electronics devices and circuit Flashcards
At room temperature the current in an intrinsic semiconductor is due to
holes and electrons
Work function is the maximum energy required by the fastest electron at 0 K to escape from the metal surface.
TRUE OR FALSE
FALSE ; minimum energy required
The most commonly used semiconductor material is
silicon
In which of these is reverse recovery time nearly zero?
Schottky diode; In schottky diode there is no charge storage and hence almost zero reverse recovery time.
A transistor has a current gain of 0.99 in the CB mode. Its current gain in the CC mode is
100
In p-n-p transistor the current IE has two components viz. IEP due to injection of holes from p-region to n-region and IE due to injection of electrons from n-region to p-region. Then
IEp»_space; IEn
In an n channel JFET, the gate is
p type; Since channel is n type gate must be p type.
The amount of photoelectric emission current depends on
intensity of incident radiation
Assertion (A): A p-n junction has high resistance in reverse direction.
Reason (R): When a reverse bias is applied to p-n junction, the width of depletion layer increases.
Both A and R are true and R is correct explanation of A
In the circuit of figure the function of resistor R and diode D are
to limit the current and protect LED against reverse breakdown voltage.
;
Resistance limits current and diode is reverse connected and therefore protects LED against reverse breakdown.
At very high temperatures the extrinsic semi conductors become intrinsic because
band to band transition dominants over impurity ionization;
Covalent bonds are broken.
When a voltage is applied to a semiconductor crystal then the free electrons will flow.
towards positive terminal;
Since electrons are negatively charged they will flow towards positive terminal.
Ferrite have
low resistivity;
Ferrite is a low density material of composition with Fe2O3 x O, where x is a bivalent metal, such as Cobart, Ni, Mn. These magnetic materials having very low loss of current and used in high frequency circuit.
In a p type material the Fermi level is 0.3 eV above valence band. The concentration of acceptor atoms is increased. The new position of Fermi level is likely to be
0.28 eV above valence band;
Addition of acceptor atom brings Fermi level closer to valence band.
In an n-p-n transistor, the majority of carriers in the base are
electrons;
Emitter is n type and emits electrons which diffuse through the base.
An LED has a rating of 2 V and 10 mA. It is used along with 6V battery. The range of series resistance is
400 Ω and above;
R= V/I
R must be at least 400Ω so that current in LED does not exceed 10 mA.
The number of doped regions in PIN diode is
2;
A PIN diode has p and n doped regions separated by intrinsic layer.
A transistor has two p-n junctions. The batteries should be connected such that
one junction is forward biased and the other is reverse biased;
Emitter-base junction is forward biased and base collector junction is reverse biased.
A silicon (PN) junction at a temperature of 20°C has a reverse saturation current of 10 pico Ampere. The reverse saturation current at 40°C for the same bias is approximately.
40 pA;
By increasing of temperature by 10°C, Io become double so by increasing temperature 20°C, Io become 4 time than initial value… and it is 40 PA.
In a bipolar transistor the barrier potential
0.7 V across each depletion layer;
Since there are two p-n junctions, there are two depletion layers and 0.7 V across each layer.
Recombination produces new electron-hole pairs
False;
Due to recombination the number of electron-hole pairs is reduced.
As compared to an ordinary semiconductor diode, a Schottky diode
has lower cut in voltage;
Cut in voltage in Schottky diode is about 0.3 V as compared to 0.7 V in ordinary semiconductor diode.
Assertion (A): When a high reverse voltage is applied to a p-n junction the diode breaks down.
Reason (R): High reverse voltage causes Avalanche effect.
Both A and R are true and R is correct explanation of A;
Avalanche breakdown occurs at high reverse voltage.
As compared to an ordinary semiconductor diode, a Schottky diode
has higher reverse saturation current and lower cut in voltage;
This is due to high electron concentration in metals.
Crossover distortion behavior is characteristic of
class B O/P stage;
It is a characteristics of class B output stage as the amplifier is biased in cut-off region.
In class B amplifier, two transistor are operated in such a way that one is amplify the half cycle and second is amplify -ve half cycle.
If aac for transistor is 0.98 then βac is equal to
49
Assertion (A): The conductivity of p type semiconductor is higher than that of intrinsic semiconductor.
Reason (R): The addition of donor impurity creates additional energy levels below conduction band.
Both A and R are true but R is not a correct explanation of A;
A refers to type semiconductor while R refers to n type semiconductor. Both A and R are correct but independent.
In an n-p-n transistor biased for operation in forward active region
base is positive with respect to emitter and collector is positive with respect to base;
In forward active mode emitter base junction is forward biased and base collector junction is reverse biased.
An increase in temperature increases the width of depletion layer.
With increase in temperature width of depletion layer decreases.
A zener diode is used in
voltage regulator circuit;
Zener diode is used only in voltage regulator circuits.
A zener diode is used in
voltage regulator circuit;
Zener diode is used only in voltage regulator circuits.
