BJT VS FET

BANSILAL RAMNATH AGARWAL CHARITABLE TRUST’S VISHWAKARMA INSTITUTE OF TECHNOLOGY

 ( An autonomous institute affiliated to Savitribai Phule pane university)

             Vishwakarma institute of technology, 666, upper Indiranagar,

                       Bibwewadi, pane, Maharashtra, india-411037.

                                  contact No.+91-20-24283001

BLOG ON BJT VS FET

Contents

1.INTRODUCTION

2. WHAT IS BJT

3. WHAT IS FET ?

4.DIFFERENCE BETWWEN BJT VS FET

5.WHICH IS FASTER BJT VS FET

6. WHY IS FET PREFERRED OVER BJT

INTRODUCTION:

The BJT VS FET also known as active semiconductor devices. They have both are two different kinds of transistor

The BJT stand for (bipolar junction transistor) and FET stand for (field effect transistor) .Both the devices available in variety

of packages and they are operating based on current, frequency, power-rating, voltage like this devices control over their work and allow a greater degree. The used of this devices like in electronics and electrical circuit also they used as an amplifier switches. The difference between BJT VS FET is the BJT carries both the charges like majority and minority but FET they charges carries only majority flow

What is BJT?

The BJT stand for (bipolar junction transistor) this is one type of transistor they used both minority and majority charges. The BJT available in two types such as PNP and NPN transistor. Both the transistor has main function is amplifies the current. This transistor can be used as amplifiers and switches. BJT is involve a wide range that including electronics devices like mobile, TV, computers, radio, transmitters, industrial control amplifiers etc. these are application of BJT

Construction of BJT

A BJT has two P-N junction they are depending on the structure the BJT is classified into two types PNP and NPN.

In NPN transistor p-type Semiconductor lightly doped is placed between two heavily doped N -type semiconductor and PNP transistors is formed placing of N-type semiconductor to P - type semiconductor.

The construction of BJTs we can show below in the figure. The collector and the emitter terminals in below structure are called N-type and P-type semiconductor namely called of 'E' And 'C' and collector terminal is called P-types semiconductor is denoted within 'B'

                       

 Regions of BJT

BJT operated through three region is cut off, active & saturation they are all discussed below

.in the cut-of region the transistor is off so that's why there is no transmission in two terminal. Like emitter and collector so IE=0 & IB=0

   In an active region when transistor is on the collector current is in controlled & comparative. Through the base control is

IC=BIC is comparatively insensitive toward as an amplifier

The saturation transistor region less chance change through a base current the collector current are depends of VCE

 

BJT Characteristics

The characteristics of BJT include the following.

In BJT O/P impedance is high and the I/P impedance is low

Because of the occurrence of minority charges carries BJT is noisy components

Because of the flow of current will be there and also both charges carries that’s why the BJT is bipolar device

Because of outflow current and reverse the saturation current the thermal capacity of BJT is low

   In BJT the collector terminal area is high as compared to FET

 

Types of BJT

The classification of BJTs on their construction like PNP and NPN.

PNP Transistor

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

NPN Transistor

NPN transistor, is two N-type semiconductor layers, but only the p-type semiconductor layer is sandwiched.

What is FET?

FET stand for (field -effect transistor) they also known as unipolar transistor. It is one type of transistor. In the FET O/P current is controlled by the electric fields the FET is totally dissimilar from BJT. The FET is and consists of three region gate, source & drain. Charge carries of the transistor are holes or electrons the flow of the terminal of source to the terminal of drain in an active channel

CONSTRUCATION OF FET

The FET is classified in two types such as JFET and MOSFET they both transistor are similar principals the p-channel JFET construction is shown below. The p- channel JFET carries majority of charges flow form the source to drain. The drain and sources terminal are denoted by D&S

The terminal of gate is connected reverse bias mode. Of a voltage of source so the depletion layer is formed across regions of the channel and the gate where the charge is flow

reverse voltage on the gate terminal is increased whenever, the depletion also increase's and stop the flow of current form the source terminal with the help of changing the voltage of at the gate terminal flow of current from sources terminal to be drain terminal is controlled

Region of FET

FETs runs through three areas such as cut-off, active & ohmic region.

In cut-off region the transistor will be turned off. in the region there in now conduction among the sources also in drain.

Active region also known as saturation region. This region the transistor is on

Voltage of gate- source is higher as compared to cut-off voltage

Controlling the drain current with the help of VGS (gate -sources voltage) & comparatively insensitive. To VDS i this region transistor works an amplifier

 ID = IDSS = (1- VGS/ VGS, off) 2

VDS is low as compared to active region that's why drain current is approximately comparative. Toward the drain- source voltage & controlled through gate voltage So, ID = IDSS

FET Characteristics

The characteristics of FET include the following.

