FETs (Field-Effect Transistors)

A FET is a three terminal semiconductor device in which the voltage across its gate and source terminals is used to control the current flow between its drain and source terminals. This behavior allows a FET to act as a voltage controlled resistance, amplifier, or switch, and this makes them applicable to both analog and digital electronics.

There are many different types of FETs available, and each type has its own specific variation of materials, configuration, and geometric arrangement. On the Digi-Key Electronics website nearly all singly packaged FETs of any type can be found under the “FETs – Single” product category. The only other categories that may contain a singly packaged FET are “JFETs (Junction Field Effect)” and “RF FETs”. Having these devices in separate categories makes sense for practical reasons. If someone needs a device from one of these other product categories they should know, and those who don’t can avoid them and stick to the more general “FETs – Single” product category.

At the time this was written, Digi-Key had more than 39,000 individual part numbers in their “FETs – Single” product category. Fortunately, Digi-Key’s parametric search allows filtering on the specifications common to all FET types. This approach naturally leads the user to the correct FET types, and eventually to those specific FETs that best fit an application, by allowing filtering based on performance requirements.

The n-Channel (NMOS) Enhancement-Mode MOSFET

The enhancement-mode MOSFET (Metal-Oxide-Semiconductor FET) is the most widely used type of FET. Because of its ubiquity, the basic construction and operation of a traditional n-channel enhancement-mode MOSFET is explained here first. The later sections for p-channel and depletion-mode devices assume the reader has read and understands all the information in the n-channel enhancement-mode section.

Basic Construction and Operation

Figure 1 shows a cross-section of an n-channel enhancement-mode MOSFET. A p-type substrate is used for the body, and two heavily doped n-type regions are formed in it. A thin, electrically insulating, oxide layer is grown over the surface of the substrate spanning the length between the two n-type regions. Layers of metallization or poly-silicon are added where the black bars are shown, which allow for connection to external conductors.

Figure 1: A cross-section of an n-channel enhancement-mode MOSFET.

With the exception of the body connection, these metallic connection points make up the familiar gate, drain, and source connections shared by all FETs.

Figure 2 shows that when sufficient positive voltage is applied from the gate to source connections, then an n-type channel is induced in the p-type body. The voltage necessary to first form a conducting channel is known as the threshold voltage, but the mobile negative charge density in this channel continues to increase with additional positive gate to source voltage.

Figure 2: The basic operation of an n-channel enhancement-mode MOSFET.

The induced channel allows electrical current to flow between the drain and the source. A more enhanced channel means greater mobile negative charge density and greater conductivity.

Current can flow in both directions through the channel, but typically current flows from the drain to the source in n-channel FET applications. The reason for this is elaborated on in the section that explains the intrinsic body diodes.

The Field-Effect

The channel’s mobile negative charge density is actually controlled by an electric field that results from the gate to source voltage. An examination of Figure 3 can aid in visualizing this field.


Figure 3: Differential charge accumulation across the oxide layer of an n-channel enhancement-mode MOSFET.

Positive charge accumulation at the gate metallization and negative charge accumulation in the induced n-channel is shown. This can be thought of as a parallel plate capacitor. The gate and channel act as the plates, and the oxide acts as the insulating dielectric. The resulting electric field in the oxide layer is the mechanism that controls the density of mobile negative charge inside, and therefore the conductivity of, the channel.

Other types of FETs will have variations of materials, configuration, or geometric arrangement, but the same field-effect mechanism of voltage controlled channel conductivity applies to any FET.

The Body Connection

An astute observer of Figure 2 and Figure 3 may have noticed an otherwise unmentioned detail. In both cases the gate’s bias voltage was referenced to the source and the body; the source and body were tied together and therefore remained at the same voltage potential.

The body’s connection to the source helps to induce a conductive channel. MOSFETs may not operate correctly without proper biasing of the body. The body to source connection is made internally in nearly all commercially sold discrete FETs, and it’s why FETs are generally considered three-terminal devices. The connection is actually a part of many circuit symbols used to represent a MOSFET, such as Figure 4.

Figure 4: A circuit symbol for an n-channel enhancement-mode MOSFET.

Discrete FETs in which the body is not connected directly to the source are rare. If the body connection were left unconnected for the designer to use, then the Figure 5 symbol would be more accurate.

Figure 5: A circuit symbol for an n-channel enhancement-mode MOSFET with an external body connection not tied to the source.

In other rare cases, discrete FET manufacturers might use the body as another degree of engineering freedom by biasing it in some way other than with a direct connection to the source.

