The physical concept of IGBT safe operating area and the failure mechanism of ultra-safe operating area

1. Introduction

There are many reasons for the failure of semiconductor power devices. It is also very difficult and complicated to perform a replacement analysis after replacement. One of the main reasons for failure is the use beyond the safe operating area (SOA). Therefore, it is very important to fully understand the SOA and control the maximum DC current IC and collector-emitter voltage Vce of the IGBT within the SOA during use. SOA is divided into forward bias safe operating area (FBSOA), reverse bias safe operating area (RBSOA), switch safe operating area (SSOA) and short circuit safe operating area (SCSOA).

2. Physical concepts of each safe working area

The SOA of an IGBT indicates its ability to withstand high voltage and high current, and is an important indicator of reliability.

2.1 Forward Bias Safe Operating Area (FBSOA)

FBSO is within the active region of the output characteristic curve where Vge>threshold voltage Vth, as shown in Figure 1. The area surrounded by ABCDO in Figure 1 is the DC safe operating area. The AB segment is the maximum DC current Ic limited by tc=80℃. The product of IC and Vce corresponding to point B is equal to the maximum dissipated power Pcm. The BC segment is the equal power consumption line. The CD segment is the boundary of the safe operating area limited by the secondary breakdown, and this segment is not equal power consumption. As Vce increases, the power consumption decreases, and the higher the Vce, the lower the power consumption. This shows that failure is more likely to occur in high voltage and strong electric field states.

As shown in Figure 1, as the pulse width decreases, the SOA expands. What needs to be explained here is that the FBSOA given in the manual is in addition to the DCSOA. The pulse SOA under a certain pulse width is the single pulse safe operating area. Moreover, FBSOA only considers conduction losses, not switching losses. Therefore, FBSOA is only applicable to Class A, Class B and short-circuit working conditions of power amplifiers without switching losses. For continuous operation with a certain pulse width and duty cycle, its safe operating area should be determined using the calculation of the transient thermal resistance curve.

2.2 Reverse Bias Safe Operating Area (RBSOA)

RBSOA是表明在箝位电感负载时，在额定电压下关断最大箝位电感电流Ilm的能力。Ilm一般是最大DC额定电流的两倍，而额定电压接近反向击穿电压。PT型IGBT和NPT型IGBT的反偏安全工作区略有不同。PT型IGBT的RBSOA是梯形SOA，NPT型IGBT的RBSO是矩形SOA。如图2所示。可见NPT型IGBT。在额定电压下关断箝位电感电流的能力强于PT型IGBT。因此，PT型IGBT不适用于电感负载电路和马达驱动等电路，而且短路持续时间TSC较短，一般不给出短路安全工作区。所以，NPT型IGBT的可靠性高于PT型IGBT。

2.3 Switch Safe Operating Area (SSOA)

开关字全工作区如图3所示。由图2和图3可见，SSOA和RBSOA相似，都是矩形的。所不同的是RBSOA只考虑关断时承受高电压大电感电流的能力。SSOA不仅考虑关断状态，同时也考虑开启瞬间。所以SSOA兼顾FBSOA和RBSOA两种状态的考虑。另外，纵坐标的电流，RBSOA是Iim ；而SSOA是最大脉冲电流Icm。一个是最大箝位电感电流，一个是最大脉冲电流。而且两者在手册中给出的数值又是相等的。现在有的公司只给出SSOA，不再给出FBSOA和RBSOA。在IGBT开启时，往往是Vce没有降下来，Ic就达到负载电流Il。在有续流作用时还要达到Ic +Ir r m。Ir r m为续流二极管的最大反向恢复电流，因此导通过程也存在高压大电流状态。

2.4 Short Circuit Safe Operating Area (SCSOA)

SCSOA is when the C-E of the IGBT is under high voltage (rated reverse voltage), and an excessively high gate voltage Vg is suddenly added between G-E. The excessively high Vg and high conduction breakdown cause a short circuit state, and its short circuit current ISC can be as high as 10 times the rated current IC. This is similar to the on state of SSOA, but ISC>Icm. During the entire short circuit time Tsc, the IGBT is always in the on state. In this state, the energy consumption of the IGBT is the largest in the four safe operating areas, and the probability of failure is also the highest. SCSOA is shown in Figure 4.

