Temperature effects on a MOSFET

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The working temperature of a MOS device is one of importance as it affects performance of the device like threshold voltage, drain current and mobility of carriers.

Effect of temperature on threshold voltage


It is observed that as the temperature increases, there is a linear decrease in the threshold voltage. The dependency is governed by the following equation
Where

Effect of temperature on drain current and carrier mobility

The dependency of the drain current and the carrier mobility is characterized by the following equation.
For a gate voltage of 3V, the following result is obtained.
For a gate voltage of 10V, the following result is obtained.
If the gate voltage is slightly larger than the threshold voltage, the drain current ratio increases as temperature increases. However, if the gate voltage is sufficiently larger than the threshold voltage, the over-drive voltage becomes approximately equal to gate voltage making the drain current ratio dependent on the mobility, and hence exhibits almost the same dependency as that of mobility on temperature. Electron mobility is dependent on the drift velocity. The primary factors determining drift velocity are effective mass and scattering time i.e. the quantity of time for which the carrier is ballistically accelerated by the electric field until it collides with another particle that changes its direction and/or energy. Typically, the sources of scattering in a semiconductor are ionized impurity scattering and acoustic phonon scattering (lattice scattering). With increasing temperature, the concentration of phonons (acoustic pressure waves generated by vibrating atoms) increases which causes increased scattering with electrons. Thus, lattice scattering decreases the electron mobility as temperature increases.

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