MOSFET model

The excellence of a properly formed Si02—Si interface and the difficulty of passivating other semiconductor surfaces has been one of the most important factors in the development of the worldwide market for siUcon-based semiconductors. MOSFETs are typically produced on (100) siUcon surfaces. Fewer surface states appear at this Si—Si02 interface, which has the fewest broken bonds. A widely used model for the thermal oxidation of sihcon has been developed (31). Nevertheless, despite many years of extensive research, the Si—Si02 interface is not yet fully understood. 

We first demonstrate the switch function of our transistor, namely we show that the system can act like a good heat conductor or an insulator depending on the control temperature. This is illustrated in Fig.lO(b), where we plot JG, Js, and Jd versus Trj. When TG increases from 0.03 to 0.135, both Jd and Js increase. In particular, at three points TG 0.04,0.09 and 0.135, Jd = Js thus JG is exactly zero. These three points correspond to off , semi-on and on states, at which Jd is 2.4 x 10-6,1.2 x 10-4 and 2.3 x 10-4, respectively. The ratio of the heat current at the on state and that at the off state is about 100, hence our model displays one important function - switch -just like the function of a MOSFET used in a digital circuit. 

The last step in the construction of the MOSFET-heater model includes the description of an appropriate heating process. Due to the source-drain current flow, the membrane is heated by resistive Joule heating in the channel region. By assuming that all electric power dissipated in the device is converted into heat, the corresponding heating power is ... 

Fig. 4.19. Comparison between measured T-versus-Vsg characteristics and the MOSFET-heater model data for a source-drain voltage of 5 V...

D. Foty. MOSFET Modeling with Spice Principles and Practice, Prentice Hall PTR, New Jersey, USA (1997). 

First, we need to create models to describe these transistors. When you look up the MOSFET models in the Cadence PSpice reference manual on the CD-ROM that accompanies this text, you will find the following equations to describe MOSFET operation ... 

The conversion between the two sets of equations is shown in Table 4-1. The PSpice model parameter names are close to the standard names used to represent MOSFET operation in many textbooks. One difference is that the PSpice model parameter Kp is twice the value of K. Thus, in our model we should set KP= 40 pA/V2. All other model parameters will be the same. 

We must now define the models for these two MOSFETs. Two separate models are required. Click the LEFT mouse button on the graphic symbol of the... 

This is the default model for all n-channel MOSFETs (no model parameters specified). This window is a text editor. Change the model name as shown ... 

After we save the library, model AU is added to the list of models contained in library SECTION 4D3Jlb. Select File and then Exit to the schematic. Notice that the model for the driver MOSFET has changed from MbreaHN to MX ... 

We are calling the new model My. Note that the symbols for enhancement- and depletion-mode MOSFETs are the same. PSpice knows that a MOSFET is either an enhancement- or a depletion-mode MOSFET by the value of the threshold voltage parameter, VtO. 

A Parametric Sweep allows us to sweep a parameter. We would like to sweep the width of the driver MOSFET in the circuit of the previous section. We are assuming that you have followed that procedure and have set up the MOSFET models and DC Sweep. We must first set up a parameter for the width of the driver. We will start with the completed circuit from the previous section. It is repeated here for convenience. 

EXERCISE 7-Z Change the model parameters of Ml in the circuit above to define an enhancement MOSFET with the parameters K = 20 pA/V2 and VT = 3 V. MOSFET operation in the saturation region is governed by the equation ... 

Two different models are given to specify n- and p-type MOSFETs. Both models use the same parameters. These... 

Many semiconductor manufacturers have data sheets and PSpice models available online for easy access by engineers. In this section we will show how to obtain those models so that we can use them in a simulation. The types of models we will show can be split into two types. The first type of models are primitives that use only a. model statement. Examples of these are diodes, bipolar junction transistors (BJT), and MOSFETs. The second type of models we will download are subcircuit models such as op-amps, IGBT s, Darlington transistors, and MOSFET subcircuit models. 

Mosfet N Subckt D N-type MOSFET subcircuit model with an anti-parallel diode... 

Note that if you wish to use a MOSFET model that does not contain an anti-parallel diode, you can use the standard MOSFET breakout parts such as MbreakN and MbreakP. 

As a first example, we will show how to download a model for a Schottky diode. This example will be the same for any device that uses a. model statement. Thus, you can use this same procedure for BJT s, MOSFETs, and jFETs. For this example, we will obtain the model from International Rectifier. Run your browser and open site www.irf.com ... 

The models for the MOSFETs have tolerances in their threshold voltages and transconductances. The models for the MOSFETs are given below ... 

The results in Table 5.1 suggest that the loop characteristics are heavily dependent on the MOSFET characteristics, which were not modeled as accurately as they could be by the available FET models. 

The excellent performance characteristics of MOSFETs are conditional on having well-designed gate drive circuitry. This chapter will aid the designer in modeling these circuits and utilizing MOSFETs to their fullest extent. 

Generally, a linear SPICE primitive capacitor is not the best way to model the nonlinear capacitance of a MOSFET. The capacitance of the MOSFET s gate is dependent on the gate-to-source voltage, and to a lesser extent, the drain-to-source voltage. However, the fixed capacitance used in this simulation is adequate for our purposes. 

In the Micro-Cap simulation, the model for the IRF140 MOSFET was substituted for the IRF150 MOSFET. The IRF150 MOSFET model showed an unusually long turn-off time, which is believed to be in error. 

Figure 21. Example of two MOSFET channel B implants performed through a poly gate I oxide structure and annealed at 600 °C for 30 min and at 700 °C for 30 min. The substantial enhanced diffusion is shown modeled with calculations from the Predict program. Data are from Mele et al. (61). Abbreviation and symbols S/MS, secondary ion mass spectrometry I, measured after implant A, measured after anneals. (Reproduced with permission from reference 59. Copyright 1988 Institute of Electrical and Electronics Engineers,...

Pd MOS STRUCTURES The Pd MOS device (capacitor and field effect transistor) has been extensively studied as a model chemical sensor system and as a practical element for the detection of hydrogen molecules in a gas. There have been two outstanding reviews of the status of the Pd MOS sensor with primary emphasis on the reactions at the surface (7,8). In this section, the use of the device as a model chemical sensor will be emphasized. As will be seen, the results are applicable not only to the Pd based devices, they also shed light on the operation of chemfet type systems as well. Because of its simplicity and the control that can be exercised in its fabrication, the discussion will focus on the study of the Pd-MOSCAP structure exclusively. The insights gained from these studies are immediately applicable to the more useful Pd-MOSFET. 

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