Junction Field-Effect Transistor (JFET)

Junction Field-Effect Transistor (JFET)

 The JFET is a three-terminal device with one terminal capable of controlling the current between the other two. In our discussion of the BJT transistor  the npn transistor was employed through the major part of the analysis and design sections, with a section devoted to the impact of using a pnp transistor. For the JFET transistor the n-channel device will appear as the prominent device, with paragraphs and sections devoted to the impact of using a p-channel JFET.

Junction Field-Effect Transistor are two types

1.  n-channel JFET

2.  p-channel JFET

n-channel JFET:

The basic construction of the n-channel 
JFET is shown in Fig. 4. Note that the
major part of the structure is then-type
material that forms the channel
 between the embedded layers of p-type

material.The top of the n-type channel is
connected through an ohmic contact to a 
terminal referred to as the  drain (D), while
 the lower end of then same material is 
connected through an ohmic contact to 
a terminal referred to as the source (S). 
The two p-type material are connected

together and to the gate (G) terminal. 

                                                                         

        In essence, therefore, the drain and source are connected to the ends of the n-type channel and the gate to the two layers of p-type material. In the absence of any applied potentials the JFET has two p-n junctions under no-bias conditions. The result is a depletion region at each junction as shown in Fig.5  that resembles the same region of a diode under no-bias conditions. Recall also that a depletion region is that region void of free carriers and therefore unable to support conduction through the region.

 

IDSS is the maximum drain current for a JFET and is defined by the condition VGS  =0 V and VDS |Vp|.










 Voltage-Controlled Resistor:

The region to the left of the pinch-off locus of Fig. 6 is referred to as the ohmic or voltage-controlled resistance region. In this region the JFET can actually be em-

ployed as a variable resistor (possibly for an automatic gain control system) whose

resistance is controlled by the applied gate-to-source voltage. the slope of each curve and therefore the resistance of the device between drain and source for VGS < Vp is a function of the applied voltage As VGS becomes more and more negative, the slope of each curve becomes more and more horizontal, corresponding with an increasing resistance level. The following equation will provide a good first approximation to the resistance level in terms of the applied voltage VGS.

 

FIELD-EFFECT TRANSSISTOR

Difference between BJT and FET:

The primary difference between the two types of transistors is the fact that the  BJT transistor is a current-controlled device as depicted in Fig. 1(a), while the JFET transistor is a voltage-controlled device as shown in Fig. 1(b). In other words, the current  IC in Fig. 1(a) is a direct function of the level of  IB. For

the FET the current  I will be a function of the voltage VGS applied to the input circuit as shown in Fig. 1(b). In each case the current of the output circuit is being controlled by a parameter of the input circuit in one case a current level and in the other an applied voltage.

 Just as there are npn and pnp bipolar transistors, there are n-channel and p-channel field-effect transistors. However, it is important to keep in mind that the BJT transistor is a bipolar device the prefix bi- revealing that the conduction level is a function of two charge carriers, electrons and holes. The FET is a  unipolar device depending solely on either electron (n-channel) or hole (p-channel) conduction.

FIELD-EFFECT TRANSISTOR:

The field-effect transistor (FET) is a three-terminal device used for a variety of applications. The operation of the Field-effect transistor (FET) can be explained in terms of only majority-carrier (one-polarity) charge flow; the transistor is therefore called unipolar. There are two types of field-effect transistors, the Junction Field-Effect Transistor (JFET) and the “Metal-Oxide Semiconductor”  Field-Effect Transistor (MOSFET), or Insulated-Gate Field-Effect Transistor (IGFET). The principles on which these devices operate (current controlled by an electric field) are very similar — the primary difference being in the methods by which the control element is made. This difference, however, results in a considerable difference in device characteristics and necessitates variances in circuit design, which are discussed in this note.  

Field-Effect Transistor are two types.

   1.   Junction Field-Effect Transistor (JFET)

    2.  Metal-Oxide Semiconductor Field-Effect Transistor (MOSFET)

 JFET CONSTRUCTION AND SYMBOL

The physical arrangement of, and symbols for,the two kinds of JFET are shown in Fig.4-1.Conduction is by the passage of charge carriers from source(S)t o drain (D) through the channel Between the gate (G) elements. The transistor can be an n channel device (conduction by electrons) or a p channel device (conduction by holes); a discussion of n-channel devices applies equally to p-channel devices if complementary(opposite in sign) voltages and currents are used.Analogies between the JFET and the BJT are shown in Table 4-1. Current and voltage symbology for FETs parallels that given in Table 3-1.

