Electronic Technology Forum
The temperature characteristics of MOS transistors and the detailed explanation of temperature effects
In the characteristic equations and main parameters of MOS devices, almost all are related to the conductivity factor κ and threshold voltage VT, and these two parameters change with temperature. Therefore, the change of temperature directly affects the MOS device and MOS. The working performance and reliability of the circuit. Therefore, in the circuit design, the factors that change the parameters of the device with temperature must be taken into consideration.
1.Change of conductivity factor with temperature
The expression of conductivity factor κ is:
Among them, the surface mobility of carriers is the main factor that changes with temperature. The temperature characteristics of MOS transistors have been proved by theory and practice: For N-channel and P-channel MOS devices, the relationship between the mobility of electrons and holes in the inversion layer and temperature is approximately:
That is, as the temperature increases, the mobility of carriers in the inversion layer decreases, as shown in Figure 1-36 and Figure 1-37. This is because the temperature rises and the probability of carriers being scattered in the channel increases.
The μ and κ values at a temperature of T℃ can be determined from the following formula:
Where T is the working temperature，
and
is25℃time value，It can be calculated by measuring the saturation current of the sample tube.
The following shows the relationship curve of 
with temperature, which can be used for the circuit Refer to when designing.
2. Threshold voltage changes with temperature
From the expression of the threshold voltage:
As you can see, the main factor that obviously changes with temperature is the Fermi potential.
And space charge area charge
The degree of change with temperature is minimal and can be completely ignored.
For N-channel MOS devices:
Because 
The relationship with temperature is:
Therefore 
It decreases with the increase of temperature. At the same time, the surface charge density of the space charge region is:
Its absolute value also decreases with increasing temperature. Therefore, the VT of the N-channel MOS tube increases with the temperature Reduced. As shown in Figure 1-39.
It’s not difficult to understand that the temperature characteristics of MOS transistors, for P-channel MOS transistors, follow the rise in temperature of |VT| is also reduced. As shown in Figure 1-40.
As the temperature rises, the |VT| Decrease, will cause the leakage current to increase. So when designing dynamic MOS circuits, special attention should be paid.
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