Transient Thermal Impedance and the Safe Operating Temperature of Semiconductor Devices

Key Takeaways

• The transient thermal impedance depicts the ability of a semiconductor device or integrated circuit to diffuse heat from junctions to the remainder of the device’s body.

• Usually, transient thermal impedance is presented as junction-to-case thermal impedance in datasheets, with ℃/W as the unit.

• The transient thermal impedance value is significant in determining the pulse load operation of a semiconductor device. 

Thermal information is included in a semiconductor device’s datasheet to detail the operational temperature limits of the device

As it is being used, a semiconductor device generates internal heat and the temperature of the device junctions can increase dangerously. If the temperature of any of the junctions crosses the temperature limit listed in the device’s datasheet, the device will be damaged. The safe operating area of a semiconductor device is limited by the thermal environment surrounding it. The device lifetime and operational parameters are also dependent on the operating temperature, and thus on the thermal parameters.

Thermal parameters—such as transient thermal impedance and thermal resistance—help engineers understand the thermal characteristics of a semiconductor device. The transient thermal impedance and resistance describe the device’s efficiency at moving internal heat to the ambient during the transient state and steady-state. When selecting semiconductor components for an application, it is important to consider the device’s transient thermal impedance and thermal resistance. 

Transient Thermal Impedance

When a semiconductor device is operating, electrical energy is dissipated in internal junctions in the form of heat. This self-heating phenomenon increases the device’s internal temperature. There is a relationship between power dissipation and temperature rise in the junction. The transient thermal impedance gives the ratio of the temperature rise (for a single pulse applied) to the power dissipated in the junction due to it.

Usually, transient thermal impedance is presented as junction-to-case thermal impedance in datasheets, with ℃/W as the unit. It is the parameter that connects the heat-up time, temperature rise, and power dissipation in the junctions of a semiconductor device, especially for the pulsed loads. 

What Is the Difference Between Transient and Steady-State Thermal Impedance?

Steady-state thermal impedance and transient thermal impedance are two different parameters used to describe semiconductor device thermal behavior.

Steady-state thermal impedance gives the efficiency of heat diffusion from the device to the ambient.

Transient thermal impedance corresponds to the heat diffusion from the hot junction to the device mass.

The value of transient thermal impedance is always less than steady-state thermal impedance.  The transient thermal impedance value is significant in determining the pulse load operation of semiconductor devices such as MOSFETs and IGBTs. For the given conditions, such as initial temperature, pulse magnitude, and duration of the pulse, the transient thermal impedance value changes. From the transient thermal impedance graph given in a device’s datasheet, engineers can calculate the current limit of a semiconductor device for different pulse durations. 

The Importance of Transient Thermal Impedance

Transient thermal impedance depicts the ability of a semiconductor device or integrated circuit to diffuse heat from junctions to the remainder of the device’s body. The safe operating internal temperature of a device can be calculated, provided the transient thermal impedance and the power dissipated inside the device during a given duration of time are known. This is how transient thermal impedance influences the reliability of a semiconductor device’s operation.

The transient thermal impedance is an inevitable parameter when modeling electrothermal macro models of semiconductor devices. To obtain accurate nonisothermal characteristics from electrothermal macro models of semiconductor devices, an estimation of transient thermal impedance is critical.

In mission-critical circuits, the failure of semiconductors or ICs due to a temperature rise is unacceptable. The transient thermal impedance of these semiconductor devices is an important thermal parameter to consider for extending the normal operating lifetime of mission-critical systems.

Cadence software, including the Celsius Thermal Solver featured in the video below, provides electrical-thermal co-simulation for the extended lifetime of critical systems.

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