Monolithic integrated circuits differ from discrete FETs and will generally have a common body (bulk substrate) tied to one of the supply rails of the device. Also, their schematics often use simpler FET symbols. This avoids schematic clutter when a large number of individual FETs must be drawn.

The Intrinsic Body Diodes and BJT

Once again, an astute observer of Figure 4 and Figure 5 may have noticed an otherwise unmentioned detail. The symbols have diodes in them pointing from the body to the drain and source. Figure 4 doesn’t show the body to source diode because it is shorted out by the internal body to source connection.

These diodes are appropriately named body diodes, and with the exception of JFETs, they are intrinsic to any FET type. Where the body diodes come from is indicated in Figure 6. There are p-n junctions in the FET structure between the substrate and the doped regions.

Figure 6: Cross section of an n-channel enhancement-mode MOSFET with the intrinsic body diodes indicated symbolically.

Current typically flows from the drain to the source in n-channel FET applications because of the body diode polarity. Even if a channel has not been induced, current can still flow from the source to the drain via the shorted source to body connection and the body to drain diode. Because of this, a typical n-channel FET cannot block current flow from its source to its drain.

In some applications, such as certain DC-to-DC converters, the body diode is actually relied upon for normal circuit operation. In contrast, it doubles the number of FETs necessary in other applications, such as certain power source selectors, where current must be blocked in both directions.

The two back-to-back p-n junctions also result in an intrinsic BJT (Bipolar Junction Transistor) within the FET; however, with a shorted body to source connection it effectively ceases to exist.

The p-Channel (PMOS) Enhancement-Mode MOSFET

If the previous explanations for an n-channel enhancement-mode MOSFET are understood, they can be used as a starting point to make understanding a p-channel enhancement-mode MOSFET easier. If the following changes are made the explanations are essentially the same.

  • Regions that were n-type become p-type, and vice-versa.
  • Polarities of voltages and diodes are reversed.
  • Any instances of negative charge are substituted for positive charge, and vice-versa.
  • Although current can still flow both ways through the induced channel, typically current flows from the source to the drain in p-channel FET applications.

With these changes in mind, Figure 7 shows a simple p-channel enhancement-mode MOSFET.

Figure 7: A cross-section of a p-channel enhancement-mode MOSFET.

Figure 8 shows that when sufficient negative voltage is applied from the gate to source connections, then a p-type channel is induced in the n-type body. The voltage necessary to first form a conducting channel is known as the threshold voltage, but the mobile positive charge density in this channel continues to increase with additional negative gate to source voltage.

Figure 8: The basic operation of a p-channel enhancement-mode MOSFET.

Figure 8 assumes that the threshold voltage is given as a negative voltage value, and therefore the gate to source voltage must be equal to or more negative (less) than the threshold voltage in order to form a conductive channel. Sometimes FET manufacturers use this same convention. Other times they provide the threshold voltage as a positive number, but for an enhancement-mode p-channel FET this must be understood to be a magnitude value.

The induced channel allows electrical current to flow between the source and the drain. A more enhanced channel means greater mobile positive charge density and greater conductivity.

The electric field mechanism of control for the channel conductivity is the same, but opposite in polarity. Figure 9 shows this.

Figure 9: Differential charge accumulation across the oxide layer of a p-channel enhancement-mode MOSFET.

An internal body to source connection is still the norm. The symbol is slightly different; the directions of the body arrow and body diode are reversed.

Figure 10: A circuit symbol for a p-channel enhancement-mode MOSFET.

There are still intrinsic body diodes and a BJT, but again the BJT and body diode between the source and body effectively cease to exist with an internal body to source connection.

Figure 11: Cross section of a p-channel enhancement-mode MOSFET with the intrinsic body diodes indicated symbolically.

Current typically flows from the source to the drain in p-channel FET applications because of the body diode polarity. Even if a channel has not been induced, current can still flow from the drain to the source via the drain to body diode and the shorted body to source connection. Because of this, a typical p-channel FET cannot block current flow from its drain to its source.

Depletion-Mode MOSFETs

Depletion-mode MOSFETs have one major physical difference from the already discussed enhancement-mode devices; depletion-mode MOSFETs have a channel physically implanted between the drain and source regions. This means that depletion-mode devices have a conductive path between their drain and source even at zero gate to source voltage. This channel can be enhanced in the same way as enhancement-mode devices to increase the channel conductivity, but it can also have a charge carrier depletion region induced in the channel to reduce or eliminate the channel conductivity.

