Today is the age of FETs, down at the level of integrated circuits and as high-level as power electronics systems. BJTs are much less common in advanced power systems and RF systems, having been largely replaced by FETs in most applications. Within FETs, there are different types of FETs that differ in terms of component construction and electrical behavior.
If you’re unsure which type of FET you need for a certain application, this guide can help you determine the best course of action. The three best-known types of FETs are MESFETs, MOSFETs, and JFETs. For high-frequency systems, there is another type of FET, known as HEMTs. We’ll examine these different FETs and their ideal applications in this article.
Four Types of FETs
Everyone should be familiar with the standard functionality of a FET: a voltage is applied to the gate region, which then modulates current flow between the source and drain terminals in the component. Current flow in the OFF state is possible across the body of a FET, and this is represented in MOSFETs with a body diode across the source and drain terminals. This is essentially the basic functionality of FETs, and in particular MOSFETs.
Within FETs, there are four different types of FETs:
- Metal-oxide semiconductor FETs (MOSFETs, also known as insulated gate FETs)
- Metal-semiconductor FETs (MESFETs)
- Junction FET (JFET)
- High electron mobility transistor (HEMT)
These FETs are used in a diverse range of application areas, spanning from power delivery and motor control, to wireless systems for power amplification and active filtering.
MOSFETs, MESFETs, and HEMTs can operate in two different modes: enhancement mode and depletion mode. In enhancement mode, applying a gate voltage turns the device ON, while in depletion mode, a gate voltage must be applied to turn the device OFF. The channel polarity will determine the gate voltage polarity required to modulate the FET’s channel conductance. Note that JFETs can only operate in depletion mode.
MOSFETs are general-purpose FETs and they probably receive use in the broadest range of systems. They are commonly used for general-purpose switching at low power, as well as in areas like power delivery and motor control. They are also used in low-frequency MMICs as switching elements or as part of amplifiers (often as push-pull amplifiers with DC offset).
The channel polarity (n-type or p-type) will determine the current flow direction in the ON state, and the polarity required to modulate the MOSFET. A summary of the operating modes of MOSFETs is shown below.
Finally, FETs can have a body diode connecting the source and drain terminals through the bulk semiconductor below the channel in the MOSFET. The bulk channel conductance is rectifying and allows some current to flow in one direction, even if the MOSFET is OFF. This could allow some reverse current to flow via the body diode’s one-way conductance.
Junction FETs (JFETs) can be either n-channel or p-channel devices, which will determine the gate voltage polarity required to modulate the device. JFETs only operate in depletion mode, so the gate voltage can only act to modulate a JEFT into the OFF state. The voltages required are:
- N-channel device: negative gate-source voltage
- P-channel device: positive gate-source voltage
Unlike MOSFETs, JFETs will not have a body diode and cannot pass current in reverse bias. Aside from this point, JFETs are comparable to MOSFETs in most respects and can be a suitable replacement for a MOSFET.
These FETs are formed by direct metalization of the gate electrode onto the channel, forming a Schottky junction at the gate electrode. This means the electrical characteristics (threshold voltage, rectification, saturation, etc.) all depend on the Schottky barrier rather than doping in a pn junction.
MESFET structure on bulk semiconducting substrate.
Silicon MESFETs can operate at high frequency, but these MESFETs have been more often built from GaAs or SiGe. This class of MESFETs is sometimes lumped into another type of FET: high electron mobility transistors (HEMTs).
RF devices, such as integrated amplifiers and switches, may also use HEMTs in the device to provide harmonic power delivery at microwave frequencies. The aggressive scaling of Si MOSFETs for logic devices in recent years has led to their improved performance in RF systems, but not with the desired power handling deep in mmWave bands. In these cases, HEMTs are built from alternative materials that can balance heat, power delivery, and fast operation at high frequencies.
The primary materials used to build HEMTs at commercial scale are GaAs and GaN. Other III-V materials like InP have been researched for use in HEMTs, but they have not been commercialized yet. GaAs has been the major material platform used to build MMIC components operating above WiFi frequencies. GaN is a new candidate that is useful in power applications and RF systems due to its high power/heat handling ability as well as its fast switching capabilities.
When you’ve determined the types of FETs you plan to use in your design, you can design and simulate your circuits with the simulation tools in PSpice from Cadence. PSpice users can access a powerful SPICE simulator as well as specialty design capabilities like model creation, graphing and analysis tools, and much more.