JFET TERMINAL CHARACTERISTICS:

 The JFET is almost universally applied in the common-source (CS) two-port arrangement of 

Fig.4-1, where vGS maintains a reverse bias of the gate-source pn junction. The resulting

gate leakage current is negligibly small for most analysis (usually less than 1A), allowing the

gate to be treated as an open circuit. Thus, no input characteristic curves are necessary.

Typical output or drain characteristics for an n-channel JFET in CS connection with vGS 0 are

given in Fig. 4-2(a). For a constant value of vGS, the JFET acts as a linear resistive device  

(in the ohmic region) until the depletion region of the reverse-biased gate-source junction

extends the width of the channel (a condition called pincho?). Above pincho? but below

avalanche breakdown, drain current  iD.

 
 

 

   

Introduction to metal

Types of metal:

Insulator:  The metal which is bad conductor of electricity is called insulator. Some examples of insulators include plastic, rubber, glass, porcelain, air, paper, cork, mica, ceramics and certain oils.

Conductor: The metal which is good conductor of electricity is called conductor. All metals are conductors and some examples include copper, aluminum, brass, platinum, silver, gold and carbon.

 Semiconductor: The metal which is neither good conductors nor good insulator is called semiconductor. Some examples of semiconductors include Ge, Si, GaAs, CdS, GaN.

There are two classes of semiconductor materials.

                                      1. Single crystal:----------- Ge , Si.

                                      2. Compound crystal:-----  GaAs , CdS .

                 

The three semiconductors used most frequently are Ge , Si, GaAs .

Type of semiconductor:

 1.  Intrinsic semiconductor

 2. Extrinsic semiconductor

Intrinsic semiconductor :

A semiconductor in on extremely pure form is called intrinsic semiconductor.In an intrinsic 

semiconductor, even at room temperature, hole- electron pairs are created.When electric

field is applied across an semiconductor, the current conduction takes place by two process,

namely: by free electrons and holes.

Extrinsic semiconductor:

Doping:

The intrinsic semiconductor has little current conduction capability at room temperature. To 

be  useful on electronics devices , the pure semiconductor must be altered so as to significantly

increase its conducting properties. This is achieved by adding a small amount of suitable impurity

to a semiconductor . It is then called impurity or extrinsic semiconductor . The process of adding 

impurities to a semiconductor is known as doping.

Depending  upon the type of impurity added , extrinsic semiconductors are classified into :

 i) n-type semiconductor      

 ii) p-type semiconductor

 n-type semiconductor:

When a small amount of pentavalent impurity

is added to a pure semiconductor , it is know

 as n-type  semiconductor.To produce a n-

type semiconductor a small amount of pent-

avalent impurity is added to a pure semico-

nductor . The addition of pentavalent impurity

provides a large number of free electrons  in 

the semiconductor.

p-type semiconductor:


When a small amount of trivalent impurity is added to a pure semiconductor it is know as p-type

 semiconductor.

                                                                            
 





                












To produce a p-type semiconductor a small amount of trivalent impurity is added to a pure 

semiconductor. The addition of trivalent impurity provides a large number of free electrons 

in the semiconductor.

Majority and Minority Carriers:


n-type material :  


When a  tetravalent semiconductor material and a pentavalent material is doped , there is a free

electron which don’t make bond. So , in an n-type material the electron is called the majority 

carrier and the hole is called the minority carrier.

Example: 

p-type material :  


When a tetravalent semiconductor material and a trivalent material is doped , there is a lack of

electron which cannot make bond and make a hole. So , In a p-type material the hole is the 

majority carrier and the electron is the minority carrier.


PN junction:


When a p-type semiconductor is suitably joined to n-type semiconductor ,the contact 

surface is called PN junction. Most semiconductor devices contain  one or more PN 

junction .The PN junction is of great importance because it is in effect, The control 

element for semiconductor devices.


Properties of PN junction:

To explain the properties of a PN junction,

consider two types of materials; one P-

type and other N type as shown in figure. 

In this figure, left side material  is a N-type 

semiconductor having negative acceptor 

ions and positive charged holes. The right 

side material is P-type semiconductor having

positive donor ions and free electrons.

 
 

                  

                     •distribution of the charge carriers before 

         

                      the diffusion   

                    • distribution of the charge carriers after   

                      the diffusion of the charge carriers 

   

                    • junction barrier

                    • charge distribution in the junction barrier

 
 
 

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