A particular green LED emits light of wavelength 5490, Å, the energy bandgap of the semiconductor material used there is .. h = 6.6 x 10-34 J sec.
2.26 eV;
Plank equation/ 1.6 x 10-19.
In a zener diode
sharp breakdown occurs at a certain reverse voltage;
When reverse voltage equals breakdown value it starts conducting and voltage does not increase further.
In a bipolar transistor which current is the largest
emitter current;
Emitter current is larger,
Collector current is slightly less than emitter current
Base current is very small.
The v-i characteristics of a FET is shown in figure. In which region is the device biased for small signal amplification
BC;
Small signal amplifier operation is in constant current region of characteristics.
Secondary emission is always decremental.
FALSE;
Sometimes it can be useful also.
In a degenerate n type semiconductor material, the Fermi level,
is in conduction band;
This is due to high level of doping.
The types of carriers in a semiconductor are
2;
Holes and electrons.
A potential of 7 V is applied to a silicon diode. A resistance of 1 K ohm is also in series with the diode. The current is
6.3 mA;
Silicon = 0.7V Germanium = 0.3V I= V/A I= 7-0.7/11k
Assertion (A): The reverse saturation current in a semiconductor diode is 4nA at 20°C and 32 nA at 50°C.
Reason (R): The reverse saturation current in a semiconductor diode doubles for every 10°C rise in temperature.
Both A and R are true and R is correct explanation of A;
At 20°C, 4 nA, at 30°C, 8 nA, at 40°C, 16 nA, at 50°C, 32 nA.
Calculate the stability factor and change in IC from 25°C to 100°C for, β = 50, RB/ RE = 250, ΔIC0 = 19.9 nA for emitter bias configuration.
42.53, 0.85 μA;
stability factor and change in IC
A periodic voltage has following value for equal time intervals changing suddenly from one value to next… 0, 5, 10, 20, 50, 60, 50, 20, 10, 5, 0, -5, -10 etc. Then rms value of the waveform is
31 V
Work function of oxide coated cathode is much lower than that of tungsten cathode.
TRUE;
Therefore emission current from oxide coated cathode is more.
The word enhancement mode is associated with
MOSFET;
MOSFET may be depletion mode or enhancement mode.
In which region of a CE bipolar transistor is collector current almost constant?
Active region;
It is used as an amplifier when it operates in this region.
A p-n junction diode has
low forward and high reverse resistance
a non-linear v-i characteristics
zero forward current till the forward voltage reaches cut in value
Which of the following is true as regards photo emission?
Maximum velocity of electron increases with decreasing wave length;
As wavelength decreases, frequency increases and maximum velocity of electron increases.
The power dissipation in a transistor is the product of
collector current and collector to emitter voltage;
Maximum power dissipation occurs at collector junction.
The power dissipation in a transistor is the product of
collector current and collector to emitter voltage;
Maximum power dissipation occurs at collector junction.
The minority carrier life time and diffusion constant in a semiconductor material are respectively 100 microsecond and 100 cm2/sec. The diffusion length is
0.1 cm;
Diffusion length
An incremental model of a solid state device is one which represents the
ac property of the device at desired operating point
What is the correct sequence of the following step in the fabrication of a monolithic, Bipolar junction transistor?
- Emitter diffusion
- Base diffusion
- Buried layer formation
- E pi-layer formation
4, 3, 2, 1
For an n-channel enhancement type MOSFET, if the source is connected at a higher potential than that of the bulk (VSB > 0), the threshold voltage VT of the MOSFET will
remain unchanged;
VT depends upon MOSFET construction, hence it will Independent from MOSFET parameters.
Which of the following is used for generating time varying wave forms?
UJT;
Its output is used to trigger SCR.
Calculate the resistivity of n-type semiconductor from the following data, Density of holes = 5 x 1012 cm-3. Density of electrons = 8 x 1013 cm-3, mobility of conduction electron = 2.3 x 104 cm2/ V-sec and mobility of holes = 100 cm2/V-sec.
0.34 Ω-m;
Resistivity(r) = 1/ σ
σ = e(neue + nnun)
Conductivity= 1.6x10^-19 * (5x10^12 * 100 + 8x10^13 * 2.3x10^4) = 0.29488.
Resistivity = 1/conductivity,
Since this is in cm multiply it by 100 to get value in m which is 29.488,
Resistivity = 0.034 ohm-meter.
n-type semiconductors
are produced when phosphorus is added as impurity to silicon;
n type semiconductor is produced when pentavalent impurity is added.
In all metals
conductivity decreases with increase in temperature;
In all metals conductivity decreases (and resistance increases) with increase in temperature.
The voltage across a zener diode
Zener diode is always reverse biased.
Assertion (A): Two transistors one n-p-n and the other p-n-p are identical in all respects (doping, construction, shape, size). The n-p-n transistor will have better frequency response.
Reason (R): The electron mobility is higher than hole mobility.
Both A and R are true and R is correct explanation of A;
Therefore mostly npn transistors are used.
