Reg. No.

Subject Code: 14BEEC204

KARPAGAM UNIVERSITY

(Under section 3 of UGC Act 1956)

COIMBATORE - 641 021.

(For the candidates admitted from 2013 onwards)

B.E DEGREE EXAMINATION, AUGUST 2014

SECOND SEMESTERELECTRONICS AND COMMUNICATION ENGINEERINGELECTRONICS DEVICES

Time : 3 hours

Maximum : 100 MarksQUESTION & ANSWERS

PART A (15X2 = 30 Marks)(Answer Any Fifteen Questions)1. Differentiate between drift and diffusion semi-conductor.Drift Current:

The drift current is defined as the flow of electric current due to the motion of the charge carriers under the influence of an external electric field.

Drift current due to the charge carriers such as free electrons and holes are the current passing through a square centimeter perpendicular to the direction of flow.

Diffusion Current:In a semiconductor material the change carriers have the tendency to move from the region of higher concentration to that of lower concentration of the same type of charge carriers. Thus the movement of charge carriers takes place resulting in a current called diffusion current.

2. What is mass action law?Under thermal equilibrium the product of the free electron concentration and the free hole concentration is equal to a constant equal to the square of intrinsic carrier concentration. The intrinsic carrier concentration is a function of temperature.

The equation for the mass action law for semiconductors is:

3. Write the Einsteins relationship for semi-conductor.In physics (specifically, in kinetic theory) the Einstein relation (also known as EinsteinSmoluchowski relation) is a previously unexpected connection revealed independently by Albert Einstein in 1905 and by Marian Smoluchowski in 1906 in their papers on Brownian motion. The more general form of the equation is[4]

where

D is the diffusion constant;

is the "mobility", or the ratio of the particle's terminal drift velocity to an applied force, = vd / F;

kB is Boltzmann's constant;

T is the absolute temperature.

This equation is an early example of a fluctuation-dissipation relation.4.A sample of n type semiconductor has a hall coefficient of 160 cm^3/coulomb. If its resistivity is 0.16 ohm cm, estimate the electron mobility in the sample.

5. What is barrier voltage?

The barrier voltage is the amount of electromotive force required to start current through the P-N junction. Barrier voltages for silicon typically range from 0.5 - 0.7 volts. Barrier voltages for germanium typically range from 0.2 - 0.3 volts.6. The current flowing in a certain PN junction diode at room temperature 2*10^-7 amps, when the large reverse voltage is applied. Calculate the current flowing, when 0.1v forward bias is applied at room temperature. Since PN junction diode only conducts current in forward bias. The barrier voltage for silicon is 0.7 v and for germanium 0.3 v. The applied forward bias voltage is 0.1 v only, the diode does not conducts any current because its falls below the barrier voltage. The current flowing through the PN diode is 0 v.7. Write some applications of photodiode.

A photodiode is a semiconductor device that converts light into current. The current is generated when photons are absorbed in the photodiode. A small amount of current is also produced when no light is present. Photodiodes may contain optical filters, built-in lenses, and may have large or small surface areas. Photodiodes usually have a slower response time as their surface area increases.8.What is the unique feature of PIN DIODE?A PIN diode is a diode with a wide, undoped intrinsic semiconductor region between a p-type semiconductor and an n-type semiconductor region. The p-type and n-type regions are typically heavily doped because they are used for ohmic contacts.The wide intrinsic region is in contrast to an ordinary PN diode. The wide intrinsic region makes the PIN diode an inferior rectifier (one typical function of a diode), but it makes the PIN diode suitable for attenuators, fast switches, photo detectors, and high voltage power electronics applications.

9. When does the transistor act as a switch?If the circuit uses the Bipolar Transistor as a Switch, then the biasing of the transistor, either NPN or PNP is arranged to operate the transistor at both sides of the I-V characteristics curves we have seen previously.The areas of operation for a Transistor Switch are known as the Saturation Region and the Cut-off Region. This means then that we can ignore the operating Q-point biasing and voltage divider circuitry required for amplification, and use the transistor as a switch by driving it back and forth between its fully-OFF (cut-off) and fully-ON (saturation) regions as shown below.

Operating Regions

10. Derive the relationship between current gain (alpha and beta).

The expressions for both Alpha, and Beta, the mathematical relationship between these parameters and therefore the current gain of the transistor can be given as:

11.A certain-transistor has alpha=0.98, ico=5 micro amps and ib=100 micro amps. Find the values of collector and emitter current.

