Semiconductors and Electronic Devices Test - 1

CONCEPT:

Light Emitting Diode:

• It is a heavily doped p-n junction that under forward bias emits spontaneous radiation. The diode is encapsulated with a transparent cover so that emitted light can come out.
• When the diode is forward biased, electrons are sent from n to p (where they are minority carriers) and holes are sent from p to n (where they are minority carriers).
• At the junction boundary, the concentration of minority carriers increases compared to the equilibrium concentration (i.e., when there is no bias).
• Thus at the junction boundary on either side of the junction, excess minority carriers are there which recombine with majority carriers near the junction.
• On recombination, the energy is released in the form of photons. Photons with energy equal to or slightly less than the bandgap are emitted.
• When the forward current of the diode is small, the intensity of light emitted is small.
• As the forward current increases, the intensity of light increases and reaches a maximum. Further, an increase in the forward current results in a decrease in light intensity.
• LEDs are biased such that the light-emitting efficiency is maximum.
• The V-I characteristics of a LED are similar to that of a Si junction diode.
• But the threshold voltages are much higher and slightly different for each colour.
• The reverse breakdown voltages of LEDs are very low, typically around 5V.
• So care should be taken that high reverse voltages do not appear across them.
• LEDs can emit red, yellow, orange, green, and blue light that are commercially available.
• The semiconductor used for fabrication of visible LEDs must at least have a bandgap of 1.8 eV (spectral range of visible light is from about 0.4 μm to 0.7 μm, i.e., from about 3 eV to 1.8 eV).
• The compound semiconductor Gallium Arsenide – Phosphide is used for making LEDs of different colours.
• Gallium Arsenide is used for making infrared LED.

EXPLANATION:

• We know that LED's can emit red, yellow, orange, green, and blue light are commercially available. Hence, option 4 is correct.

CONCEPT:

• Conductor: The material that allows the electric current to flow through them easily is called a conductor.
  • Example: Metals like -iron, silver, copper, etc.
• Insulator: The material that doesn't allow the electric current to flow through them is called an insulator.
  • Example: Rubber, wood, etc.
• Semiconductor: The material having electric current conducting tendency between conductor and insulator is called a semiconductor.
  • Example: Silicon, germanium, etc.
• Valence band: The region in which the valence electrons of the atom move is called the valence band.
• Conduction band: The region in which free electrons remain is called the conduction band.
  • In the case of conductors like metals, the valence band and conduction band overlap each other and almost all the charge carriers are found in the conduction band; This is the reason they are good conductors of electricity.

EXPLANATION:

• In metallic conductors, the conduction band overlaps on the valence band. So option 3 is correct.

CONCEPT:

Photodiode: A photodiode is a semiconductor device with a P-N junction that converts photons (or light) into electrical current. The P-layer has an abundance of holes (positive), and the N-layer has an abundance of electrons (negative ).

​The energy (E) of the radiation is given by:

E = h ν 

Where h is Planck constant and ν is frequency.

CALCULATION:

p - n photodiode is a semiconductor diode that produces a significant current when illuminated. It is reversed biased but is operated below the breakdown voltage.

Energy of radiation = Band gap Energy

i.e. E = hν = 2.0 eV

 or, ν = ( 2.0 × 1.6 × 10^-19) / 6.6 × 10^-34.

Frequency (ν) = 5 × 10¹⁴  Hz. 

option 4 is the answer.

Additional Information

• A Photon can strike an atom within the device and release an electron if the photon has enough energy. This creates an electron-hole pair (e- and h+) where a hole is simply an "empty space" for an electron.
• If photons are absorbed in either the P or N layers, the electron-hole pairs will be recombined in the materials as heat if they are far enough away (at least one diffusion length) from the depletion region.
• Photons absorbed in the depletion region (or close to it) will create electron-hole pairs that will move toward the positive potential on the Cathode, and the holes will move toward the positive potential on the Cathode, and the holes will move toward the negative potential on Anode.
• These moving charge carriers form the current (photocurrent)  in the photodiode.

CONCEPT:

• A rectifier is a device that converts an alternating current into a direct current.
• A p-n junction can be used as a rectifier because it permits current in one direction only.
• In half-wave rectifier, there is only one diode, so during the positive half cycle diode conducts and gives the output similarly in the negative half cycle diode don’t conduct and gives no output.
• The output frequency (ω) for the half-wave rectifier is the same as that of ac.

