Degradation of Electrical Parameters of Power Semiconductor Devices – Process Influences and Modeling
4.2 In-situ temperature measurement
4.2.1 Drain current
In order to determine the temperature of the device during poly-heater use, the dependence of the drain current on the chuck temperature is needed. With this an increase of the drain current due to power dissipation in the poly-heater can be calculated to an according temperature increase. See the left hand side of Fig. 4.3 for a sample dependence of on the chuck temperature.
For this an operating point must be chosen which should be different from the temperature compensation point of the MOSFET [Spi+02]. From the full transfer characteristic of the device at every chuck temperature an operating point can be chosen where can be approximated by a low order polynomial to ease the to conversion. This dependence can later be used to map every increase of the drain current with poly-heater power supply to an increase of the device temperature [PobegenTDMR13; Aic+10c] as shown at the right hand side of Fig. 4.3.
4.2.2 Body diode current
The same concept for device temperature determination for the drain current can also be used to determine the body temperature with the current through the body diode, which is the current from source and drain to the bulk. In Fig. 4.4 the temperature measurements with either the drain or the body diode current are compared to each other.
The graph shows that the different currents give vastly different temperature values at the same poly-heater power values. The temperature obtained from the body diode does not reflect the temperature of the interface because of the temperature gradient within the device stack [Aic+10c], cf. also the finite element method (FEM) simulations subjacent depicted in Fig. 4.14. Furthermore, there is a difference in the temperature obtained by biasing the body diode in forward direction compared to the reverse direction. This difference most probably results from the fact that different areas within the semiconductor or pn-junction are responsible for either current. The reverse current exhibits a lower temperature as it averages over the large space charge region which extends into colder parts of the semiconductor. Generally speaking, even though the body diode current is predominately used to determine the device temperature [Mut+03; MW04; Sch+07; Boi+12], the use of the drain current is more appropriate for most studies because it directly reflects the temperature of the active device region [PobegenTDMR13; Wan+06; KWS07; Aic+10c].
The change of the resistance of the poly-heater can be used to obtain the temperature of the poly wires. This temperature is always much larger than the device temperature, see Fig. 4.5.
The reason why the temperature of the poly wires is always much higher than the device lies in the finite thermal resistances between the heater and the device. As will be described later, the correct understanding of the characteristics of these thermal resistances is important for an accurate determination and extrapolation of the device temperature.