Test: Basic Electronics- 2 - Electrical Engineering (EE) MCQ

At lower temperatures, the electrons in valence band do not have sufficient energy to jump into conduction band. Thus carrier concentration decreases at lower temperatures and semiconductors behaves as insulators

In an intrinsic semiconductor (un doped silicon) the number of electrons and the number of holes are equal. Hence recombination rate depends on both the number of electrons & number of holes.

In case of doped semiconductors, the recombination rate of the majority carriers are dependent on minority carrier density.

The fermi level (E_F) in an intrinsic semiconductor is given by

Where E_c = Energy of conduction band minima

E_v = Energy of valence band maxima

K = Boltzman constant

T = Temperature

N_c = density of states in conduction band

N_v = density of states in valence band

The built in potential V₀ of a junction is given by

So built-in potential depends on both doping densities & temperature

The reverse bias characteristics of a p – n junction diode is shown in the figure

From the figure it is clear that the reverse current is independent of voltage.

The holes from p-region cross the p-n Junction and recombine with electrons on the n-region. Similarly the electrons form n-region cross the junction & recombine with holes in p-region. Leaving immobile ions behind.

Thus the region near the junction is devoid of any mobile charge carriers, due to their recombination. Hence Depletion region of p-n Junction consists of immobile charges. 

Since the output of full wave rectifier is an even function. Hence from Fourier series Analysis we know that even function have even harmonics thus output of FWR contains only even harmonics.

When the photodiode is forward biased it acts as a normal diode and the incident light intensity does not affect the diode current

Tunnel diode characteristics

In Region between Peak Point & valley Point the current in Tunnel diode decrease with increase in forward Bias voltage. Thus Tunnel diode shows -ve resistance between peak point & valley point

Thus if the ripple factor is less, the power supply has less AC components and power supply output is more pure (i.e more DC without much fluctuations)

Thus ripple factor is indication of purity of output of power supply

The I - V characteristic of Zener diode is shown

When Zener diode is reversed Biased the voltage across it is constant (V₂).

Hence Zener Diode on reverse Biasing acts as voltage Stabilizer

Av = gain of Amplifier

Miller Capacitance

C_M = C_C (1 + A_V)

The increase in input capacitance decreases the upper cut off frequency

In case of CB configuration there is no miller capacitance between input and output & hence upper cut off frequency (f_H) is high & higher Bandwidth

Input Terminal Emitter – Base (E_B)

Output Terminal Collector – Base (C_B)

Since for Amplification Application in BJT

E_B Junction → Forward Biased (Low Impedance)

C_B Junction → Reversed Biased (High Impedance)

Input Impedance is low

Output Impedance is high

The 3 configurations in BJT are

The term “Common” refers to the Common terminal through which input is given and from which output is withdrawn i.e. common terminal between input and output.

The term common current is logically incorrect and is not a BJT Configuration.

Different modes of BJT operations

For switching Application BJT is operated in cut off (off switch) and saturation (on switch)

For any amplification, (voltage or current), the transistor should be operated in active region ie. emitter base junction is forward biased and collector base junction is reverse biased

The Upper cut off frequency of R-C coupled multistage Amplifier is

Thus upper – cut off frequency depends on coupling capacitors

Hence cascade is CE – CB Configuration

It is a majority carrier device. It is a semiconductor diode with a very fast switching action, but a low forward voltage drop. When a current flows through the diode there is a small voltage drop across the diode terminals. In a normal diode, the voltage drop is between 0.6 to 1.7 volts, while in a Schottky diode the voltage drop normally ranges between 0.15 and 0.45volts. This lower voltage drop provides higher switching speed and better system efficiency.

In Schottky diode, a semiconductor-metal junction is formed between a semiconductor and a metal, thus creating a Schottky barrier. The N-type semiconductor acts as a cathode and the metal side acts as the anode of the diode. The Schottky barrier diode is a unidirectional device conducting current flows only in one direction.

Carrier concentrations depends on the energy gap in the intrinsic semiconductor. In the case of Ge and Si the intrinsic carrier concentration of Ge is higher than that of Si because the energy gap in Ge is smaller than that of Si.