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Avalanche failure mode

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Avalanche failure mode

Power MOSFETs have an intrinsic bipolar transistor in their structure. This vertical device,

as illustrated in Figure 1, consists of the P+ diffusion, the N- epitaxial layer and the N+

substrate with the base-emitter junction shorted by the source metalization, forming the

“Body Diode”. At the OFF-state, this non-symmetrical structure is reverse bias. The

maximum reverse bias voltage that can be applied to a p-n body diode is limited by

breakdown. When the applied voltage exceeds breakdown, a critical electrical field is

reached and the carriers in the transition region are consequently accelerated to energies

sufficient to free electron-hole pairs via collisions with bound electrons. This mechanism,

known as Avalanche phenomenon, causes an electrical current multiplication that can allow

very large currents within materials. Basically, the breakdown mechanism is not destructive

for a p-n junction, however, heating caused by the large breakdown current and high

breakdown voltage can damage a MOSFET device. In particular, two mechanisms can

generate the failure of MOSFETs. The first one occurs because of the creation of thermally

generated carriers in the epitaxial/bulk region and hence the creation of hot spots. The

second one depends on the avalanche current: if this current creates an increasing voltage

drop across the RB resistor (see Figure 1) sufficient to forward bias the parasitic BJT, it turns

on, with potentially catastrophic results as control of the switch is lost. Due to these “failure

without any bipolar latch up.

Figure 1. MOSFET inside structure