Know the Major Difference between BJT and FET

BJTs and FETs are two different kinds of transistors and also known as active semiconductor devices. The acronym of the BJT is Bipolar Junction Transistor and FET stands for Field Effect Transistor. BJTS and FETS are available in a variety of packages based on the operating frequency, current, voltage, and power ratings. These types of devices allow a greater degree of control over their work. BJTS and FETs can be used as switches and amplifiers in electrical and electronics circuits. The major difference between BJT and FET is that in a field-effect transistor only majority charge carries flows, whereas in BJT both majority and minority charge carriers flow.

Difference Between BJT and FET

The main difference between BJT and FET is discussed below, which includes what is BJT and FET, construction and working of BJT and FET.

What is BJT?

The BJT is one type of transistor that uses both majority and minority charge carriers. These semiconductor devices are available in two types such as PNP and NPN. The main function of this transistor is to amplify current. These transistors can be used as switches and amplifiers. The applications of BJTs involve a wide range that includes electronic devices like TVs, mobiles, computers, radio transmitters, audio amplifiers, and industrial control.

Construction of BJT

A bipolar junction transistor comprises two p-n junctions. Depending on the structure of the BJT, these are classified into two types such as PNP and NPN. In an NPN transistor, a lightly doped P-type semiconductor is placed between two heavily-doped N-type semiconductors. Equally, a PNP transistor is formed by placing an N-type semiconductor between P-type semiconductors. The construction of a BJT is shown below. The emitter and collector terminals in the below structure are called n-type and p-type semiconductors which are denoted with ‘E’ and ‘C’. While the remaining collector terminal is called a p-type semiconductor denoted with ‘B’.

When a high voltage is connected in reverse bias mode across both the base and collector terminals. This roots a high depletion region to form across the BE junction, with a strong electric field that stops the holes from the B-terminal to the C-terminal. Whenever the E and the B terminals are connected in forwarding bias, the flow of electrons direction will be from the emitter terminal to the base terminal.

In the base terminal, some electrons recombine with the holes, but the electric field across the B-C junction attracts electrons. Most electrons end up overflowing into the collector terminal to create a huge current. Since the flow of heavy current through the collector terminal can be controlled by the small current through the emitter terminal.

If the potential difference across the BE junction is not strong, then the electrons are not able to get into the collector terminal so, there is no flow of current through the collector terminal. Due to this reason, a bipolar junction transistor is used as a switch as well. The PNP junction also works with the same principle, but the base terminal is made with an N-type material and the majority of charge carriers in the PNP transistor are holes.

Regions of BJT

BJT can be operated through three regions like active, cut-off & saturation. These regions are discussed below.

The transistor is ON in-active region, then the collector current is comparative & controlled through the base current like IC = βIC. It is comparatively insensitive toward VCE. In this region, it works as an amplifier.

The transistor is OFF in the cut-off region, so there is no transmission among the two terminals like the collector & the emitter, so IB = 0 so IC = 0.

The transistor is ON in the saturation region, so the collector current changes extremely less through a change within the base current. The VCE is small and the collector current mainly depends on VCE not like in the active region.

BJT Characteristics

The characteristics of BJT include the following.

• The i/p impedance of BJT is low whereas the o/p impedance is high.
• BJT is a noisy component because of the occurrence of minority charge carriers
• BJT is a bipolar device because the flow of current will be there because of both the charge carriers.
• The thermal capacity of BJT is low because the outflow current otherwise reverses the saturation current.
• Doping within emitter terminal is maximum whereas in the base terminal is low
• The collector terminal’s area in BJT is high as compared with FET

Types of BJT

The classification of BJTs can be done based on their construction like PNP and NPN.

PNP Transistor

In the PNP transistor, in between two p-type semiconductor layers, only the n-type semiconductor layer is sandwiched.

NPN Transistor

In an NPN transistor, in between two N-type semiconductor layers, only the p-type semiconductor layer is sandwiched.

What is FET?

The term FET stands for Field-effect transistor and it is also named a Unipolar transistor. FET is one type of transistor, where the o/p current is controlled by electric fields. The basic type of FET is totally dissimilar from BJT. FET consists of three terminals namely source, drain, and gate terminals. The charge carriers of this transistor are holes or electrons, which flow from the source terminal to the drain terminal via an active channel. This flow of charge carriers can be controlled by the voltage applied across the source and gate terminals.

Construction of FET

Field-effect transistors are classified into two types such as JFET and MOSFET. These two transistors have similar principles. The construction of p-channel JFET is shown below. In p-channel JFET, the majority of charge carriers flow from the source to drain. Source and drain terminals are denoted by S and D.

The gate terminal is connected in reverse bias mode to a voltage source so that a depletion layer can be formed across the regions of the gate and the channel where charges flow. Whenever the reverse voltage on the gate terminal is increased, the depletion layer increases. So it can stop the flow of current from the source terminal to the drain terminal. So, by changing the voltage at the gate terminal, the flow of current from the source terminal to the drain terminal could be controlled.

Regions of FET

FETs operated through three regions such as cut-off, active & ohmic region.

