What is FET?

By
FET is a device in which the flow of current through the conducting region is controlled by an electric field, Hence the name Field Effect Transistor (FET). As current conduction is only by majority carriers, FET is said to be a unipolar device.
As the output voltage of a FET is controlled by the gate input voltage, FET is called as the voltage control device.

Types of FET - There are two main types of FET. They are JFET & MOSFET.
Junction Field Effect Transistor (JFET)- JFET is a three terminal semiconductor device in which current Conduction takes place only due to the flow of majority charge carriers.
There are three terminal of JFET are source,drain & gate. 
Source (S)- Source terminal provides majority charge carriers,whose movement generates a current through the device.
Drain (D)- Drain terminal is present at the opposite end of the source terminal.The majority charge carriers move from an end to other and get collected at the drain terminal of the Transistor.
Gate (G)- Gate terminal is formed by a combined connection of the two heavily diffused regions over the substrate.The voltage that controls the level of current is provided at the gate terminal.
Channel- It is a region present between gate & source terminal through which movement of majority carrier takes place.
Types of JFET - There are two types of JFET.They are N-Channel JFET & P-Channel JFET.

N-Channel JFET - As in an N-Channel JFET, the N-type silicon substrate is taken & two P-type semiconductor bars are diffused on it,There by generating a channel for the flow of electron.Thus known as a N-Channel JFET
Operation of N-Channel JFET - Source voltage VGG is applied between gate to source, to make P-N junction reverse bias & forward bias is applied between drain & source terminal.
We can consider following conditions to observe working of N-Channel JFET

Case(1) - When VDS is forword bias & VGS=0 :
If VDS is applied between drain to source & no bias is applied between gate and source.Now the majority carriers (electrons) flows from source to drain through the narrow channel.Therefore the conventional current (drain current)flows from drain to source.

Case(2) - When VGS is reverse biased & VDS=0 : When reverse bias voltage VGS is increased the depletion region becomes more wide.This reduces the effective width of the channel & therefore controls the flow of current through channel.
 When gate to source voltage increased further a stage is reduced at which two depletion region touch each other. At this  stage the channel is completely blocked or pinch off & drain current reduced to zero.
The gate to source voltage at which the drain current is zero is called as Pinch off voltage.

V-I Characteristic of JFET Transistor - The JFET has different characteristics at different stages of operation depending on the input voltage.Mainly the JFET operates
In ohmic, saturation,cut off & breakdown region.
 Ohmic Region - If VGS=0 then the depletion region of the channel is very small & in this region the JFET acts as a voltage controlled resister.
Pinch off region - The gate to source voltage at which the drain current is zero,is called pinch off voltage & this region is called pinch off region.
Saturation or Active region - In this region the channel acts as a good Conductor which is controlled by the gate voltage (VGS).
Breakdown region - The voltage between drain & source (VDS) is high enough to causes the JFET's resistive channel to breaksdown & pass uncontrolled maximum current.

The drain current flowing through the channel is zero when applied voltage VGS is equal to pinch off voltage. The drain current flowing through the channel can be calculated as follows.

Comments

Post a Comment

Popular posts from this blog

What is a Capacitor?

By
Image
Capacitor is a passive component that has the ability to store the energy in the form potential difference between two plates. which form to be positive & negative depending upon the direction of charge storage .   Symbol &Unit   - The capacitor symbol is represented by drawing two parallel lines close to each other, but not touching. It consists of two terminals. Standard units for capacitor is Farads. The most common subunits of capacitance are micro farad, nano farad (nF), pico farad (pF) & femo farad(fF) etc.  1 micro farad = 10–⁶ farads 1 nano farad = 10–⁹ farads 1 pico farad = 10–¹² farads 1 femo farad = 10–¹⁵ farads Construction of capacitor  - A capacitor has the  ability to store electric energy in the form of electric field. A non conducting region is present between two plates which is called as dielectric material   (Insulator such as aluminum, paper,air,& ceramic etc). When two plates having same area A, & distance between two pa
Read more

What is rectifier?

By
Image
 A rectifier is an electrical device that converts alternating current (AC), which periodically reverses direction,to direct current (DC),which flows in only one direction.This process is known as rectification. Types of rectifier  - There are mainly three types of rectifier. ( 1) Half wave rectifier  - A type of rectifier that converts only the half cycle of the alternative current (AC) into direct current (DC) is known as half wave rectifier & only require a single diode to construct.   A high AC voltage is applied to the primary side of the step down transformer & it will get a low voltage, which will be applied to the diode. During the positive half cycle of the AC voltage,the diode will be forward biased, for an ideal diode, the forward voltage drop is zero.So the whole input voltage will appear across the load resistance. During the negative half cycle of the AC voltage,the diode will be reverse biased & the current flow
Read more

What is Complementary MOSFET(CMOS)?

By
Image
Complementary Metal Oxide semiconductoris a type of transistor fabrication process that uses complementary & symmetrical pairs of P-type & N-type MOSFET for logic functions . Generally,the CMOS technology is associated with VLSI (Very Large Scale Integrated) circuit,where a few million or even billions of transistors (MOSFETs to be specific) are integrated into a single chip or die. The reasons for the dominant use of CMOS technology in the fabrication of VLSI chips are reliability, low power consumption, considerably low cost & most important scalability . These are four important CMOS process technology (1) N- well process  - In the N-well process,an N-type well or tub is diffused in the P-type substrate. The PMOS transistor M2 is fabricated within this well. The well region acts as the substrate for the PMOS transistor.The additional processing step are the formation on N-region & the ion-imlantation step of P-type source & drain regions.A typical CMOS inverter
Read more