WKT, = /1- =0.98/1-0.98. =Ic/Ib. Hence, Ic= x Ib=49x100 amps.

Ie= Ic/ =4900 amps/0.98 = 50 mill amps.

12.Draw the hybrid model of a common emitter transistor.

13. Write the effect of gate to source voltage on drain characteristics.

14. Differentiate between FET and BJT.

1. The BJT is a current-controlled device since its output is determined on the input current, while FET is considered as a voltage-controlled device, because it depends on the field effect of the applied voltage.

2. The BJT (Bipolar Junction Transistor) uses both the minority and majority carriers (holes and electrons), while FETs, which are sometimes called unipolar transistors, uses either holes or electrons for conduction.

3. BJTs three terminals are named the base, emitter, and collector, while FETs are named the source, drain, and gate.

4. BJTs are the first type to be commercially massed produced.

15.Why JFET is often called square law device.

The input-output transfer characteristic of the JFET is not as straight forward as it is for the BJT.In a JFET, the relationship (Shockleys Equation) between VGS (input voltage) and ID (output current) is used to define the transfer characteristics, and a little more complicated (and not linear):

As a result, FETs are often referred to square law devices.16.Why N-channel MOSFET device is always preferred more than P-channel devices.The only real reason is that electron mobility is higher than hole mobility. All of the physical advantages fall out of that (gate-capacitance, channel resistance, cost, size, thermal rating, etc).Every other difference is just a difference. Let's consider polarity. If you are using the transistor as a switch, p-type is "on" under conditions opposite those of the n-type.

17. List the methods to turn off an SCR.There are three methods of switching off the SCR, namely natural commutation, reverse bias turn-off, and gate turn-off.

18. Draw the equivalent circuit of UJT.

19.Differentiate between SCR and TRIAC.Silicon Controlled Rectifier

An SCR is a modified diode. A diode is a device that conducts electricity in one direction, blocking it from going the other way. The diode is a two-lead device; the leads are called the cathode and the anode. The SCR has a third lead called the gate. Normally, the device does not conduct until it receives a voltage at the gate; then it remains on until the voltage across the cathode and anode drops past a critical point. It can switch large currents many thousands of times per second.

Triac

A triac, like an SCR, has three leads and acts as a current switch. Its construction and operation are somewhat more complex than an SCR's, as it conducts electricity in two directions. This makes the triac more useful in alternating current (AC) circuits than an SCR, as the current direction for AC changes 120 times per second.

20. What are the advantages of ICs over discrete circuits?

Size: Sub-micron vs. millimeter/centimeter. Speed and Power: Smaller size of IC components yields higher speed and lower power consumption due to smaller parasitic resistances, capacitances and inductances.

Lower power consumption ripple effect => less heat => cheaper power supplies=> reduced system cost. Integrated circuit manufacturing is versatile.

Simply change the mask to change the design.PART B (5X14=70 Marks)Answer All the Questions

21. a. Derive the equation of drift and diffusion current.

The flow of charge (ie) current through a semiconductor material are of two types namely drift & diffusion.

(ie) The net current that flows through a (PN junction diode) semiconductor material has two components

(i) Drift current

(ii) Diffusion current

Drift current :-

(a) Exess hole concentration varying along the axis in an N-type semiconductor bar

(b) The resulting diffusion current

When an electric field is applied across the semiconductor material, the charge carriers attain a certain drift velocity Vd , which is equal to the product of the mobility of the charge carriers and the applied Electric Field intensity E ;

Drift velocity Vd = mobility of the charge carriers X Applied Electric field intensity.

Holes move towards the negative terminal of the battery and electrons move towards the positive terminal of the battery. This combined effect of movement of the charge carriers constitutes a current known as the drift current .

Thus the drift current is defined as the flow of electric current due to the motion of the charge carriers under the influence of an external electric field.

Drift current due to the charge carriers such as free electrons and holes are the current passing through a square centimeter perpendicular to the direction of flow.

(i) Drift current density Jn , due to free electrons is given by

Jn = q n n E A / cm2(ii) Drift current density JP, due to holes is given by JP = q p p E A / cm2

Where, n - Number of free electrons per cubic centimeter.

