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• N Mosfet Constant Current Circuit For Led
• N Mosfet Design

1. This is a simple model of a n-type MOSFET. Gate and drain voltages can be controlled using the sliders at the right. Basically no current flows if the gate voltage is below the threshold voltage When you raise it above that, current begins to flow.
2. Together, these control the control terminal on the pass transistor- in this case, an n-channel FET. So let's look at the schematic and show the losses using the standard NMOS topology. The losses are actually quite few. With an NMOS transistor, there is essentially no gate current requirement.

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Those FETs which uses a thin silicon dioxide as the insulator is known as the Metal Oxide Semiconductor (MOS) transistor or Metal Oxide Semiconductor Field Effect Transistor (MOSFET). Based on the channel formed beneath the insulating layer, MOS transistors are classified as N-channel transistor (NMOS) and P-channel transistor (PMOS).

The following information describes how the various MOSFET models from SPICE and Spectre are translated to the corresponding ADS models.

SPICE Models

All Mosfet devices in SPICE reference a model by its instance name. Each Mosfet model in SPICE has a keyword NMOS or PMOS, as well as a Level parameter.

The NMOS/PMOS keyword is used to determine what device to place in the schematic, MOSFET_NMOS or MOSFET_PMOS. The Level is used to determine which model is placed and what value is set for Idsmod.

In the ADS netlist, the model is always called MOSFET, with the appropriate keywords NMOS and PMOS set to [0|1], and the parameter Idsmod set as specified in the following table.

The only exception to this is the Mosfet device which refers to an HSpice Level 50 model, the Phillips MOS9 model. In this case, the device placed in the schematic will be MM9_NMOS or MM9_PMOS and the model will be MOS_Model9_Process. The netlist component is called MOS9. For both the MM9_NMOS and the MM9_PMOS, the translator sets the parameter Type=2 to indicate that it is a process-based model.

SPICE Level Parameters Mapping Table (MOSFET)

Dependence Parameters

There are certain model parameters listed in the BSIM Models that reference additional parameters. These additional parameters are denoted in parenthesis using the letters L, W and/or P. As an example, the Dwg (L,W, P) parameter in the BSIM3 Model defines four separate parameters:

• Dwg: Coefficient of Weff's gate dependence
• LDwg: Length dependence of Dwg
• WDwg: Width dependence of Dwg
• PDwg: Cross dependence of Dwg

Each letter indicates a sensitivity parameter that exists in HSpice and ADS. These parameters are length(L), Width(W) and Cross(P). Refer to your HSpice and ADS component documentation for details.

HSpice Automatic Model Selection

N Mosfet Constant Current Circuit For Led

Automatic Model Selection, also known as binning, is an HSpice feature that allows for the definition of a library of Mosfets over a range of lengths and widths. The following netlist fragment is an example of automatic model selection in HSpice. It is currently used only for Mosfets. Notice that the model name is a root name plus an extension, separated by a period (that is: NCHAN.2). Also note the special model parameters, LMIN, LMAX, WMIN and WMAX. They are used only for automatic model selection.

The ADS equivalent of this functionality is provided by a component called BinModel. Since ADS component names cannot include a period, the models are renamed nchanx2 and nchanx3. Then a BinModel component is placed, which lists the models by name, as well as listing the special length and width parameter ranges, as defined by LMIN, LMAX, WMIN and WMAX. The name of the BinModel component is the same as the root name of the models, in this case nchan. The ranges of all models with the same root name will be recorded on one BinModel component as shownin the following figure.

The ADS BinModel Component

Also note that the Mosfet devices that are placed in the schematic do not refer to the Mosfet models as they normally would. Instead they refer to the BinModel. During simulation, the length and width specified on the device are sent to the BinModel and checked against the ranges specified for each model, to determine which model to use. If the length and width on the device fall within the range of the length and width on more than one model, the first one that matches is used.

Spectre Models

All Mosfet devices in Spectre reference a model by its instance name. Each Mosfet model in Spectre has a name value pair type=[n|p].
The type value pair keyword is used to determine what device to place in the schematic, MOSFET_NMOS or MOSFET_PMOS. The Level is used to determine which model is placed and what value is set for Idsmod.

In the ADS netlist, the model is always called MOSFET, with the appropriate keywords NMOS and PMOS set to [0|1], and the parameter Idsmod set as specified in the following table.

The only exception to this is the Mosfet device which refers to the Phillips MOS902 model. In this case, the device placed in the schematic will be MM9_NMOS or MM9_PMOS and the model will be MOS_Model9_Process. The netlist component is called MOS9. For both the MM9_NMOS and the MM9_PMOS, the translator sets the parameter Type=2 to indicate that it is a process-based model. Ludacris theater of the mind zip.

Spectre Level Parameters Mapping (MOSFET)

Dependence Parameters

N Mosfet Design

There are certain model parameters listed in the BSIM Models that reference additional parameters. These additional parameters are denoted in parenthesis using the letters L, W and/or P. As an example, the Dwg (L,W, P) parameter in the BSIM3 Model defines four separate parameters:

• Dwg: Coefficient of Weff's gate dependence.
• LDwg: Length dependence of Dwg.
• WDwg: Width dependence of Dwg.
• PDwg: Cross dependence of Dwg.

Each letter indicates a sensitivity parameter that exists in Spectre and ADS. These parameters are length(L), Width(W) and Cross(P). Refer to your Spectre and ADS component documentation for details.

The following MOSFET models are translated in ADS:

Binning Process

In order for ADS to translate the binning process, each binning definition is translated as a separate model.

Binning Example

The following is an example Spectre binning statement for a bsim model:

The Netlist Translator separates each binning reference into an ADS model. The example would result in three models with the names ModelName_1, ModelName_2, and ModelName_3. The models are then tied together by creating an ADS 'BinModel' component and configuring the appropriate min/max values.

MOSFET Transistors or Metal Oxide-Semiconductor (MOS) are field effect devices that use the electric field to create a conduction channel. MOSFET transistors are more important than JFETs because almost all Integrated Circuits (IC) are built with the MOS technology.

There are two kinds of MOSFET transistors:

• N channel MOSFET Transistor or NMOS
• P channel MOSFET Transistor or PMOS

At the same time they can be enhancement transistors or depletion transistors. In the present days the last ones are not used. In these tutorials we will describe only the enhancement MOS transistor.

NMOS and PMOS Symbols

The following image shows the different symbols used to describe the MOS transistor.

The image shows the curves of electrical characteristics of an NMOS transistor with the different regions of operation. These regions of operation are briefly described below.

NMOS FET Cutoff region

We can verify that VGS < VT and the current ID is zero.

NMOS FET Linear region

The transistor behaves as a nonlinear resistive element, controlled by voltage. Check the following equations:

where:

is a characteristic parameter of the MOS transistor, which depends on the k constant and the size of the transistor gate (width W and length L).

NMOS FET Saturation region

The NMOSFET transistor behaves as a voltage controlled current source VGS. Check the following equations:

where ß = K (W/L)

In this region, the quadratic relationship between VGS and ID is shown in the left part of the picture. In a similar way to the JFET transistors, it can be used to identify, by graphical methods, the bias point of the transistors. This method is rarely used.

NMOS FET Breakdown region

A MOS transistor can be affected by the avalanche phenomena in the drain and source terminals. The MOS transistor can also be affected by breaks in the thin oxide layer of the gate that may destroy the device.

Finally, note that the table above shows the differences in sign and direction of the currents and voltages between NMOS and PMOS transistors.

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