Transistor heater

Fig.4.16. Heating approaches for monolithicaUy integrated microhotplates (pHP) (a) shows a resistive heater with power transistor and (b) shows a PMOS transistor heater fiheat denotes the heating resistor R is the metal-oxide chemiresistor, and Rj is a resistor used as temperaturesensor (see Fig. 4.4)...

A schematic view of the microhotplate with transistor heater is shown in Fig. 4.17 [125]. In order to ensure a good thermal insulation, only the dielectric layers of the CMOS process form the membrane. The inner section of the membrane includes an... 

Fig.4.17. Cross-section of a microhotplate with integrated MOS-transistor heater, D and S denote drain and soiu ce of the transistor...

Starting with the basic model assumptions, the analytical heater model is developed in several steps [126]. The equations include common model equations such as the Shichman-Hodge model [127], the LEVELS model [128] and the BSlMS.vS model [129]. Only selected components of these partly complex models were taken to yield a set of equations that is suitable for modelling the transistor heater. The variables and parameters have been defined in accordance to standard notations. First, a model has been estabhshed that describes the unheated transistor, then, temperature dependencies have been introduced, and, finally, the electrothermal coupHng to the microhotplate has been considered. The result is an implicit equation, which can be iteratively solved. The considered model will be compared to measurement data in Sect. 4.4.4. 

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). 

Fig. 6.13. Chip micrograph showing the membrane and the integrated PMOS transistor heater...

The third microhotplate design included a MOS-transistor heater embedded in a silicon island (Sect. 4.4). One advantage of this configuration is the reduction of the... 

M. Graf, S.K. Muller, D. Barrettino, and A. Hierlemann. Transistor heater for microhotplate-based metal-oxide microsensors , JEEE Electron Device Letters 26 (2005), 295-297. 

S. Muller. CMOS-Micro-Hotplate with MOS Transistor Heater for Integrated Metal Oxide Gas Sensors, Diploma thesis, ETH Zurich, Switzerland (2002). 

We suggested a more spectacular, two dimensional, development of integrated sensor arrays a few years ago to create what could be called an "olfactory camera" [6]. The olfactory camera is simply a chip with an array of field effect transistors, heaters and temperature sensors. The sensing transistors are coupled as diodes into which a constant... 

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