Hardik Patel & Jatin Patel Topic :- JFET & MOSFET Guides by Hardik Patel & Jatin Patel GROUP BY Patel Tulsi M. = 150983111010 Joshi Trupti B. = 150983111004 Patel Reni B. = 150983111009

Introduction of FET The field effect transistor (FET) is a three terminal device used for a variety of applications. These application are very much similar to those of a bipolar junction transistor(BJT). The three terminals of an FET are name as drain (D) ,source (S) ,and gate (G) ,out of which gate acts as a controlling terminal. FET is uni-polar device i.e. operation depends on only one type of charge carriers (h or e) . It is a Voltage controlled Device (gate voltage controls drain current)

Unipolar or Bipolar ? FET is a unipolar device, that means that means the current flowing through it is only due to one type of charge particles, holes or electrons. Transistor on the other hand is a bipolar device as holes and electron both contribute to the flow of current. Just as there are npn and pnp transistors, there are n-channel and p-channel filed effect transistors. One of the most important characteristics of the FET is its very high input impedance. FETs are more temperature stable as compared to the BJT and it requires less the space than that for a BJT.

Classification of Filed Effect Transisitors p-channel Types of FET JFET n-channel MOSFET p-channel n-channel enhancement depletion

Basic of Junction Field Effect Transisitor The field-effect transistor (FET) controls the current between two points but does so differently than the bipolar transistor.  The FET operates by the effects of an electric field on the flow of electrons through a single type of semiconductor material.  This is why the FET is sometimes called a unipolar transistor. Junction Filed effect Transistor main two types == P channel N channel

Construction of a JFET A schematic representation of an n channel JFET. An n-type channel is formed between two p- type layers which are connected to the gate. Majority carrier electrons flow from the source and exit the drain, forming the drain current. The pn junction is reverse biased during normal operation, and this widens the depletion layers which extend into the n channel only (since the doping of the p regions is much larger than that of the n channel). As the depletion layers widen, the channel narrows, restricting current flow.

Construction of N Channel the structure of an N channel filed effect transistor . A semiconductorbar of N type material is taken ohmic contacts are made to the end of the bar. These are terminal named DREAIN and SOURCE . On the both sides of N type bar, heavily doped (p+) regions have been format by alloying or by diffusion to create a P-N junction. both this p+ regions are connected together and via an ohmic contact the gate terminal is brought out.

The supply voltage is connected between the DRAIN and SOURCE terminal of a JFET hence current is cased to flow along the length the N type bar. The current is due to the majority carrier which in the case are electrons. channel is the N type material between regions. The majority carriers (electrons here) move through this channel from source to drain . Since the channel is med of N type material , this FET called as N channel JFET. The electrons enter the channel through the " source " terminal and leave through the "drain" terminal.

Construction of P Channel The only difference between the P channel and N channel JFETs is that the P type semiconductor bar is being used with to N type gate regions. In a P channel JFET, current flows due to the holes. this is becausa holes are mejority carriers in a P type semiconductoer bar. in n channel JFET current flows due to flow of electrons.

Drain curves of an N channel JFET The gate voltage is negative and kept variable. the drain to source voltage is always positive. Drain characteristics or drain curves of a n channel JFET. Drain characteristics is a plot of a drain current Iᴅ versus drain to source voltage Vᴅѕ at different values of gate to source voltage Vԍѕ. The drain curve can be divided in to three regions viz. cutoff, saturation and ohmic region.

Cutoff Region With increase in negative Vԍѕ voltage , the channel width available for conduction decreases. At a certain voltage called “Vԍѕ(off)” the deletion region touch each other to close the channel completely. Thus cutoff region corresponds to Iᴅ = 0 and Vԍѕ > Vԍѕ(off). Gate cutoff voltage Vԍѕ(off) The value of Vԍѕ that makes the drain current Iᴅ approximately equal to zero is called as cutoff and cutoff voltage and it is donated by Vԍѕ(off). A JFET must be operated between Vԍѕ = 0 and Vԍѕ(off). For this range Iᴅ will change between Iᴅѕѕ and 0 respectively.

Saturation Region (Active Region) The region to the left of the pinch-off point is called the ohmic region. The JFET can be used as a variable resistor, where VGS controls the drain-source resistance (rd). As VGS becomes more negative, the resistance (rd) increases.

Saturation region is that portion of characteristics where Iᴅ remains fairly constant and does not change with changes in Vᴅѕ. This “saturation” is entirely different than the “saturation” in a transistor. In order to use the JFET as an amplifier it is operated in the saturation region. This region is also called as active region of JFET. The value of drain current corresponding to Vԍѕ = 0V is called as the source saturation current and it is denoted by Iᴅѕѕ. It is also called as the maximum gate current. Pinch off voltage (Vр) :- The “pinch off” voltage is the value of Vᴅѕ, at which the drain current reaches its constant saturation value.

