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(ang. MOSFET secondary breakdown). This document describes the secondary breakdown of a power MOSFET.

MOSFET secondary breakdown

This section describes the secondary breakdown limit indicated by the [[wikipedia-en:|]] (SOA) curves of a MOSFET.

Safe operating area of a MOSFET

Figure 1: Safe operating area of a MOSFET

The SOA is the voltage and current conditions over which a MOSFET operates without self-damage or degradation. The MOSFET must not be exposed to conditions outside the safe operating area even for an instant. In their early history, MOSFETs were known for the absence of secondary breakdown, which was a failure mode specific to bipolar transistors. The SOA of a MOSFET was bound only by the maximum drain-source voltage, the maximum drain current, and a thermal instability limit between them. However, due to device geometry scaling, recent MOSFETs exhibit a phenomenon resembling secondary breakdown. It is therefore necessary to determine whether the operating locus of the MOSFET falls within the SOA.

The SOA of a MOSFET is divided into the following five regions (Figure 1):

1. Thermal limit - This is the area limited by the power dissipation P_D (absolute maximum rating). The maximum permissible power dissipation is determined by the guaranteed channel temperature and thermal resistance. Therefore, the SOA has a slope that represents the maximum permissible power dissipation (which has a slope of -1 on a double logarithmic graph). The thermal limit depends on the operating conditions of a MOSFET since the maximum permissible power dissipation and the device temperature vary with the conduction time and ambient temperature of the MOSFET.
2. Secondary breakdown limit
3. Current limit - This defines the area limited by the maximum drain current rating. For continuous-current (DC) operation, the SOA is limited by the I_D (absolute maximum rating). For pulsed operation, the SOA is limited by the I_DP (absolute maximum rating).
4. Drain-source voltage limit - This defines the area limited by the drain-source voltage V_DSS (absolute maximum rating).
5. On-state resistance limit - This defines the area that is theoretically constrained by the on-state resistance R_DS(ON)(max). I_D is equal to V_{DS /RDS(ON)}(max).

MOSFET secondary breakdown

Secondary breakdown is a failure mode in bipolar transistors in which negative resistance (current concentration) occurs under high-voltage and high-current conditions. Current concentration causes local heating, resulting in a small hotspot. The impedance of the hotspot decreases, causing further current concentration. This cycle called thermal runaway leads to device degradation and destruction.

In this regard, the secondary breakdown limit in the SOA of a power MOSFET can be considered in the same manner as that of a bipolar transistor. However, the secondary breakdown of a power MOSFET is not defined by the operation of a parasitic bipolar transistor in the MOSFET structure. To be precise, the MOSFET failure mode should not be called secondary breakdown, but the same term has traditionally been used for both bipolar transistors and power MOSFETs.

Mechanism of MOSFET secondary breakdown

This subsection describes the mechanism of MOSFET secondary breakdown. When the MOSFET gate is forward-biased, charge carriers are attracted to the interface between the gate electrode and the gate oxide, forming an inversion layer. The inversion layer provides a channel through which current can pass between source and drain terminals. The gate voltage at which this occurs is called the threshold voltage V_th. The MOSFET drain current is controlled by the amount of charge carriers transported to the interface between the gate electrode and the gate oxide.

Since the number of charge carriers increases with temperature, V_th decreases with temperature. The channel resistance is in inverse proportion to the difference between the gate voltage V_GS and V_th (i.e., V_GS - V_th). (As (V_GS - V_th) increases, the number of carrier charges increases. This causes an increase in the charge carrier density and therefore a decrease in channel resistance.

As mentioned above, V_th decreases with temperature since the number of charge carriers increases with temperature. This means that the channel resistance decreases as (V_GS - V_th) increases. The following paragraphs discuss the mechanism of MOSFET secondary breakdown based on these facts:

1. As the MOSFET temperature increases, the gate threshold voltage V_th decreases, reducing the channel resistance.
2. Current concentrates in the channel with reduced resistance, causing a further temperature rise, which results in a further decrease in the gate threshold voltage V_th.
3. Consequently, further current concentration occurs. This cycle eventually leads to device destruction.

As a result of the foregoing, the SOA is limited by the secondary breakdown line, considering changes in channel resistance due to temperature changes. The line constrained by the maximum permissible power and heat dissipation (i.e., ambient temperature and thermal resistance) and the line bound by secondary breakdown, which includes channel resistance (i.e., temperature dependence of the gate threshold voltage) as one of the causative factors, have different slopes.

As described above, the secondary breakdown of a MOSFET is device destruction caused by current concentration. Therefore, MOSFETs with high transconductance g_m (i.e., with a high current gain) and those that exhibit significant changes in drain current due to changes in V_th in the high V_DS region are highly susceptible to destruction.

While current is flowing because of avalanche breakdown, the gate is off, and therefore no current flows through the gate channel. Consequently, the MOSFET is not susceptible to secondary breakdown because it is not affected by the temperature-dependency characteristics of the gate threshold voltage V_th.