P - Number of holes per cubic centimeter

n Mobility of electrons in cm2 / Vs

p Mobility of holes in cm2 / Vs

E Applied Electric filed Intensity in V /cm

q Charge of an electron = 1.6 x 10-19 coulomb.

Diffusion current :-

It is possible for an electric current to flow in a semiconductor even in the absence of the applied voltage provided a concentration gradient exists in the material.

A concentration gradient exists if the number of either elements or holes is greater in one region of a semiconductor as compared to the rest of the Region.

In a semiconductor material the change carriers have the tendency to move from the region of higher concentration to that of lower concentration of the same type of charge carriers. Thus the movement of charge carriers takes place resulting in a current called diffusion current.

ORb. With Fermi-level energy band diagram explain intrinsic and extrinsic semiconductor.

Pure form of semiconductors with no other impurity atoms is called as intrinsic semiconductor and possesses poor conductivity. It has equal numbers of negative carriers (electrons) and positive carriers (holes). A silicon crystal is different from an insulator because at any temperature above absolute zero temperature, there is a finite probability that an electron in the lattice will be knocked loose from its position, leaving behind an electron deficiency called a "hole". Since that deficiency has a tendency to attract an electron to that spot hole (deficiency) have positive charge. This type of semiconductors has less number of charge carriers they cant be used for semiconductor device manufacturing.

Fermi Level In Intrinsic Semiconductor

Total number of electron in conduction band & hole in valence band can be found from FermiDirac function.

EXTRINSIC SEMICONDUCTOR

Conductivity of an intrinsic semiconductor can be increased by means of adding impurity, impure semiconductor are called as extrinsic semiconductor. The process of adding impurity into the crystal is called as doping. Dopant is of two types pentavalent impurity and trivalent impurity.

N-type

The dopant atoms added to the semiconductor crystal in this case are donor atoms.

For silicon, we can use phosphorus (P), arsenic (As) or antimony (Sb) as donors. These are group V elements, with five electrons in their outermost shell. When these atoms are included in the silicon crystal, one of the electrons in this shell can easily jump to the conduction band, leaving a positively charged donor ion behind. This process is sometimes called activation or ionization of the donor atoms. The positively charged donor atom that is left behind after ionization is immobile and does not contribute to conduction. The electron leaving the atom by ionization does, and is counted in the electron concentration n.

Acceptor level

Donor levelVariation of fermi level of n type semiconductor with temperature and impurity concentration:

At 0K Fermi level lies exactly in middle of donor energy level and bottom of conduction band energy. As temperature raises from zero donor atoms starts to donate electrons & this process dominates intrinsic carrier generation process. Since it is difficult for an electron to jump from top of valence band to bottom of conduction band at low temperature. As temperature raises donor must have donated its entire free electron to conduction band and this level becomes empty. At this point intrinsic carrier generation process starts to dominate the donor contribution there on. So Fermi level moves down as in case of intrinsic semiconductor.

When doping concentration is increased to 1028 atoms/m3 then extrinsic behavior can be retained at higher temperature as shown in the figure this means the donor contribution will be there at higher temperature and Fermi level will be in middle of donor & conduction energy level. Fermi level will move to intrinsic level only at very high temperature.

Variation of fermi level of p type semiconductor with temperature and impurity concentration:At 0K Fermi level lies exactly in middle of acceptor energy level and top of valence band energy level. As temperature rises from zero acceptor atoms starts to accepts electrons and this process start to pull Fermi level up. So Fermi level moves close to intrinsic Fermi level in between conduction band valence band.

When doping concentration is increased the acceptor contribution will be there even at higher temperatures and Fermi level will be in middle of acceptor & valence energy level. Fermi level will move to intrinsic level only at very high temperature.

22.a. With necessary diagram analyze the characteristics of tunnel diode and tunneling phenomenon. A Tunnel Diode is s pn junction that exhibits negative resistance between two values of forward voltage

Theory

The tunnel diode is basically a pn junction with heavy doping of p type and n type semiconductor materials. In normal PN diode impurity concentration is about 1 parts per 108 pure atoms. In zener diode doping is in the range of 1 part per 103 pure atoms.

Heavy doping results in large no of majority carriers because of large no of carriers, most are not used during initial recombination that produces depletion layer. It is very narrow. Depletion layer of tunnel diode is 100 times narrower.

Operation of tunnel diode depends on the tunneling effect. The movement of valence electrons from the valence energy band to the conduction band with little or no applied forward voltage is called tunneling.

