The Thermal Resistance vs. the Thermal Conductivity of Semiconductor Devices

Key Takeaways

• The relationship between thermal resistance and thermal conductivity is prominent when considering the internal construction of a semiconductor device and how heat is radiated to the ambient from the semiconductor junction.

• Thermal resistance is the measure of the thermal performance of the device, expressed in °C/W in datasheets. 

• The thermal conductivity of a semiconductor material is one of the parameters that defines the thermal resistance of each heat flow path. 

Thermal metrics are valuable inputs for increasing the thermal reliability of integrated circuits

High power density demands, downsizing, a wide range of ambient temperature operations, and limited airflow all pose thermal challenges to semiconductors and integrated circuits. In particular, rising temperature is detrimental to the performance of a semiconductor device.

The electrical and thermal characteristics of semiconductor devices are mutually inclusive.  Operating a device within specified electrical and thermal ratings is important to maintaining device lifetime and reliability. Thermal metrics provided in a semiconductor’s datasheet should be used by engineers to ensure they are properly operating their device, thus increasing its thermal reliability. Thermal resistance and thermal conductivity are two such thermal metrics that are helpful for the user utilizing the semiconductor device or integrated circuit up to its maximum limits. 

Let’s explore thermal resistance vs. thermal conductivity in semiconductor devices.

The Thermal Characterization of Semiconductor Devices

Thermal stability is important for the semiconductor device’s electrical performance. Temperature range is specified in all semiconductor devices to inform the user about the thermal limitations of the device. The derating of electrical characteristics starts as the device’s operating temperature rises above the ambient. A device’s package design and thermal management strategy should be developed by considering the thermal stability of the semiconductor die material.

The thermal characterization of a semiconductor device is crucial to ensure a long lifetime by avoiding thermal runaways. Thermal characterization defines proper thermal operating ranges and limits under steady-state, periodic, and transient thermal conditions. Thermal resistance and thermal conductivity are thermal metrics commonly used for the thermal characterization of semiconductor devices. 

Thermal Resistance vs. Thermal Conductivity 

Thermal resistance and thermal conductivity are investigated based on the packaging style used in a semiconductor device. The relationship between thermal resistance vs. thermal conductivity is more prominent when considering the internal construction of a semiconductor device and how heat is radiated to the ambient from semiconductor junctions.

Let’s take a closer look at the similarities and differences of thermal resistance vs. thermal conductivity.

Thermal Resistance 

Thermal resistance (θ) is analogous to electrical resistance, and indicates the resistance offered by the semiconductor device to the flow of heat generated internally to the ambient. It is the measure of the thermal performance of a device and is expressed in °C/W in datasheets. The thermal resistance is defined for junction-to-case, junction-to-ambient, junction-to-top of the package, junction-to-bottom of the package, etc.  It is useful for comparing the thermal reliability of a device of the same packaging style from different manufacturers.

If you are identifying two low voltage dropout regulators of similar ratings and input-output voltage ranges, the comparison of thermal resistances defined for the above-mentioned sections gives you a clear picture of each one’s thermal reliability. A smaller value of thermal resistance indicates better heat radiation for the given package. In the component selection process, thermal resistance comparison is necessary for increasing the thermal reliability of the circuit. To enable the thermal comparison of semiconductor devices, the semiconductor industry follows the standardized test procedure set by the Joint Electron Device Engineering Council (JEDEC) to calculate thermal resistance.  

Thermal Conductivity 

The heat generated in the semiconductor junctions of a device take different parallel heat flow paths to leave the package. The thermal conductivity of the semiconductor material is one of the parameters that defines the thermal resistance of each heat flow path.

Thermal conductivity is the measure of ease by which heat flows through a specific material. It is a material property that is independent of the material thickness. In devices with unipolar conduction (either electrons or holes), thermal conductivity is more or less the same as electrical conductivity. However, in devices with bipolar conduction (both holes and electrons), the thermal conductivity is different from electrical conductivity. 

Thermal conductivity is a critical parameter that determines the maximum power under which the semiconductor device is to be operated. The relationship between thermal resistance and thermal conductivity can be given as follows, where l is the length of the heat flow path, A is the cross-sectional area of the heat flow path, and k is the thermal conductivity of the material:

The importance of knowing the thermal resistance and thermal conductivity is undeniable for both the operational reliability and device design of semiconductors and integrated circuits.  Cadence software offers thermal analysis tools, including the Celsius Thermal Solver featured in the video below, that help in designing thermally reliable semiconductor devices.

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