The transistor will be turned OFF in the cut-off region. So there is no conduction among the source as well as the drain when the voltage of gate-source is higher as compared with cut-off voltage. (ID = 0 for VGS > VGS,off)

The active region is also known as the Saturation region. In this region, the transistor is ON. The controlling of drain current can be done through the VGS (gate-source voltage) & comparatively insensitive to VDS. So, in this region, the transistor works as an amplifier.

So, ID = IDSS = (1- VGS/ VGS,off)2

The transistor is activated in the Ohmic region; however, it performs like a VCR (voltage-controlled resistor). Once VDS is low as compared with the active region, then drain current is approximately comparative toward the source-drain voltage & is controlled through the gate voltage. So, ID = IDSS

[2(1- VGS/ VGS,off) (VDS/ -VDS,off) – (VDS/ -VGS,off)2]

In this region,

RDS = VGS,off/ 2IDss (VGS- VGS,off) = 1/gm

Types of FET

There are two main types of junction field-effect transistors like the following.

JFET – Junction Field Effect Transistor

IGBT – Insulated-Gate Field Effect Transistor and it is more commonly known as MOSFET – Metal Oxide Semiconductor Field Effect Transistor)

FET Characteristics

The characteristics of FET include the following.

• The input impedance of FET is high like 100 MOhm
• When FET is used as a switch then it has no offset voltage
• FET is comparatively protected from radiation
• FET is a majority carrier device.
• It is a unipolar component and provides high thermal stability
• It has low noise and more suitable for input stages of low-level amplifiers.
• It provides high thermal stability as compared to BJT.

Difference between BJT and FET

The difference between BJT and FET is given in the following tabular form.

Key Difference between BJT and FET

• Bipolar junction transistors are bipolar devices, in this transistor, there is a flow of both majority & minority charge carriers.
• Field-effect transistors are unipolar devices, in this transistor, there are only the majority charge carriers flows.
• Bipolar junction transistors are current-controlled.
• Field-effect transistors are voltage controlled.
• In many applications FETs are used than bipolar junction transistors.
• Bipolar junction transistors consist of three terminals namely emitter, base, and collector. These terminals are denoted by E, B, and C.
• A field-effect transistor consists of three terminals namely source, drain, and gate. These terminals are denoted by S, D, and G.
• The input impedance of field-effect transistors has high compared with bipolar junction transistors.
• The manufacturing of FETs can be done very smaller to make them efficient in the designing of commercial circuits. Basically, FETs are available in small sizes and they use low space on a chip. Smaller devices are more convenient to use and user friendly. BJTs are larger than FETs.
• FETs particularly MOSFETs are more costly to design as compared with BJTs.
• FETs are more extensively used in different applications and these can be manufactured in small size and uses less power supply. BJTs are applicable in hobby electronics, consumer electronics and they generate high gains.
• FETs provide several benefits for commercial devices in large-scale industries. Once it is used in consumer devices, then these are preferred because of their size, high i/p impedance & other factors.
• One of the largest chip designing companies like Intel uses FETs to power billions of devices around the world.
• A BJT needs a small amount of current to switch on the transistor. The heat dissipated on bipolar stops the total number of transistors that can be fabricated on the chip.
• Whenever the ‘G’ terminal of the FET transistor has been charged, no more current is required to keep the transistor ON.
• The BJT is responsible for overheating due to a negative temperature coefficient.
• FET has a +Ve temperature coefficient for stopping overheating.
• BJTs are applicable for low current applications.
• FETS are applicable for low voltage applications.
• FETs have low to medium gain.
• BJTs have a higher max frequency and a higher cutoff frequency.

Why is FET Preferred over BJT?

• Field-effect transistors provide high input impedance as compared with BJTs. The gain of FETs is lesser as compared with BJTs.
• FET generates less noise
• The radiation effect of FET is less.
• The offset voltage of FET is zero at zero drain current & therefore it makes an outstanding signal chopper.
• FETs are more temperature stable.
• These are voltage-sensitive devices including high input impedance.
• The input impedance of FET is higher, so it is preferred to use like the i/p stage to a multi-stage amplifier.
• One class of field-effect transistor produces less noise
• Fabrication of FET is simple
• FET responds like a voltage-controlled variable resistor for tiny drain-to-source voltage values.
• These are not sensitive to radiation.
• Power FETs dissipate high power as well as they can switch large currents.

Which is Faster BJT or FET?

• For low power LED driving & same devices from MCU (Micro Controllers Unit), BJTs are very suitable because BJTs can switch quicker as compared with MOSFET because of low capacitance on the control pin.
• MOSFETs are used in high-power applications; as they can switch quicker as compared with BJTs.
• MOSFETs utilize small inductors within switch-mode supplies to increase efficiency.

Thus, this is all about the comparison between BJT and FET, includes what is BJT and FET, Construction of BJT, construction of FET, differences between BJT and FET. Both the transistors like BJT and FET were developed through various semiconductor materials like P-type as well as N-type. These are utilized in the design of switches, amplifiers as well as oscillators. We hope that you have got a better understanding of this concept. Furthermore, any queries regarding this concept or electronics projects please comment in the comment section below. Here is a question for you, what are the applications of BJT and FET?

Photo Credits:

• BJT ibiblio
• Construction of BJT wikimedia
• FET circuitstoday
• Construction of FET elektroda