Tunnel diode

Symbol

A

K

At zero bias

At small forward voltage

At high forward bias (peak voltage)

At Decreasing Current REGION

At higher forward bias

V-I CHARACTERISTICS - Forward bias operation

Under normal forward bias operation, as voltage begins to increase, electrons at first tunnel through the very narrow p-n junction barrier because filled electron states in the conduction band on the n-side become aligned with empty valence band hole states on the p-side of the pn junction. As voltage increases further these states become more misaligned and the current drops this is called negative resistance, because current decreases with increasing voltage. As voltage increases yet further, the diode begins to operate as a normal diode, where electrons travel by conduction across the pn junction, and no longer by tunneling through the 2 pn junction barrier. Thus the most important operating region for a tunnel diode is the negative resistance region. When operated in the negative region used as oscillator.ORb. i) write the principle operation of the following

1. Photo Diode

Photodiode is a 2 lead semiconductor device that transforms light energy to electric current. Suppose anode and cathode of a photodiode are wired to a current meter. When photodiode is placed in dark, the current meter displays zero current flow.

When the photodiode is expose to light, it acts a a current source, causing current flow from cathode to anode of photodiode through the current meter. Photodiodes have very linear light v/s current characteristics. Commonly used as light meters in cameras. Photodiodes often have built-in lenses and optical filters. Response time of a photodiode slows with increasing surface area. Photodiodes are more sensitive than photo resistor.

2. Photo voltaic cell

Solar cells are photodiodes with very large surface areas.

Compared to usual photodiodes, the large surface area in photodiode of a solar cell yields

a device that is more sensitive to incoming light. a device that yields more power (larger current/volts). Solar cells yield more power. A single solar cell may provide up to 0.5V that can supply 0.1A when exposed to bright light.

3. LED

In a diode formed from a direct band-gap semiconductor, such as gallium arsenide, carriers that cross the junction emit photons when they recombine with the majority carrier on the other side. Depending on the material, wavelengths (or colors) from the infrared to the near ultraviolet may be produced. The forward potential of these diodes depends on the wavelength of the emitted photons: 1.2 V corresponds to red, 2.4 to violet. The first LEDs were red and yellow, and higherfrequency diodes have been developed over time. All LEDs are monochromatic; 'white' LEDs are actually combinations of three LEDs of a different color

4. LCD

Aliquid crystal displayis special thin flat panels that can let light go through it, or can block the light. (Unlike anLEDit does not produce its own light). The panel is made up of several blocks, and each block can be in any shape. Each block is filled withliquid crystalsthat can be made clear or solid, by changing the electric current to that block. Liquid crystal displays are often abbreviatedLCDs.

Liquid crystal displays are often used inbattery-powered devices, such as digital watches, because they use very littleelectricity. They are also used for flat screenTV's. Many LCDs work well by themselves when there is other light around (like in a lit room, or outside in daylight). Forsmartphones,computer monitor, TV's and some other purposes, a back-light is built into the product.

5. LDR

Light sensitive variable resistors OR Light dependant resistor.

Its resistance depends on the intensity of light incident upon it.

Under dark condition, resistance is quite high (M: called dark resistance & Under bright condition, resistance is lowered (few hundred ).

Response time:

When a photoresistor is exposed to light, it takes a few milliseconds, before it lowers its resistance & When a photoresistor experiences removal of light, it may take a few seconds to return to its dark resistance.

ii) Explain the breakdown mechanism of Zener diode.

AZener diodeis also a PN diodethat permitscurrentnot only in the forward direction like a normal diode, but also in the reverse bias if the voltage is larger than thebreakdown voltageknown as "Zener voltage".

The device was named afterClarence Zener, who discovered this electrical property.A conventional solid-statediodewill not allow significant current if it isreverse-biasedbelow its reverse breakdown voltage.

When the reverse bias breakdown voltage is exceeded, a conventional diode is subject to high current due to avalanche breakdown. Unless this current is limited by circuitry, the diode will be permanently damaged.

A Zener diode exhibits almost the same properties under forward bias, except the device is specially designed so as to have a greatly reduced breakdown voltage, the so-called Zener voltage. By contrast with the conventional device, a reverse-biased Zener diode will exhibit a controlled breakdown and allow the current to keep the voltage across the Zener diode at the Zener voltage.