The Ohmic region Breakdown region The drain current Iᴅ varies with variation in the drain to source voltage Vᴅѕ. The JFET is therefore said to be operating as a voltage variable resistance (VVR) in ohmic region. The resistance offered by the JFET decreases with in the value of negative gate to source bias voltage negative Vԍѕ. Breakdown region FET Breakdown :- When a JFET is operating in the saturation region , Iᴅ does not change with change in Vᴅѕ up to a certain value of Vᴅѕ called Vᴅѕ(max). If Vᴅѕ is increased further beyond this value , the gate channel junction breakdown due to avalanche effect and the drain current shoots up suddenly . This can damage the device . The value of break down voltage does not remains constant .Infact it decreases with increase in negative values of Vԍѕ.

Transfer characteristics Transfer characteristics or transconductance curve is the plot of output current Iᴅ versus the input controlling quantity which is Vԍѕ in the case.

Transfer characteristics or transconductor curve is the plot of output current Iᴅ versus the input controlling quantity which is Vԍѕ in this case. A bipolar transistor the relation between output current Iс and input controlling quantity Iв is given by Iс = βᴅс Iв Where βᴅс is considered to be a constant. The transfer characteristics defined by them shockley’s equation are unaffected by the network in which the JFET is connected. The transfer curve can be obtained using shockely’s equation or from the drain characteristics a transfer characteristics can be plotted from the drain characteristics.

MOSFET The MOSFET has become one of the most important devices used in design and constructions of integrated circuit for digital computers. MOSFET is the short form of Metal Oxide Semiconductor Field Effect Transistor.

Depletion type MOSFET A P type of semiconductor material (silicon) is used a substrate. Usually the substrate is internally connected to the source terminal but some time it is taken out as a separate terminal termed “SS”. The drain and source terminal are connected to the end type regions through the metallic contacts. These n type regions are linked with each other by a N channel. The gate terminal is insulated from the N channel by a thin silicon dioxides layer

Operation of D- MOSFET OPERATION OF Vԍѕ = 0V It shows that the gate, source , and subtract terminals are connected together to the ground point thus Vԍѕ = 0 volt a positive voltage Vᴅᴅ is applied between drain and source. Due to the positive voltage applied to the drain terminal, free electron from the channel are attracted to the drain and the drain current starts flowing. OPERATION OF DEPELTION MOSFET WITH NEGETIVE Vԍѕ Negative voltage applied gate and cathode terminals, the gate will tend to repel the free electrons towards the P type subtract and attract the holes form the subtract. These electrons and holes will recombine in side the channel this will reduced the number of free electrons available for conduction. Therefore the drain current will decrease with the increase negative value of Vԍѕ thus as -Vԍѕ increase , Iᴅ decrease for Vᴅѕ.

The higher the negative bias, the more the recombination the less is the drain current. APPLICATION OF D – MOSFET DMOSFET can be used as an amplifier It can be used as a switch.

ENHANCEMENT MOSFET N CHANNEL E-MOSFET The basic construction of an N channel enhancement type MOSFET. A slab of P type semiconductor is used as subtract. The subtract is a some time to connected to the source other wise it is brought out as the fourth terminal . The drain and source terminal are connected to the N type doped regions through the metallic contact. But most important point to be noted is that the “channel ”is absent here. OPREATION OF N CHANNEL E – MOSFET. The operation can be explain with two different conditions :- 1 operation with Vԍѕ = 0v 2 operation when Vԍѕ is positive.

OPERATION WITH Vԍѕ = 0 volt If Vԍѕ = 0 and a positive voltage is applied between its drain and source, than due to absence of the N type channel, a zero drain current will result. This is totally different from what happen in the depletion region type MOSFET , where Iᴅ = Iᴅѕѕ at Vԍѕ = 0. OPERETION WHEN Vԍѕ IS POSISTIVE The positive potential at the gate terminal will repel the holes present in the P type subtracted. This result in creation of depletion region near the SiO₂ insulating layer. But the minority carrier the electron in the P type substrate will be attracted toward the gate terminal and gather near the substrate of SiO₂ The electron concentration near SiO₂ layer increases to such and extend that its creats an induced N channel which connect the N type doped regions.

P CHANNEL E-MOSFETs CARACTERISTICS OF P CHANNEL E-MOSFET That the drain current increase in the negative gate to sourece voltage (Vԍѕ). That it is exactly the mirror image of the transfer characteristics of an N channel E-MOSFET The drain current Iᴅ remains zero up to voltage Vgs > Vt and increase as negative vgs become greater than Vt.

APPICATION OF E-MOSFET E-MOSFET can be used as a linear amplifier . As an inverter . As an active load (in integrator circuit )