before the T-IGBT, because the gate to collector capacitance of the P-IGBT and the BP-IGBT is smaller. However, this feature does not affect the E_OFF of the devices.

The main device characteristics of the three devices are listed in

Table 2. Comparison of the device characteristics.

	P-IGBT	T-IGBT	BP-IGBT	Unit
VON	5.2	3.4	3.6	V
BV	26.5	26.09	26.15	kV
Eox-m	2.14	8.02	2.36	MV/cm
EOFF	0.65	0.65	0.62	J/cm2

for comparison. The proposed BP-IGBT obtains high performance, to the level of the SiC trench IGBT, and is a promising candidate of power switches in ultrahigh voltage power applications.

4. Conclusions

In this paper, we use a new SiC planar IGBT (BP-IGBT) to approach the high performance level of a SiC trench IGBT (T-IGBT). The proposed BP-IGBT features buried p-layers and obtains a low on-state voltage (V_ON) of 3.6 V, close to the 3.4 V of the T-IGBT. The buried p-layer is aligned with the p-body, and thus they can share the same mask set. The breakdown voltage of the BP-IGBT is nearly the same as the T-IGBT. The proposed BP-IGBT avoids a high off-state gate oxide field, which often is an issue in the T-IGBT. The dynamic V_ON characteristics of the BP-IGBT are not degraded.