For example, a diode with a Zener breakdown voltage of 3.2 V will exhibit a voltage drop of 3.2 V even if reverse bias voltage applied across it is more than its Zener voltage.

The Zener diode is therefore ideal for applications such as the generation of a reference voltage (e.g. for anamplifierstage), or as a voltage stabilizer for low-current applications.

Zener breakdown is a break down in which direct rupture of covalent bond takes place because of high electric field that exists due to high doping. Due to high doping depletion width will be small & E will be high since E= V/d.

In avalanche mechanism voltage drop across the junction increases in reverse bias. Carriers are generated by absorbing the heat across the junction; these carriers acquire sufficient energy while travelling inside the electric field & collide with stable atoms resulting in covalent bond breaking.

New carriers gain energy from electric field & again collide with other ions. This process result in avalanche of charge carriers & as result current increases suddenly.

23. a. i) Derive the Ebers moll equation for a transistor with its emitter base junction forward biased and collector base junction reverse biased.The bipolar junction transistor can be considered essentially as two p-n junctions placed back-to-back, with the base p-type region being common to both diodes. This can be viewed as two diodes having a common third terminal as shown in Fig..

Fig. Bipolar Transistor Shown as Two Back-to-Back p-n JunctionsHowever, the two diodes are not in isolation, but are interdependent. This means that the total current flowing in each diode is influenced by the conditions prevailing in the other. In isolation, the two junctions would be characterized by the normal Diode Equation with a suitable notation used to differentiate between the two junctions as can be seen in Fig. 2.2. When the two junctions are combined, however, to form a transistor, the base region is shared internally by both diodes even though there is an external connection to it. As seen previously, in the forward active mode, (F of the emitter current reaches the collector. This means that (F of the diode current passing through the base-emitter junction contributes to the current flowing through the base-collector junction. Typically, (F has a value of between 0.98 and 0.99. This is shown as the forward component of current as it applies to the normal forward active mode of operation of the device. Note this current is shown as a conventional current in Fig. 2.2. It is equally possible to reverse the biases on the junctions to operate the transistor in the reverse active mode. In this case, (R times the collector current will contribute to the emitter current. For the doping ratios normally used the transistor will be much less efficient in the reverse mode and (R would typically be in the range 0.1 to 0.5.

Fig. The n-p-n Transistor Considered as Combined p-n Junctions

EBERS-MOLL EQUATIONS

The Ebers-Moll transistor model is an attempt to create an electrical model of the device as two diodes whose currents are determined by the normal diode law but with additional transfer ratios to quantify the interdependency of the junctions as shown in Fig. 2.3. Two dependent current sources are used to indicate the interaction of the junctions. The interdependency is quantified by the forward and reverse transfer ratios, (F and (R. The diode currents are given as:

EMBED Equation.3 Applying Kirchoffs laws to the model gives the terminal currents as:

IE = IF - (RIR

(F = 0.98 0.99 typically

IC = (FIF IR

(R = 0.1 0.5 typically

IB = IE - IC

This gives:

These are called the Ebers-Moll Equations for the bipolar transistor (see Fig.).

Fig. The Ebers-Moll Model of an n-p-n Bipolar Junction Transistor

ii) Draw the hybrid equivalent circuit of a CB, CC, CE transistor.COMMON BASE

COMMON COLLECTOR

COMMON EMITTER

ORb. Explain the input and output characteristics of a NPN transistor.An NPN bipolar transistor is so called because the outer layers are N-type semiconductors, while the base is a P-type. N stands for negative charge carriers or electrons, and P for positive charge carriers or holes.General Characteristics

A common emitter or CE circuit is used for amplification. A small signal introduced into the base produces a larger signal at the output. It has the emitter lead connected to ground. It is usually built with at least two resistors, with one at the base and the other at the collector.

The circuit has two loops, where one is called the base loop and the other the collector loop. The loops are found by using Kirchoff's Law to follow the path between the supplied voltage and the transistor leads. Ohm's Law is also used. It is V = IR, where V is the voltage, I the current and R is the resistance.The transistor gain, or dc beta, is the ratio of the collector current IC to the base current IB, and is symbolized as Bdc, where B is the Greek letter beta. It is also called Hfe. The gain tells how much the input signal is amplified. It is a constant that depends on the transistor type.

