Thermal Management in SoCs with Thermal Voltage Diodes

Key Takeaways

• A thermal diode is the pseudonym of semiconductor diodes or Bipolar Junction Transistors (BJTs) when they are employed for thermal management.

• The three types of thermal diodes are ballistic phonon thermal diodes, junction thermal voltage diodes, and phase-transition junction thermal voltage diodes.

•  In SoCs, the die-temperature is measured using thermal diodes and temperature sensors. The thermal transistor reduced to a thermal diode is incorporated in SoC design.

Thermal management enhances integrated circuit performance.

Poor thermal energy transport has a large impact on a System-on-Chip (SoC), as their size shrinks on a daily basis. High thermal load in chips can result in thermal runaways, hot carrier injections, hotspots, reduced clock speed, electromigration, IR drops, and stress migration. As the electronics industry advances, market demands for efficient thermal management grows, and companies have begun to invest in research to find solutions for these heating problems. 

Well-organized heat control can improve the performance of SoCs. Thermal Interface Materials ❲TIM❳ such as thermal pads, thermal grease, and passive components such as thermal resistors and thermal capacitors are the traditional thermal solutions in electronic circuits. In SoCs and embedded systems, thermal resistors and capacitors offer linear and static thermal solutions. 

The performance of a heat management system is enhanced when a non-linear active device is included. A thermal voltage diode is one such switchable active component, employed in chips for thermal management. A thermal voltage diode, or thermal diode, is a dynamic device that holds a relationship between its voltage and absolute temperature. The growth of VLSI technology has allowed electronics systems to be embedded into chips, making the incorporation of thermal voltage diodes in SoCs for monitoring and controlling temperature a common thermal management technique. 

Thermal Voltage Diodes Ensure the Reliability of an Operation

Thermal diodes are the pseudonym of semiconductor diodes or Bipolar Junction Transistor (BJT) when they are employed for thermal management. Thermal management plays a key role in ensuring the reliability of processors and sensitive electronic circuits. Thermal voltage diodes are generally fixed in embedded systems and SoCs as a part of thermal management. The basic principle of thermal diodes is the dependency of its voltage to absolute temperature. The forward voltage of the thermal voltage diode increases with temperature. This voltage measurement can be used for sensing the temperature of the substrate, or die. Thermal voltage diodes are placed on the spots expected to encounter heating issues, and the voltage across thermal diodes can give the exact temperature there.

How Do Diodes and Transistors Work as Thermal Diodes?

The junction forward voltage in the semiconductor diode and the base-emitter junction voltage (V_BE) in the BJT are the electrical quantities that make thermal diodes a feasible component in the electronics industry. The working principle that converts the BJTs and diodes to thermal diodes is the change in junction forward voltage (in diodes) or VBE (in BJTs) for two different current values is directly proportional to the absolute temperature. Whenever a diode or transistor is used as a thermal diode, the electrical quantity of interest is the PN junction forward voltage, which varies linearly with the die temperature. The equation governing temperature measurement using a semiconductor diode is:

Constructional Details of Thermal Voltage Diodes

The constructional asymmetry created by P and N-type material junction and the resultant nonlinearity in the device operation make the semiconductor junction diode a thermal solution. Thermal diode construction looks for asymmetry in the internal structure and non-linear device behavior. Some thermal diodes are introduced here in this section: 

Ballistic phonon thermal diode- The asymmetrical scattering of the phonons by the pyramid and the saw-tooth-shaped internal structures present in an element opens a window to the research of the thermal solutions using them. An asymmetrically etched silicon membrane structure exhibiting non-linear characteristics is a good thermal diode choice. 

Junction thermal voltage diode- Composite materials with different thermal resistance or thermal conductivity ❲k ❲T❳❳ are required to provide the spatial asymmetry and non-linearity in this type of thermal voltage diode. 

Phase-transition junction thermal voltage diode- The phase-transition thermal diode is composed of phase variant and phase invariant materials. In this type of thermal diode, phase variant material undergoes a phase transition with temperature variation. Table 1, shown below, gives the details of some of the phase transition thermal diodes.

Details of phase-transition junction thermal diodes. 

Measure Temperature in SoCs Using Thermal Diodes

In SoCs, typically a pin is dedicated to measure the die temperature. The internal connection of this temperature pin leads to a thermal diode. The thermal diode and temperature sensor makes a combo, (usually digital sensors) for the temperature measurement in SoCs. The SoC layout and thermal diode parameters influence the accuracy of temperature measurement. The location of the thermal diode in SoC is crucial, as the temperature varies across the die length. The trace resistance in an SoC package probe errors into a thermal diode-based temperature measurement. The thermal diode and sensor should be placed as adjacent as possible to minimize the temperature measurement errors due to trace resistance.

The two factors under the diode parameter category that affects the accuracy of the thermal diode-based temperature measurement are the ideality factor and the series resistance of the thermal diode. The ideality factor (n_f) and the junction voltage are directly proportional and this factor changes with each diode. The temperature error caused by the thermal diode ideality factor can be negative or positive depending upon the value of the ideality factor to which the temperature sensor is trimmed. 

The series resistance of the thermal diode is the next major concern especially when the current variations in the device are frequent. The voltage drop due to thermal diode series resistance results in a temperature measurement greater than the actual value. The remedy for the temperature errors evolved from the diode parameter variations is to program the offset register of the digital temperature sensor so that the final temperature measurement is close to the actual die temperature. 

The general trend in SoCs is to use a thermal transistor instead of a thermal diode. The temperature pin gets internally connected to the emitter-base junction, and the collector-base is connected to the SoC ground. The voltage drop across the emitter-base junction ❲V_BE❳ gives information about the die temperature which is given to a temperature sensor. If overheating is found upon measurement, the SoC calls for interrupts, alarms, or a direct shut-down. 

Whether you choose a ballistic phonon thermal diode, junction thermal voltage diode, or a phase-transition junction thermal voltage diode, this useful component will be a great asset to your work. With the help of thermal diodes, you can detect the die temperature and can protect the entire system from heating issues.