Basic concepts

TN1156

4/10

DocID025511 Rev 1

the carriers in the channel and V

TH

change theirs values. In particular, either μ or V

TH

decrease when the temperature increases. From the derivative of (equation

2) against T, the

thermal coefficient, TC, of the device can be achieved by:

Equation 4

TC is the main thermal-electro parameter used to monitor the thermal instability

phenomenon. In fact, in linear zone, the electro-thermal stability of each device is evaluated

by considering a graph where TC is achieved versus I

D

. In equation

4, the first term,

depending on the derivative of K, tries to make TC negative while, vice versa, the second

term, depending on the derivative of V

TH

, tries to make the same coefficient positive. If the

first one is higher than the second one, TC becomes negative and no failure occurs, vice

versa, TC becomes positive and a thermal run-away phenomenon could occurs. However,

even if TC is positive, the device could work in safety region. This depends on the capability

of the whole die thermal system to catch the heat per unit area and time developed by the

electrical power pulse. If the heat produced by unit time can be totally extracted from the

device, then the Power MOSFET works in safety conditions. Otherwise, the heat increases

the internal energy of the system causing a die temperature rise until T reaches the

maximum allowable value (localized silicon) leading the device to fail.

I

D

∂

T

∂

--------

I

D

K

-----

K

∂

T

∂

------

V

DS

c onst

=

=

2KI

D

V

TH

∂

T

∂

-------------

V

DS

con s t

=

–