Working with MOSFET SPICE Models in Circuit Analyses

September 13, 2019 Cadence PCB Solutions

MOSFETs on black background

 

Transistors are wonderful devices, and it is highly likely that you would not be able to read this sentence on your computer without them. There are many types of transistors, but MOSFETs are by far the most widely used type of transistor used in analog and digital circuits for a variety of applications.

As a fundamental transistor architecture in power electronics and integrated circuits, you’ll likely need to use MOSFET SPICE models in your circuit analyses. Although we like to think that every model for a circuit element is universally applicable, you’ll find that there are different MOSFET models that are applicable for different types of MOSFETs and different specific components. So which analyses can you conduct with these different types of models? Here’s where you’ll need to choose the right circuit model for use in your SPICE simulations.

What is a MOSFET?

If you are familiar with the basic ideas of a transistor, then you are aware that any transistor is intended to act like a purely electronic switch with two inputs and one output. Current will flow through the output (either into or out of the output, depending on the type of transistor), depending on the potential applied between each input and the output. A common type of field effect transistor (FET) is the metal oxide semiconductor FET (MOSFET). MOSFETs are widely used in integrated circuits with high switching speed (i.e., short rise time). The conductance of the channel in a MOSFET is modulated by applying voltages to the source and gate. Note that the gate electrode is insulated from the underlying gate semiconductor by a thin oxide layer. 

MOSFETs actually have four inputs (source, drain, gate, and body or substrate), although the circuit symbol for most MOSFETs normally includes the first three of the aforementioned inputs. The threshold voltage and channel conductance in a MOSFET depends on the source to body voltage. A nonzero source-to-body voltage changes the threshold voltage from its ideal value; this is known as the body effect or back-gate effect.

If the body voltage is equal to the source voltage (i.e., the body and source are shorted), then electronic conduction will only be modulated by the gate potential and you will not be able to observe the body effect. However, if the body is held at a lower potential than the source (in NMOS), then electrons will need a higher gate potential to conduct through the channel, i.e., the threshold voltage is larger (again, in NMOS). This is the essence of the body effect, which is normally quantified using a substrate bias coefficient (see below). A complimentary effect arises in PMOS when body is at a higher potential than the source.

Which MOSFET SPICE Models Should I Use?

Circuit simulations are extremely useful for analyzing a variety of circuits and understanding their ideal behavior, but you can lose some perspective on the unique characteristics of a particular circuit element unless your simulation is built correctly and you use the right model for each component. Even passive components have some equivalent series resistance (ESR) and equivalent series inductance (ESL) that cause certain effects like self-resonance in a capacitor.

Similarly, different MOSFET SPICE models take account of different device parameters that govern various physical phenomena in a MOSFET during its operation. In general, there are three generations of MOSFET SPICE models, where each model takes account of successively more phenomena one observes in a MOSFET. The standard BSIM models are physical MOSFET models that allow a component designer to define important dimensional and processing parameters such as channel, gate oxide, and junction dimensions, substrate doping concentration, and other parameters. Newer generations can account for short channel effects, sub-threshold operation, leakage due to tunneling through the gate, temperature variations, and noise.

 

MOSFET SPICE model parameters

Important parameters in MOSFET SPICE models

 

The earlier generation of MOSFET SPICE models (Levels 1-3) are normally applicable to MOSFETs with gate lengths exceeding 0.1 mm, which are typically used in power electronics and other applications where a single MOSFET might run at high voltage/current. It is best to take a component-based approach and choose MOSFET SPICE models for specific components you intend to use in your next device rather than try to adapt a specific BSIM model to different components. With the right schematic drawing program, it is a simple matter to swap out one MOSFET for another component and compare the performance of each circuit.

Circuit Analyses Involving MOSFET SPICE Models

If you look at MOSFET SPICE models, you’ll find that the model only includes three terminals rather than four. In other words, the body terminal is omitted and it is implied that the body and source are held at the same potential. This is an important point to note as not all simulators allow you to examine body effects in a simulation. Later generations (i.e., newer BSIM versions) will allow you to define a specific body voltage in your simulation.

Perhaps the most fundamental analysis involving a MOSFET is determining its transfer characteristics. This shows how the drain-source current varies over a range of applied source-to-drain voltages and at specific gate voltage values. A family of DC sweep simulations is the standard tool one uses to examine the behavior of a MOSFET and the circuit to which it is connected. In total, there are three parameters that would need to be examined: the source, gate, and body voltage values. 

When dealing with varying voltages on any of the inputs, small-signal analysis is useful for examining how the behavior of a circuit with a changes around some specific bias point. In power electronics, MOSFETS can dissipate a significant amount of power and reach high temperature in some situations. This is where a DC temperature sweep or similar analysis can be useful for determining the electrical-thermal equilibrium.

 

MOSFET with a heat clip attached

Many transistors for power electronics include a metal clip to attach a heat sink

 

For switching amplifier applications, you’ll need to perform an AC frequency sweep to examine how your circuit responds over different frequencies. When the amplifier is brought to saturation, you’ll need to use a mix of frequency sweeps and small-signal analyses to examine the behavior. A good amplifier should run in the linear regime, and you should try to explore the linear limits of your device using small-signal analysis.

If you are building a new product that will take advantage of the unique characteristics of different types of MOSFETs, you need access to a broad array of MOSFET SPICE models for use in your circuit analyses. The PSpice Designer for OrCAD can incorporate a variety of MOSFET SPICE models in simulations, giving you accurate analysis capabilities as the design becomes more complex. This unique set analysis package takes data directly from your schematic and gives you a full view of the behavior of your circuits.

If you’re looking to learn more about how Cadence has the solution for you, talk to us and our team of experts.

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