The threshold voltage of an n-channel enhancement mode MOSFET is 0.5 when the device is biased at a gate voltage of 3V. Pinch off would occur at a drain voltage of
2.5 V;
It is used with reverse bias.
Vds = Vgs-Vt.
=>Vds = 3-0.5 = 2.5(B).
For n-channel mosfet.
Vgs< Vt - cutoff region.
Vgs> Vt & Vds>Vgs - linear or triode or ohmic region.
Vgs> Vt- saturation region – constant current region.
Which of these has degenerate p and n materials?
Tunnel diode;
Tunnel diode has heavily doped p and n layers called degenerate p and n materials.
A Schottky diode clamp is used along with switching BJT for
reducing the switching time;
Schottky diode has very low switching time.
In a piezoelectric crystal, applications of a mechanical stress would produce
electrical polarization in the crystal;
In piezoelectric materials mechanical stress produces electric polarization.
In which of the following is the width of junction barrier very small?
Schottky diode;
Schottky diode has very small depletion layer.
If the reverse voltage across a p-n junction is increased three times, the junction capacitance
will decrease by an approximate factor of about 2;
Increase of reverse voltage widens the depletion layer and junction capacitance decreases. However the decrease in capacitance is not proportional to increase in voltage.
Which of these has highly doped p and n region?
Tunnel diode;
Tunnel diode has heavily doped p and n regions leading to very thin depletion layer.
Measurement of Hall coefficient enables the determination of
type of conductivity and concentration of charge carriers;
If a potential difference is developed across a current carrying metal strip when the strip is placed in transverse magnetic field.
Hall effect is very weak in metals, but it is large semiconductors.
The units for transconductance are
siemens;
same as the units of conductance
The amount of photoelectric emission current depends on the frequency of incident light.
False;
It depends on intensity of incident light.
When a p-n junction is forward biased
the width of depletion layer decreases;
Forward voltage decreases the width of depletion layer leading to low resistance.
The carriers of n channel JFET are
free electrons;
In n type semiconductors carriers are electrons.
The depletion layer around p-n junction in JFET consists of
immobile charges;
Depletion layer always has immobile charges
Junction temperature is always the same as room temperature.
FALSE;
When the device is being used, junction temperature is higher than room temperature.
When a p-n-p transistor is properly biased to operate in active region the holes from emitter.
diffuse through base into collector region;
The reason that collector current is nearly equal to emitter current.
Assertion (A): Silicon is preferred over germanium in manufacture of semiconductor devices.
Reason (R): Forbidden gap in silicon is more than that in germanium.
Both A and R are true and R is correct explanation of A;
Wider forbidden gap in silicon makes it less sensitive to temperature than germanium.
Assertion (A): A decrease in temperature increases the reverse saturation current in a p-n diode.
Reason (R): When a diode is reverse biased surface leakage current flows.
A is false but R is true;
A is wrong because decrease in junction temperature decreases reverse saturation current.
At room temperature a semiconductor material is
slightly conducting;
At 0 K a semiconductor is perfect insulator. At room temperature it is slightly conducting.
In an n channel JFET
ID, IS and IG are considered positive when flowing into the transistor;
All currents are assumed positive when flowing into JFET.
The concentration of minority carriers in a semiconductor depends mainly on
temperature;
Minority carriers are generated due to thermal excitation.
Which of the following has highest conductivity?
Silver;
Silver has highest conductivity (and lowest resistivity) in all metals.
In a bipolar junction transistor the base region is made very thin so that
recombination in base region is minimum;
Since recombination in base region is minimum, I - IE.
Compared to bipolar junction transistor, a JFET has
high input impedance and low voltage gain;
JFET is voltage controlled device. Therefore its input impedance is high. But voltage gain is lower than in BJT.
The drain characteristics of JFET in operating region, are
almost flat;
The drain current is almost constant. Therefore, characteristics is flat.
As temperature increases
the forbidden energy gap in silicon and germanium decrease;
Therefore, conductivity increases.
When a reverse bias is applied to a p-n junction, the width of depletion layer.
increases;
Therefore, the resistance is very high.
Which of the following devices has a silicon dioxide layer?
MOSFET;
The SiO2 layer provides very high input impedance.
Which statement is false as regards holes
Holes exist in conductors as well as semiconductors;
Holes do not exist in conductors.
Photo electric emission can occur only if
wave length of incident radiation is less than threshold value;
The frequency of incident radiation has to be more than threshold value. Wavelength is inversely proportional to frequency.
Therefore wavelength of incident radiation must be less than threshold value.
The reverse saturation current of a diode does not depend on temperature.
FALSE;
It increases with increase of temperature.
In a piezoelectric crystal, application of a mechanical stress would produce
electric polarization in the crystal;
Commonly material used as piezoelectric crystal is Barium Nitrate. Generally ferroelectric crystals and piezoelectric
The value of a in a transistor
is less than 1 but more than 0.9;
a is about 0.98.