NPN transistors may be modeled as two back-to-back diodes in what is called the Ebers-Moll model. The base-emitter behaves like a forward-biased diode, while the base-collector behaves like a reverse-biased diode. Forward biased means that the voltage is applied is in a conducting direction, while reverse-biased means that the voltage is applied against easy current flow.

Input Characteristics

Input characteristics are found by considering the base loop. A graph of the base current IB versus VBE, which is the voltage between the base and the emitter, looks like that of an ordinary diode. The current is zero until VBE reaches 0.7 volts, where it then increases very suddenly.

The base voltage forward biases the emitter. The equation to find the voltage across the resistor RB is VBB -- VBE, where VBB is the base voltage. The current IB is found using VBB -- VBE / RB.

Output Characteristics

Output characteristics are found by considering the collector loop.

A graph of the collector current IC versus the collector-emitter voltage VCE shows much the same shape for different transistors, though the numbers will be different. When VCE is zero, so is IC. As VCE increases, IC will remain zero and then suddenly shoot up when the voltage reaches a certain value, much the same way as IB. Unlike IB, IC will reach a plateau and then remain basically constant as VCE increases. The graph illustrates that IC = Bdc * IB, or that a small increase in IB leads to a large increase in IC.IB will be constant until the breakdown region of the transistor is reached. This region is where the transistor will become damaged when the voltage is too large, and is dependent on the transistor type. IB will rapidly increase when the breakdown voltage is reached. The collector voltage reverse-biases the collector. The collector-emitter voltage is equal to the collector voltage minus the voltage across the collector resistor. It is VCE = VC -- IC * RC.

24.a. Describe the construction and operation of the depletion type MOSFET.Construction:

Heavily doped two n type semiconductors is placed inside p-substrate and named as source & drain. Depending up on the applied voltage either terminal can act as source or drain. Gate terminal is made up of metal & is separated from substrate by silicon-di- oxide layer.Sio2 layer acts as insulator and prevents movement of charge carriers outside the substrate. Lightly doped n type semiconductor is placed between source and drain and named as channel.

Gate

silicon dioxide layer

Source

Drain

Conducting channel (N type)

Working:

Unlike Enhancement mode MOSFET Depletion mode has channel buried underneath the gate during fabrication of device. When drain terminal is given positive voltage with respect to source the device start to conduct ie electron starts to move from source to drain & current start to flow from drain to source even at zero gate voltage.

Vgs=0v

Vds

Dr ain

Conducting channel(N type)

When gate voltage is applied more electron are attracted in to channel from substrate & current increases.

Vgs

Vds

Dr ain

Conducting channel (N type)

When gate voltage is negative electrons in channel are repelled deep in to the surface leaving a positive charge inside the channel. Hence the electrons in the channel reduce there by reducing the current.

Vgs

Vds

Dr ain

Conducting channel (N type)

Symbol and Characteristics:

OR

b. i) Explain the operation of JFET.JFET operation is like that of agarden hose. The flow of water through a hose can be controlled by squeezing it to reduce thecross section; the flow ofelectric chargethrough a JFET is controlled by constricting the current-carrying channel. The current also depends on the electric field between source and drain (analogous to the difference inpressureon either end of the hose).

Constriction of the conducting channel is accomplished using thefield effect: a voltage between the gate and source is applied to reverse bias the gate-source pn-junction, thereby widening thedepletion layerof this junction (see top figure), encroaching upon the conducting channel and restricting its cross-sectional area. The depletion layer is so-called because it is depleted of mobile carriers and so is electrically non-conducting for practical purposes.[1]When the depletion layer spans the width of the conduction channel, "pinch-off" is achieved and drain to source conduction stops. Pinch-off occurs at a particular reverse bias (VGS) of the gate-source junction. The pinch-off voltage (Vp) varies considerably, even among devices of the same type. For example, VGS(off)for the Temic J202 device varies from0.8 Vto4 V.[2]Typical values vary from0.3 Vto10 V.

To switch off ann-channel device requires anegative gate-source voltage (VGS). Conversely, to switch off ap-channel device requirespositive VGS.

In normal operation, the electric field developed by the gate blocks source-drain conduction to some extent. Some JFET devices are symmetrical with respect to the source and drain.