3. Failure Mechanism of Hyper-SOA

As the safe operating area, it is safe to work within the SOA, and it will be unsafe or cause failure if it exceeds the SOA. Due to the different bias states of the four safe operating areas, the failure mechanism beyond the SOA is also different. The opening state of FBSOA, SCSOA and SSOA is forward biased, while RBSOA is reverse biased. It is well known that the main cause of IGBT failure is the latch-up of the parasitic SCR and the burnout caused by the super junction temperature tj operation.

（1）RBSOA的失效：在额定电压下关断箝位电感电流Ilm时，由于关断来自IGBT发射极的沟道电子电流，寄生PNP管发射极注入到高阻漂移区（PNP管的是基区）的少子空穴一部经过PNP管的基区从IGBT的发射极流出。当该空穴电流Ih在NPN管的基区电阻R b上压降Ih·R≥0.7V时，NPN管导通，其共基极放大系数αnpn迅速增大。同时由于PNP管的集电极处于高压，集电结耗尽层宽度（Xm）很宽，使PNP管的有效基区Wb变窄，α pnp也增大。当α npn+α pnp1时出现动态锁定而烧毁。因此直角安全区是IGBT可靠性的重要标志。由图2可见NPT型IGBT具有直角SOA，而PT型IGBT是梯形安全工作区。这说明PT型IGBT在额定电压下关断的箝位电感电流Ilm比NPT型IGBT要小。其抗高压大电流冲击能力和短路能力都不如NPT型IGBT。

The turn-off failure mechanism of SSOA is the same as that of RBSOA.

When FBSOA, SCSOA and SSOA are turned on, they all work in the high voltage and high current state in the active region. Because they are in forward bias, the instantaneous current is 2-10 times the DC rated current. The α npn and α pnp of the parasitic NPN and PNP tubes in the IGBT increase with the increase of the operating current. When α npn+α pnp1, static lock burnout occurs.

(2) Failure of SCSOA: Since the short-circuit current ISC may be as high as 10 times the DC rated current, the Joule heat generated during the short-circuit time TSC is excessive and cannot be dissipated in time, resulting in thermal burnout.

For example: 100A 1200V NPN IGBT, when TSC=10μs, the energy generated is:

ESC=Vce·Ic·Tsc=12 joules.

This energy is generated in the depletion layer Xm of the P-well PN junction, and the electric field in the depletion layer is ε=1200V/Xm. At this time, Xm (1200V) is about 200μm, so ε=6×104V/cm. Define εm≥3×104V/cm as a strong electric field. Now, the drift velocity of electrons in the strong electric field reaches saturation when ε>εm. The reason for saturation is the optical wave phonon scattering under the strong electric field, and the energy of the external electric field is transferred to the scattered lattice through the optical wave phonon scattering. Quantum physics puts forward a basic fact: "Although electrons in solids are moving at high speed between dense atoms, as long as these atoms are arranged in a strict periodicity, the high-speed movement of electrons does not suffer from scattering." The defects in Si single crystals and epitaxial wafers are the destruction of the periodic arrangement of the lattice. The scattering cross section of the part with a large defect density is large. At this time, the energy received from the external electric field is more, and the lattice vibration of this part is violent, which increases the lattice temperature t1. When t1 is greater than the melting point of silicon (1415°C), Si melt holes appear and burn out. This is why Si melt holes are found in all burned devices after dissection. Here we have made the above analysis of the burnout mechanism using the application exceeding SCSOA as an example. We have made the above analysis of the burnout mechanism using the application exceeding SCSOA as an example. For applications exceeding FBSOA, SSOA and RBSOA, as long as the ratio of the bias voltage to the depletion layer width Xm corresponding to the bias voltage is greater than 3×104V/cm, the above burnout may occur.

The dissection found that the area of ​​the Si melt hole A si is about 100μm2~1mm2. The lattice temperature is:

T1=Ic·Vce·Tsc/Dsi ·Csii·Asi·X m       （1）

Where Dsi and Csi are Si specific gravity and heat ratio respectively. Csi = 0.7 joules/gram °C, Dsi = 2.328 grams/cm3. We assume that 10% of the energy generated during the short-circuit time of 10μs is absorbed by the strong scattering area, and take Asi = 1mm2. Substituting the relevant data into (1) yields: t1 = 3600℃. This temperature is much higher than the melting point of Si, 1415℃. No wonder a melt hole appears in the Si wafer after burning.