Electrothermal Characterization and Comparison with Simulations

The microhotplate with the transistor heater was electrothermally characterized similarly to the procedures presented in Sect. 4.1.3. Special care was taken to exclude wiring series resistances by integration of on-chip pads that allow for accurate determination of Fsg and sd- With the two on-chip temperature sensors in the center (Tm) and close to the transistor (Tt) the temperature homogeneity across the heated area was assessed as well. Both sensors were calibrated prior to thermal characterization. The relative temperature difference (Tj - Tm)/Tm was taken as a measure for the temperature homogeneity of the membrane. The measured thermal characteristics of a coated and an uncoated membrane are summarized in Table 4.6. The experimental values have been used for simulations according to Eq. (4.10). 

The saturation current, fs j, is plotted versus the temperature in Fig. 4.20. In this case, the temperature is the independent variable, which is adjusted by the source-gate voltage. The plot thus shows how well the transistor model in Eq. (4.3) takes into account the temperature dependence. 

The relative deviation between measurement results and the temperature-depen-dent MOS transistor model data was less than 10% above 100 °C. In the case of a source-drain bias of 5 V it appeared that the model described the real situation well up to 300 °C, but then started to deviate. 

Since the uncertainty in model parameters Hmits the model accuracy, a deviation of 10% is generally considered a very good result. The measurements therefore demonstrate the validity of the presented model, in particular when considering that the temperature-dependent parameters are extracted from data that are only valid in a temperature range between 0 °C and 100 °C. 

It is noteworthy, that, in the case of the MOSFET heater, the transistor temperature is a function of the drain current, and, therefore, the leakage current also strongly depends on the drain current. As it is shown in Fig. 4.21 the leakage current amounts 

---