Mathematical modelThe current in N-JFET due to a small voltage VDS(that is, in the linear ohmic region) is given by treating the channel as a rectangular bar of material ofelectrical conductivity:[3]

where

ID= drainsource current

b= channel thickness for a given gate voltage

W= channel width

L= channel length

q= electron charge = 1.6 x 1019C

n=electron mobilityNd= n-type doping (donor) concentration

The drain current in thesaturation regionis often approximated in terms of gate bias as:[3]

where

IDSSis the saturation current at zero gatesource voltage, i.e. the maximum current which can flow through the FET from drain to source at any (permissible) drain-to-source voltage; see e. g. the I-V characteristics diagram above.

In thesaturation region, the JFET drain current is most significantly affected by the gatesource voltage and barely affected by the drainsource voltage.

If the channel doping is uniform, such that the depletion region thickness will grow in proportion to the square root of (the absolute value of) the gatesource voltage, then the channel thicknessbcan be expressed in terms of the zero-bias channel thicknessaas:

where

VPis the pinchoff voltage, the gatesource voltage at which the channel thickness goes to zero

ais the channel thickness at zero gatesource voltage.

Then the drain current in the linear ohmic region can be expressed as:

or (in terms of):

ii) Analyze the VI characteristics of UJT

The static emitter characteristic (a curve showing the relation between emitter voltage VEand emitter current IE) of aUJTat a given inter base voltage VBBis shown in figure. From figure it is noted that for emitter potentials to the left of peak point, emitter current IEnever exceeds IEo. The current IEocorresponds very closely to the reverse leakage current ICoof the conventional BJT. This region, as shown in the figure, is called the cut-off region. Once conduction is established at VE= VPthe emitter potential VEstarts decreasing with the increase in emitter current IE. This Corresponds exactly with the decrease in resistance RBfor increasing current IE. This device, therefore, has a negative resistance region which is stable enough to be used with a great deal of reliability in the areas of applications listed earlier. Eventually, the valley point reaches, and any further increase in emitter current IEplaces the device in the saturation region, as shown in the figure. Three other important parameters for the UJT are IP, VVand IVand are defined below:

Peak-Point Emitter Current. Ip. It is the emitter current at the peak point. It represents the rnimrnum current that is required to trigger the device (UJT). It is inversely proportional to the interbase voltage VBB.

Valley Point Voltage VVThe valley point voltage is the emitter voltage at the valley point. The valley voltage increases with the increase in interbase voltage VBB.

Valley Point Current IVThe valley point current is the emitter current at the valley point. It increases with the increase in inter-base voltage VBB.

Special Features of UJT.The special features of a UJT are :

1. A stable triggering voltage (VP) a fixed fraction of applied inter base voltage VBB.

2. A very low value of triggering current.

3. A high pulse current capability.

4. A negative resistance characteristic.

5. Low cost.

25.a. Analyze the working principle of SCRConstruction:

As the terminology indicates, the SCR is a controlled rectifier constructed of a silicon semiconductor material with a third terminal for control purposes. Silicon was chosen because of its high temperature and power capabilities. The basic operation of the SCR is different from that of an ordinary two-layer semiconductor diode in that a third terminal called a gate, determines when the rectifier switches from the open-circuit to short-circuit state. It is widely used as a switching device in power control applications. SCR is a three-terminal four-layer semiconductor device, the layers being alternately of P-type and N-type. The junctions are marked J1, J2 and J3 (junctions J1and J3 operate in forward direction while middle junction J2 operates in the reverse direction) whereas the three terminals are anode (A), cathode (C) and gate (G) which is connected to the inner P-type layer. The function of the gate is to control the firing of SCR.

Operation:

The Silicon Control Rectifier SCR start conduction when it is forward biased. For this purpose the cathode is kept at negative and anode at positive. When positive clock pulse is applied at the gate the SCR turns ON. When forward bias voltage is applied to the Silicon Control Rectifier SCR, the junction J1 and J3 become forward bias while the junction J2 become reverse bias. When we apply a clock pulse at the gate terminal, the junction J2 become forward bias and break over voltage comes early & SCR start conduction. The Silicon Control Rectifier SCR turn ON and OFF very quickly, At the OFF state the Silicon Control Rectifier SCR provide infinity resistance and in ON state, resistance of the SCR is typically 0.01 to 0.1 ohm. The Silicon Control Rectifier SCR is normally operated below the forward break over voltage (VBO). To turn ON the Silicon Control Rectifier SCR we apply clock pulse at the gate terminal which called triggering of Silicon Control Rectifier, but when the Silicon Control Rectifier SCR turned ON, now if we remove the triggering voltage, the Silicon Control Rectifier SCR will remain in ON state. This voltage is called Firing voltage. Alternatively the SCR can be switched off by applying negative voltage to the anode.

