A transistor is a device that controls the current flowing into the channel of a low-power electrical signal application. They are classified in two ways:
1. Bipolar junction transistors (BJTs). A current-controlled device, as a small amount of current is used to control its flow in a channel.
2. Field effect transistors (FETs). A voltage controlled device because a small voltage is used to control the current flowing in a channel.
This article focuses on the various types of FETs and includes a comparison chart.
Junction Field Effect Transistor (JFET)
In a JFET, a thin layer of N- or P-type resistive semiconductor material forms a channel. Typically, N- or P-type silicon is used, which allows most carriers to flow through two ohmic electrical connections (called the “drain” and “source”) formed at each end of the channel. A third ohmic contact (the “gate”) is also formed.
The various layers of a JFET.
A voltage applied to the gate controls the current flowing through the channel. When a drain-source voltage causes drain current to flow into the resistive channel, the current is distributed equally throughout.
However, due to its resistive nature, a small gradient forms that decreases in magnitude as the flow moves from the drain to the source terminal. This results in a PN junction with a larger reverse bias at the drain terminal and a smaller reverse bias at the source terminal.
This tendency forms a layer of exhaustion within the channel. Its width increases with the bias.
FETs are classified as:
1. N-channel JFET – doping impurities that form a negative flow current in the form of electrons.
2. P-channel JFET – acceptor impurities that form a positive flow current in the form of holes.
N-channel JFETs are preferred over P-channel JFETs because electrons provide more conduction than holes.
Forms
- As a low noise, high impedance amplifier.
- As a buffer amplifier with high input impedance and low output impedance.
- Like RF amplifiers in the receiver section of the communications unit (because a JFET is ideal in low current signal operations).
- Like analog multiplexers, which can be made using JFETs.
Metal Oxide Semiconductor Field Effect Transistor (MOSFET)
A MOSFET is a three-terminal device with a gate electrode that is a metal-oxide semiconductor, electrically isolated from the current-carrying channel by a thin layer of silicon dioxide (called “glass”).
The insulation of the gate electrode makes the input resistance extremely high – on the order of a few mega-ohms. As a result, MOSFETs are easily damaged by a high build-up of static charges.
OC construction of a MOSFET.
The gate terminal uses an electric field to change the flow of carriers in the channel. The gate electrode is placed on top of a thin insulating material with a pair of N regions below the drain and source terminals.
As mentioned, a JFET is biased and normally reverse biases the PN junction, but such conditions do not exist in a MOSFET. However, they can be influenced by a positive or negative polarity. In addition to classifying a MOSFET as N- and P-type, there are two additional types available.
1. Depletion type MOSFET – the transistor can turn on without applying gate bias, which is equivalent to a “normally closed switch.”
2. MOSFET enhancement type – the transistor can turn on with the application of gate bias, which is equivalent to a “normally open switch.”
Forms:
- Millions of MOSFETs are integrated into digital ICs, such as microprocessors and memory devices, to implement logic gates and data storage.
- Used as switching power supply and variable frequency drives.
- Used as an analog signal and power amplifier in RF to UHF amplifiers.
- Used as oscillators and mixers to convert frequencies in radio systems.
Heterojunction Field Effect Transistor (HFET)
The field-effect transistors used for high-speed applications – such as cell phones – are high electron mobility transistors (HEMT) or heterojunction field-effect transistors (HFET).
These transistors incorporate materials with different bandgaps. The channel is made of two different materials (in contrast to the doping used in MOSFETs), making it ideal for high-speed applications.
Construction of an HFET used for high-speed applications.
A variety of material combinations can be used depending on application demands. Gallium nitride (GaN) and aluminum gallium nitride (AlGaAs) are now the most common materials. Once considered to have high power performance, indium has been replaced by GaN, which has high frequency responses.
Unlike other FETs, HFETs operate differently. For effective conduction, semiconductors are doped with materials that provide a sufficient amount of mobile electrons and holes. However, these electrons lose all their energy during collisions and do not contribute much to conduction. This is not the case with HFETs.
Because HFETs use two materials – AlGaN (with a highly doped wide bandgap) and GaN (an undoped narrow bandgap) – high electron mobility is achieved when electrons from a thin layer of N-type AlGaN fall freely onto the GaN, forming a depleted AlGaN layer.
These “falling” electrons then move freely in the Gan conduction band without escaping or colliding with impurities. This is because GaN is undoped and has higher affinity. The electrons form a very thin conductive layer with higher concentration, giving the channel low resistivity and high electron mobility. This layer is called the 2D electron gas.
HFETs are:
1. pHEMT (pseudomorphic HEMT) – uses heterojunction materials with different lattice constants.
2. mHEMT (metamorphic HEMT) – has a buffer layer between the two heterojunction materials.
Forms:
- Such as low noise and small signal amplifiers, power amplifiers, oscillators and mixers.
- As RF design applications when incorporated into monolithic integrated circuits.
- Such as electronic warfare systems such as radar and radio astronomy.
Comparing JFETs, MOSFETs, and HFETs
|
JFET |
MOSFET |
HFET |
||
|
Used material |
Mainly silicon |
Mainly silicon |
III-V and Si, SiGe |
|
|
Input Impedance |
High |
Very high |
High |
|
|
gate terminal |
PN junction |
Isolated gate |
Schottky Gate |
|
|
gate chain |
leakage current |
No gate chain |
Small door chain |
|
|
Channel |
Depletion channel |
Inversion channel |
Accumulation channel |
|
|
Types |
N and P channels | N and P channels |
Mainly N-type HFET |
|
|
JFET Channel |
MOSFET channel |
|||
|
Substrate node |
Skewed |
Skewed |
Non-polarized (connected to ground) |
|
|
There is no such way |
||||
| Operation mode | Exhaustion mode | Depletion and enhancement modes | ||
|
Forms |
Analog multiplexers, |
SMPS, microprocessors |
Low noise amplifiers, RF design |
|
| Systems | RF Amplifiers | Memory Devices | Electronic warfare | |
|
|
|
|||
|
|
||||
|
|
|
|||
|
|
||||
|
|
|
|||
|
|
||||
