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Comparisons of JFET, MOSFET, HFET

Written By: 

Swetha Jayachandran

A device which controls the current flowing into the channel on application of low-power electrical signal is called a transistor. Roughly, transistors can be classified as Bipolar Junction Transistors (BJTs) and Field Effect Transistors (FETs). In BJTs, a small amount of current is used to control the current flowing into the channel and hence BJTs are also known as current controlled devices. In FETs, a small voltage is used to control the current flowing and hence called as voltage controlled devices. This article briefs about the various types of FETs with a comparison chart. Let’s get started.

Junction Field Effect Transistor (JFET)

In JFET, a thin layer of resistive N-type or P-type semiconductor material forms a channel. The commonly used material is either N-type or P-type Silicon which allows majority carriers to flow through with two ohmic regions drain and source formed at either ends of the channel. In addition to these, a third ohmic contact Gate is also formed.

Diagram Showing Various Layers of JFET

 

Fig. 1: Diagram Showing Various Layers of JFET

A voltage applied to the gate controls the current flowing through the channel. When drain-to-source voltage causes the drain current to flow into the resistive channel, the current gets distributed equally to all parts. Because of the resistive nature, a small potential gradient is created which reduces in magnitude as we move from the drain to the source terminal. This results in PN junction with a larger reverse bias at the drain terminal and smaller reverse bias at the source terminal. This bias causes a depletion layer to be formed within the channel whose width varies with the applied gate voltage. FETs are classified into two types:

· N-channel JFET – dopant impurities that makes the flow of current negative in the form of electrons.

· P-channel JFET – acceptor impurities that makes the flow of current positive in the form of holes. N-Channel JFETs are preferred over P-channel JFETs because electrons are more favorable for conduction than holes.

Applications :

·         Low noise and high impedance amplifier

·         Buffer amplifier with high input impedance and low output impedance

·         RF amplifiers in the receiver section of the communication unit because JFET is good in low current signal operation.

·         Analog multiplexers can be made using JFETs.

Metal Oxide Semiconductor Field Effect Transistor (MOSFET)

MOSFET is a three terminal device where the gate electrode is a metal oxide semiconductor, electrically insulated from the current carrying channel by a thin layer of silicon dioxide called as glass. This isolation of the gate electrode makes the input resistance very high in the order of few mega-ohms. Because of this insulation, MOSFETs gets damaged easily by high accumulation of static charges. The figure below shows the construction of a MOSFET.

Figure Explaining Construction of MOSFET

Fig. 2: Figure Explaining Construction of MOSFET

An electric field is used by the gate terminal to alter the flow of carriers in the channel. The gate electrode is placed on top of a thin insulating material and there are a pair of N-regions below the drain and source terminals. As we saw previously, JFET was biased in such a way to reverse bias the PN junction but no such conditions exists in MOSFET. They can be biased by a positive or a negative polarity. Apart from classifying MOSFET as N-type and P-type, there are two forms of MOSFETs available.

· Depletion Type MOSFET – transistor is switched ON without the application of gate bias which is equivalent to “Normally closed switch”.

· Enhancement Type MOSFET – transistor is switched ON with the application of gate bias which is equivalent to “Normally open switch”.

Applications:

·         Millions of MOSFETs are integrated in digital ICs such as microprocessors and memory devices for implementing the logic gates and data storage.

·         Switched mode power supply and variable frequency drives.

·         MOSFETs are used as analog signal and power amplifiers in RF amplifiers upto UHF.

·         Oscillators and mixers to convert the frequencies in radio systems.

Heterojunction Field Effect Transistor (HFET)

Field Effect Transistors used for high speed applications like mobile phones is the High Electron Mobility Transistor (HEMT) or Heterojunction Field Effect Transistor (HFET). As the name implies, this transistor incorporates materials with different band gaps. The channel is made of two different materials (in contrast to doping seen in MOSFETs) for high speed mobility making it attractive for high speed applications.

Diagram Showing Construction of HFET Used for High-Speed Applications

Fig. 3: Diagram Showing Construction of HFET Used for High-Speed Applications

A wide variety of combination of materials can be used demanded by the application but the most commonly used materials are Gallium Nitride (GaN) and Aluminum Gallium Nitride (AlGaAs). Usage of indium which was thought to have high-power performance was replaced by gallium nitride which has high-frequency responses. Unlike other FETs, HFET operates in a different way. For effective conduction, semiconductors are doped with materials that provide sufficient amount of mobile electrons and holes. However, these electrons lose all their energy during collision and do not contribute much to the conduction. Well, this is not the case with HFETs. Since HFETs use two materials AlGaN (highly doped wideband gap) and GaN (non-doped narrow band gap), high electron mobility is achieved when electrons from a thin n-type layer of AlGaN falls freely into GaN making a depleted AlGaN layer. These falling electrons then move freely in the conduction band of GaN without escaping and without any collision with the impurities because GaN is undoped and has higher affinity. These electrons hence form a very thin conducting layer with higher concentration giving the channel low resistivity or in turn high electron mobility. This layer is called 2D electron gas. HFETs are classified into two types:

·         pHEMT (pseudomorphic HEMT) - Heterojunction materials with different lattice constants.

·         mHEMT (metamorphic HEMT) – A buffer layer is present between two heterojunction materials.

Applications:

·         Low noise small signal amplifiers, power amplifiers, oscillators and mixers.

·         RF design applications when incorporated into monolithic integrated circuits.

·         Electronic warfare systems such as radar and radio astronomy.

Comparison of JFET, MOSFET and HFET

 

 

JFET

MOSFET

HFET

 

Material used

Mostly silicon

Mostly silicon

III-V and Si,SiGe

 

Input impedance

High

Very high

High

 

Gate terminal

PN junction

Insulated gate

Schottky gate

 

Gate current

Leakage current

No gate current

Small gate current

 

Channel

Depletion channel

Inversion channel

Accumulation channel

 

Types

N-Channel and P-

N-Channel and P-

Mostly N-type HFET

 

Channel JFET

Channel MOSFET

 

 

 

 

Substrate node

Biased

Biased

Not biased (connected to the ground)

 

 

 

 

 

 

 

Mode of operation

Depletion mode

Depletion and enhancement mode

No such mode exists

 

 

 

 

 

 

 

Analog multiplexers,

SMPS, microprocessors,

Low noise amplifiers, RF

 

 

design applications and

 

Applications

buffer amplifiers, RF

memory devices,

 

electronic warfare

 

 

amplifiers

oscillators and mixers

 

 

systems