OR

b. i) Explain the following

1. Diffusion

Diffusion of impurity atoms in silicon during processing is important for the electrical characteristics of silicon devices. Various ways of introducing dopants into silicon by diffusion are used and have been studied with the goal of controlling dopant distribution, total dopant concentration, uniformity, and reproducibility.

Diffusion is used to form base, emitter, and collector regions in bipolar device processing, to form source, drain and channel regions, and to dope poly-silicon inMOSprocessing. Dopant atoms that span a wide range of concentrations can be introduced into silicon in many ways. The most commonly used methods are:

diffusion from a chemical source in a vapor form at high temperatures,

diffusion from a doped-oxide source, and

diffusion and annealing from an ion-implanted layer. 2.Ion implantation

Ion implantation is used to introduce dopants into the silicon crystals in a very controlled way. In order to do this, atoms or molecules are ionized, accelerated in an electric field and implanted into the target material. The range of the implanted ions in the substrate depends on the mass of the implanted ions, their energy, the mass of the substrate atoms, the structure of the crystal, and the angle of incidence.The electrical activation of ion-implanted species is carried out by annealing. This causes a redistribution of the impurity atoms which should be kept as low as possible. In order to optimize the electrical behavior of the device, it is important to know how the impurities redistribute during the anneal. The development of appropriate models and simulation programs to predict the diffusion is a major topic in semiconductor technology research.

Main advantages of ion implantation compared to diffusion for the doping of semiconductors are:

Short process times, good homogeneity and reproducibility of the profiles.

Exact control of the amount of implanted ions by measuring the current. This is of particular importance for low concentrations, e.g., to adjust the threshold voltage ofMOStransistors.

Relatively low temperatures during the process.

Various materials can be used for masking, e.g., oxide, nitride, metals, and resist. Implantation through thin layers, e.g.,SiOis possible.

Low penetration depth of the implanted ions. This allows modification of thin areas near the surface with high concentration gradients.

Sequences of implantation steps with different energies and doses allow optimization of the dopant profiles.

There are also some disadvantages, such as:

The implanted ions cause damage in the substrate.

The change of material properties is restricted to the substrate domains close to the surface.

Additional effects during or after implantation, e.g., channeling or diffusion, make it difficult to achieve very shallow profiles and to theoretically predict the exact profile shapes.

3. Vapor DepositionPhysical Vapor Deposition (PVD) is a collective set of processes used to deposit thin layers of material, typically in the range of few nanometers to several micrometers.1PVD processes are environmentally friendly vacuum deposition techniques consisting of three fundamental steps.

Vaporization of the material from a solid source assisted by high temperature vacuum or gaseous plasma.

Transportation of the vapor in vacuum or partial vacuum to the substrate surface.

Condensation onto the substrate to generate thin films.

Different PVD technologies utilize the same three fundamental steps but differ in the methods used to generate and deposit material. The two most common PVD processes are thermal evaporation and sputtering. Thermal evaporation is a deposition technique that relies on vaporization of source material by heating the material using appropriate methods in vacuum. Sputtering is a plasma-assisted technique that creates a vapor from the source target through bombardment with accelerated gaseous ions (typically Argon). In both evaporation and sputtering, the resulting vapor phase is subsequently deposited onto the desired substrate through a condensation mechanism.ii) List the advantage of CMOS process.1. High input impedance. The input signal is driving electrodes with a layer of insulation (the metal oxide) between them and what they are controlling. This gives them a small amount of capacitance, but virtually infinite resistance. The current into or out of a CMOS input held at one level is just leakage, usually 1 A or less.

2. The outputs actively drive both ways.

3. The outputs are pretty much rail-to-rail.

4. CMOS logic takes very little power when held in a fixed state. The current consumption comes from switching as those capacitors are charged and discharged. Even then, it has good speed to power ratio compared to other logic types.

5. CMOS gates are very simple. The basic gate is a inverter, which is only two transistors. This together with the low power consumption means it lends itself well to dense integration. Or conversely, you get a lot of logic for the size, cost, and power.

Ic = 4900 amps

Ie = 50 milli amps

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