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.