Although they aren’t discussed here, it’s worth noting that JFETs are all inherently depletion-mode devices due to their structure.

The n-Channel Depletion-Mode MOSFET

The basic structure of an n-channel depletion-mode MOSFET is shown in Figure 12.

Figure 12: A cross-section of an n-channel depletion-mode MOSFET.

The negative charge carriers in the heavily doped regions and n-channel provide a complete conductive path between the drain and source by default at zero gate to source voltage.

The density of the negative charge carriers can be increased in exactly the same way as in an n-channel enhancement MOSFET with additional positive gate to source voltage. Just as before, this channel enhancement leads to greater conductivity. The gate voltage must be lowered relative to the source to decrease the conductivity of the channel, and the threshold voltage where the channel effectively ceases to exist is negative in value. The reduction in conductivity is a result of charge carrier depletion in the implanted channel, where negative charge accumulation at the gate repels the mobile negative charge out of the implanted channel. Figure 13 provides a visual of both enhancement and depletion in an n-channel depletion-mode MOSFET as the gate source voltage moves back and forth from above zero volts to below the threshold voltage.

Figure 13: The basic operation of an n-channel depletion-mode MOSFET.

The schematic symbol should be changed as well to properly indicate a depletion-mode device. Figure 14 shows a symbol for an n-channel depletion-mode MOSFET, and the only change from its enhancement-mode equivalent is the solid line connecting the drain and source across the body.

Figure 14: A circuit symbol for an n-channel depletion-mode MOSFET.

This solid line can be thought of as the implanted channel, whereas the previously segmented drain, body, source lines of the Figure 4 enhancement-mode device indicate the absence of an implanted channel.

The p-Channel Depletion-Mode MOSFET

Although they are not available as discrete devices, p-channel depletion-mode MOSFETs are discussed here for completeness. The basic structure of a p-channel depletion-mode MOSFET is shown in Figure 15.

Figure 15: A cross-section of a p-channel depletion-mode MOSFET.

The positive charge carriers in the heavily doped regions and p-channel provide a complete conductive path between the source and drain by default at zero gate to source voltage.

The density of the positive charge carriers can be increased in exactly the same way as in a p-channel enhancement MOSFET with additional negative gate to source voltage. Just as before, this channel enhancement leads to greater conductivity. The gate voltage must be raised relative to the source to decrease the conductivity of the channel, and the threshold voltage where the channel effectively ceases to exist is positive in value. The reduction in conductivity is a result of charge carrier depletion in the implanted channel, where positive charge accumulation at the gate repels the mobile positive charge out of the implanted channel. Figure 16 provides a visual of both enhancement and depletion in a p-channel depletion-mode MOSFET as the gate source voltage moves back and forth from below zero volts to above the threshold voltage.

Figure 16: The basic operation of a p-channel depletion-mode MOSFET.

The schematic symbol should be changed as well to properly indicate a depletion-mode device. Figure 17 shows a symbol for a p-channel depletion-mode MOSFET.

Figure 17: A circuit symbol for a p-channel depletion-mode MOSFET.

MOSFET Behavior Summary Table

n-channel enhancement p-channel enhancement n-channel depletion p-channel depletion
Symbol
In a typical application channel current flows from Drain to Source Source to Drain Drain to Source Source to Drain
Conductive channel exists at zero Gate to Source voltage No No Yes Yes
Conductivity of channel increases with Increasing Gate to Source voltage Decreasing Gate to Source voltage Increasing Gate to Source Voltage Decreasing Gate to Source Voltage
Gate to Source Threshold Voltage is Positive Negative Negative Positive
Conductive channel is cutoff when Gate to Source voltage is Less than the threshold voltage Greater than the threshold voltage Less than the threshold voltage Greater than the threshold voltage

NOTES:

  • MOSFET symbols often omit the body diode, but even if it isn’t explicitly shown it is always present.
  • Gate to source threshold voltages may be given as positive values (magnitudes) even for p-channel enhancement and n-channel depletion devices, but it should be understood that these are always negative values in practice. Digi-Key’s parametric data has all gate to source threshold voltages listed as positive values. This allows for convenience of comparison between different manufacturers, regardless of what conventions their datasheets use.
  • The threshold voltage may also be expressed as source to gate voltage rather than gate to source. This simply reverses the sign of the voltage by changing the direction of reference for the measurement; the device itself behaves no differently.
  • Although they are not available as discrete devices, p-channel depletion-mode MOSFETs are included here for completeness.

References

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