·        The input impedance of FET is high like 100 M Ohm

·        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 acceptable for input stages of low-level amplifiers.

·        It provides high thermal stability as compared to BJT.

 

                                           Difference between BJT and FET

BJT	FET
BJT stands for bipolar junction transistor, so it is a bipolar component	FET stands for the field-effect transistor, so it is a uni-junction transistor
BJT has three terminals namely base, emitter, and collector	FET has three terminals namely Drain, Source, and Gate
The operation of BJT mainly depends on both the charge carriers like majority as well as minority	The operation of FET mainly depends on the majority charge carriers either holes or electrons
The input resistance of BJT ranges from 1K to 3K, so it is very less	The input resistance of FET is relatively large
BJT is the current controlled device	FET is the voltage-controlled device
BJT has noise	FET has less noise
Any frequency changes in BJT will have direct affect on its performance	Its frequency response is high
It depends on the temperature	Its heat stability is better
It is a low cost	It is expensive
BJT size is higher when compared with FET	FET size is low
It has offset voltage	It doesn’t have offset voltage
BJT gain is more	FET gain is less
The output resistance is high due to high gain	The output resistance is low due to low gain.
When compared to emitter terminal, both the terminals of BJT like base and collector are more positive.	Its Drain terminal is positive and the gate terminal is negative when compared with the source.
Its base terminal is negative with reference to the emitter terminal.	Its gate terminal is more negative with reference to the source terminal.
It has a high voltage gain	It has a low voltage gain
It has a less current gain	It has a high current gain
Switching time of BJT is medium	Switching time of FET is fast
Biasing of BJT is simple	Biasing of FET is difficult
BJTs uses less amount of current	FETs use less amount of voltage
BJTs are applicable for low-current applications.	FETs are applicable for low voltage applications.
BJTs consume high power	FETs consume low power
BJTs have a negative temperature coefficient	BJTs have a positive temperature coefficient

Key Difference between BJT and FET

•BJT are bipolar devices, the transistor, is flow of both majority & minority charge.

•FET are unipolar devices, the transistor, and only majority charge carriers’ flows.

•  Bipolar junction transistors are current-controlled.

• Field-effect transistors are voltage controlled.

• In applications FETs are used bipolar junction transistors

•  Bipolar junction transistors consist of three terminals known as emitter, base, and collector. These terminals are substituted as E, B, and C.

•  FET consists of three terminals they source, drain, and gate. These terminals are denoted S, D, and G.

•  The input impedance of FET has high compared with bipolar junction transistors.

•   The manufacturing of FETs can be done diminutive to make them efficient in the designing of commercial circuits.

•   FETs profoundly MOSFETs are more expensive to design as compared with BJTs.

•  FETs are considerably more utilized in different applications and these can be manufactured in small size and uses less power supply. BJTs are more relevant for hobby electronics, consumer electronics and they generate high gains.

•  FETs gives numerous advantages 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 resistance & other factors.

•  A BJT needs a small amount of current to turn on the transistor. The heat disappear when bipolar stops the total number of transistors that can be fabricated on the chip.

•  The BJT is accountable for overheating due to a negative temperature coefficient.

•   FET has a +Ve temperature coefficient to stop overheating.

•   BJTs are suitable for low current applications.

•   FETs have low to medium gain.

 

Which is Faster BJT or FET?

•For impotent LED driving & same devices from MCU (Micro Controllers Unit), BJTs are of good fit because BJTs can switch rapidly as compared with MOSFET because of low capacitance on the control pin.

•MOSFETs are used in high-power applications; as they can move quicker as compared with BJTs.

•MOSFETs are utilize small inductors switch-mode supplies to increase efficiency.

 This 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 were developed through various semiconductor materials like P-type as well as N-type. These are design of switches, amplifiers & oscillators. We are hoping that you have got a brief understanding of this concept. 

Why is FET Preferred over BJT?

•    Field-effect transistors provide high input resistance compared with BJTs.

•   FET creates less noise

•  Voltage of FET is zero drain current & it makes an amazing signal chopper.

•   FETs are more temperature stable.

•  These are voltage-sensitive devices as well as high input resistance.

• The input resistance that of FET is higher, because of this it is more valued     to use like the I/P stage to a multi-stage amplifier.

•  Field-effect transistor produces less noise

•  Fabrication of FET is simple

•  FET acknowledges as voltage-controlled variable resistor for tiny drain-to-source voltage values.

•   These are not sensitive to radiation.

•   Power FETs dissipate high power and they can switch large currents.