TN1156 STMICROELECTRONICS | Alldatasheet

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  • 1 Basic concepts
  • 2 Experiments, results and discussion
  • 3 Conclusions
  • 4 References
  • 5 Revision history

Irradiated HV Power MOSFETs working in linear zone: a comparison of electro-thermal behavior with standard HV products Introduction This paper studies the thermal instability phenomenon of irradiated HV Power MOSFET devices working in linear zone operating conditions and compares their electro-thermal behavior with standard products. Experimental results show that irradiated devices have more thermal instability than standard devices, therefore, they should be used carefully in these particular operating conditions. In most of cases, Power MOSFETs are used in switching operating conditions where R DS(on) is the key parameter to evaluate the device’s performance. However, in some special cases, devices work in linear zone. A Power MOSFET works in linear zone when high-currents and high voltages are together applied to MOSFET terminals. Power MOSFETs are used in linear zone in some dedicated applications especially in automotive segment such as: standard topologies of audio amplifiers, linear DC-DC converters, DC fan controllers, electronic loads, current mirrors, smart fuses, etc… In many of these applications, typically, the LV Power MOSFETs are used. Furthermore, Power MOSFETs work in linear zone for a short period of time when they are used in switching conditions during the Miller region where high-currents and high voltages are together applied to MOSFET terminals. This means that the thermal instability must be also evaluated in applications where linear zone isn't the typical operating condition. Therefore, this phenomenon needs to be evaluated even during the slow switching process either in LV or, in particular, in HV Power MOSFETs when high inductive loads are driven. Considering a theoretical Power MOSFET FBSOA, linear zone is referred to that area delimited by maximum allowed dissipated power for different power pulse duration.

1 Basic concepts

Figure 1. Example of Power MOSFET FBSOA

the carriers in the channel and VTH change theirs values. In particular, either μ or VTH 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. ID∂ ID K----- K∂ VDS const= = 2KI D VTH∂ VDS const=

2 Experiments, results and discussion

Figure 2. Comparison between standard and irradiated HV Power MOSFETs: TC

TN1156 Experiments, results and discussion explained in equation4, during the linear zone working conditions, the device has more thermal instability.

3 Conclusions

This paper has evaluated the thermal instability phenomenon of irradiated HV Power MOSFET devices working in linear zone operating conditions comparing their electro- thermal behavior with standard products and by considering two different suppliers. A theoretical study of the phenomenon (the introduction) has been followed by an experiment: measuring the thermal coefficients of two samples in identical devices. The former includes irradiated devices while the latter includes standard devices. Results show that irradiated transistors have a higher thermal instability, in fact both the threshold voltage derating and defectiveness of interface densities are higher than those in standard ones. Higher levels of interface states in the irradiated transistors are related to the same irradiation process.

4 References

[1] Safe Operating Limits in Linear Zone - G. Consentino, G. Bazzano - PSDE, Dec. 2006 [2] Investigations on Electro-Instability of Low Voltage Power MOSFETs: Theoretical Models and Ex-perimental comparison results for different structures - G. Consentino, G. Bazzano - PET 2004 Conference, Chigaco Illinois 2004 [3] Power MOSFETs working in Linear Zone: the dangerous effect of the K gain factor on thermal in-stability - G. Consentino, IEEE/SPEEDAM 2012, Sorrento, Italy [4] A new approach to establish the thermal instability condition and the failure time during the drain current focusing process in a Power MOSFET working in linear zone - G. Consentino , IEEE/ISIE 2010, Bari, Italy

5 Revision history

Table 1. Document revision history 03-Dec-2013 1 Initial release.