# The 'thermal diode'…A diode or a transistor?

The 9/24/05 DesignLine article How To Measure Temperature In Integrated Systems, formed the foundation for using a 'thermal diode' to measure temperature and introduced sources of temperature error induced by the 'thermal diode' parameters. This article will detail these sources of error as they pertain specifically to the ambiguity of the term 'thermal diode'. The ambiguity is a result of the use of the term 'thermal diode' to represent either a true 2-terminal diode or a Bipolar Junction Transistor (BJT). When a BJT is used as the thermal diode, the base-emitter voltage difference is used to 'fit' 2-terminal diode I-V characteristics. It will be shown that errors encountered in thermal monitoring solutions are often due to not taking into consideration whether the specific 'thermal diode' is a 2-terminal diode or BJT. This article will explain the unique characteristics of a 2-terminal diode and BJT, and quantify the errors that result when these two devices are used interchangeably. Let's begin with the assumption that the 'thermal diode' is physically a 2-terminal diode, such as a discrete 1N4001. In this case, the diode V-I relationship is defined as:

**Equation 1**

where:

* η = Ideality Factor

* k = Boltzman's constant

* T = Temperature in degrees Kelvin

* q = electron charge

* I_{F} = forward diode current

* I_{S} = reverse saturation current

* R_{S} = series resistance

As described in the referenced article, temperature can be accurately determined by forcing two different, but known, values of I_{F} and measuring the corresponding V_{F} for each current. Equation 2 shows how temperature is calculated with the use of these two different I_{F} currents.

**Equation 2**

An essential point to notice in Equation 2 is that the terms η and R_{S} must be known values in order to obtain accurate thermal information from a diode. The algebraic formulas for accurately quantifying values of η and R_{S} are defined in Equation 3 and 4. Provided that the values of η and R_{S} are known, Equation 2 will yield accurate temperature calculations when a true 2-terminal diode is used. Please refer to the referenced article above for an explanation of error induced by either not knowing or incorrectly assuming values of η and R_{S}. This article will show how the 'thermal diode' parameters derived in Equation 3 and 4 are different in value when the 'thermal diode' is treated as a 2-terminal diode versus a BJT.

**Equation 3**

**Equation 4**