GOVERMENT ENGINEERING COLLEGE EC Department Sem 3 Sub - EDC

FIELD-EFFECT TRANSISTORS (FET) PART 2 PRESANTED BY : BHATT YASHASVI HIRENBHAI (130210111007)

FET AS A VOLTAGE VARIABLE RESISTOR (VVR) FET is a device that is usually operated in the constant current portion of its output characteristics for the linear application. JFET in the region just before pitch off. In the region Vds is small but Id is large. This region is marked as ohmic region in the output characteristics of a JFET. The drain to source resistance rds in this region is dependent on the gate to source voltage.

Small signal FET drain resistance rd varies with applied gate voltage Vgs and FET act like a VERIABLE PASSIVE RESISTOR.

Application of VVR AGC Automatic gain control (AGC) is used in most modern receiver. The FET used as VVR finds applications in many areas where property can be used. The VVR for example can be used to vary the voltage gain of a multistage amplifier “A” as the signal level is increased. This called automatic gain control. The gain of a CE amplifier is approximately equal to –RC/RE.

METAL INSULATOR SEMICONDUCTOR FIELD EFFECT TRANSISTOR (MISFETs) Metal insulator semiconductor field effect transistor is one of the most widely used electronic devices. Generally as a insulated gate field effect transistor (IGFET). Most of such devices are made using silicon for the semiconductor,Sio2 for the insulator, and metal or heavily doped polysilicon for the gate electrode.

The term metal oxide semiconductor field effect transistor (MOSFET) or simply MOS is commonly used. The operation of MOSFET is similar to that of JFET. All equations apply equally well to the MOSFET and JFET in amplifier connection. MOSFETs are of two types namely 1) depletion type MOSFET or DE-MOSFET 2) enhancement type MOSFET or E-MOSFET

STURCTURE OF MOSFET TYPES DE-MOSFET E-MOSFET

1) DEPLETION TYPE OF MOSFET Construction of the depletion type MOSFET is as shown in fig… A p type of semiconductor silicon is used as a substrate.

Operation The operation can be explained with three different operating conditions 1) operation with Vgs =0 2) operation with negative Vgs 3) operation with positive Vgs

The level of free electrons has been “enhanced” due to the application of positive gate voltage. Therefore the region of operation corresponding to the positive gate current is called as ‘enhancement’ region of operation and the region between cutoff and saturation is referred to as depletion region this is shown in fig

Characteristics The drain characteristics of a N channel depletion MOSFET. note that this characteristics is same as that of a JFET except for the positive part of Vgs.

Schematic Symbol

2) ENHANCEMENT MOSFET The basic construction of an n channel enhancement type of MOSFET is as shown in fig.. A slab of p type semiconductor is used as substrate.

Operation The operation can be explained with two different operating conditions 1)operation with Vgs=0 volt 2)operation with Vgs is positive

Characteristics The drain Characteristics and transfer characteristics of a n channel enhancement MOSFET are as shown in fig..

Schematic Symbol

MOSFET RESISTOR The MOSFET can be employed as a resistor. DE MOSFET can be employed as a resistor by connecting the gate to the source as shown in fig.. E MOSFET may also be employed as a resistor by connecting the gate to the drain as shown in fig.. So that gate-to-source voltage Vgs becomes equal to drain-to-source voltage Vds.

COMPARISON N channel FETs P channel FETs Current carriers – Electrons Mobility – Large Noise - Less Tranceconductance - Larger P channel FETs Current carriers – holes Mobility – Poor Noise –More Tranceconductance - Lesser

COMPARISON JFET MOSFET JFET are of two types ,p-channel and n- channel JFETs. JFETs do not have the insulated gate. Input impedance is lower than that of the MOSFETs. MOSFET MOSFETs can be of depletion type or enhancement type. MOSFETs have the insulated gate structure. Input impedance is higher than that of JFET due to the insulated gate structure.

Drain resistance is lower than that of a MOSFET. The input resistance rgs and the feedback resistance rgd are very much smaller than that of MOSFET. Biasing methods used are self bias and voltage divided bias. Drain resistance is higher than that of a JFET. rgs and rgd are very much larger than that of a JFET. Biasing circuits of depletion MOSFET are same as those used for JFET but biasing circuit for enhancement MOSFET are feedback biasing and voltage divider biasing.

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