EXPLANATION:

Given – Input frequency = 75 Hz

• As the output frequency (ω) for the half-wave rectifier is the same as that of ac. Therefore output frequency will be 75 Hz.

• The output frequency of full-wave rectifier = 2 × Frequency of input of ac.

Concept-

• The material having conductivity less than that of conductor and greater than insulator is called as semiconductor.
• The two terminal electronic device which allows the electric current to flow in only one direction is called as diode.
• When current flow easily in a diode then it is called forward biasing.
• When no current flow in the circuit due to diode then it is called reversed biasing.
• According to Ohm’s law: the potential difference across a resistor is directly proportional to the current flowing in it.

V = R I

Where V is potential difference, R is resistance and I is current

Explanation-

• In the given circuit, the diode is a forward biased. So current will flow easily.

Given that:

Resistance (R) = 1 KΩ = 1000 Ω

Higher potential = 4 V

Lower potential = - 6 V

Potential difference (V) = higher potential - lower potential = 4 - (-6) = 10 V

We will use Ohm’s law:

V = R I

CONCEPT:

LC Oscillations:

• We know that a capacitor and an inductor can store electrical and magnetic energy, respectively.
• When a capacitor (initially charged) is connected to an inductor, the charge on the capacitor and the current in the circuit exhibit the phenomenon of electrical oscillations similar to oscillations in mechanical systems.
• Let a capacitor and an inductor are connected as shown in the figure.
• Let a capacitor be charged Q_o at t = 0 sec.
• The moment the circuit is completed, the charge on the capacitor starts decreasing, giving rise to a current in the circuit.
• The angular frequency of the oscillation is given as,

\(⇒ ω_o=\frac{1}{\sqrt{LC}}\)

Where L = self-inductance and C = capacitance

• The charge on the capacitor varies sinusoidally with time as,

⇒ Q = Q_ocos(ω_ot)

• The current in the circuit at any time t is given as,

⇒ I = Iosin(ωot)

Where I_o = maximum current in the circuit

• The relation between the maximum charge and the maximum current is given as,

⇒ I_o = ω_oQ_o

EXPLANATION:

• We know that the angular frequency of the LC oscillation is given as,

\(⇒ ω_o=\frac{1}{\sqrt{LC}}\)     -----(1)

Where L = self-inductance and C = capacitance

• Hence, option 4 is correct.

CONCEPT:

• Zener diode: It is a highly doped p - n junction which is not damaged by high reverse current.
• It can operate continuously, without being damaged in the region of reverse background voltage.
• It is represented by

EXPLANATION:

• When a Zener diode is operated in the reverse breakdown region, a voltage across it remains practically constant (equal to the break voltage V_Z)  for a large change in the reverse current.
• Thus based on this fact Zener diode is used as a voltage regulator.

Concept-

• The three basic single stage bipolar junction transistor which is used as a voltage amplifier is called CE amplifier.
• The input of CE amplifier is taken from the base terminal and the output is collected from the collector terminal.
• The emitter terminal is common for both the terminals in CE amplifier.

Explanation-

• In CE amplifier, the input is applied emitter-base junction with the forward biased.
• This amplifies the signal thus the input ac signal to be amplified is applied across forward biased emitter-base junction.
• So option 2 is correct.

CONCEPT:

   TRANSISTOR:

• An electronic device made using semiconductor material like Si
• Transistors are used to amplify the electronic signals 
• Transistor can be also operated as a switch 

   TRANSFER CHARACTERISTICS :

•  These characteristics show the behavior of an electronic device.
•  Transfercharctersitics can be drawn between input and output variables. 
•  Output Voltage transfer characteristic  is  plotted V₀ against  V_i
•  A transistor can be used as a switch in the cutoff region  

   EXPLANATION:

• When a transistor works as a switch it works in cut-off and saturation regions.
• In the cut-off state, both emitter-base junction and collector-base junctions are reverse biased.
• But in the saturation region, both junctions are forward biased.
• Thus, the region I & III  will be the two regions where we can operate a transistor as a switch. Therefore option 3 is correct.               

CONCEPT

Logic gates: 

• It is an electric circuit, which works on simple Boolean algebra to perform a logical operation for one or more binary inputs that produce a single binary output.

Types of Logic gates:

AND Gate: If both the inputs are high, it produces a high output.

• The Boolean algebra for AND gate is X = A. B

Explanation:

From the above explanation we can see that, for AND gate the output will be high (1)  only if both inputs are high.

i,e., the output is 1 when input is (1,1)