SSC JE Basic Electronics Questions with Detailed Solutions

Concept: The phenomenon called "Early effect" describes how the effective width of a BJT's base changes when the reverse bias voltage across the collector-base junction changes.
Impact of voltage change: When the base-collector voltage increases, the depletion region at the collector-base junction expands, effectively narrowing the base region. This change in base width alters the transistor's current gain. 

In p-n junctions, breakdowns can occur through two main mechanisms:
1. Avalanche Breakdown: This occurs due to the collision of charge carriers gaining enough energy to cause additional carriers to be released in an avalanche-like process.
2. Tunnelling Breakdown: This occurs when charge carriers quantum tunnel through the depletion region barrier.
So, the correct option is (c) Avalanche and tunnelling breakdowns.

- In a bipolar junction transistor (BJT), the emitter is heavily doped to increase emitter current.
- The base is lightly doped to allow for high base resistance.
- The collector is moderately doped to withstand high reverse bias voltage.
- This doping profile helps achieve the desired transistor characteristics.

- When p-type and n-type semiconductor materials are joined to form a diode junction, diffusion of majority charge carriers (holes in the p-type and electrons in the n-type) occurs.
- This diffusion leads to the depletion of majority charge carriers in the region near the junction, creating a depletion region.
- The depletion region has a lack of free charge carriers, resulting in the formation of a potential barrier or built-in electric field at the junction.

Common base current gain, α=0.98

The relationship between common base current gain (α) and common collector current gain (γ) is:

$$\gamma = \frac{1}{1 - \alpha} \\[6pt]\gamma = \frac{1}{1 - 0.98} = \frac{1}{0.02} = 50 \\[10pt]\textbf{Additional Information:} \\[6pt]\text{Common emitter current gain } (\beta): \quad \beta = \frac{I_C}{I_B} \\[6pt]\text{Common base current gain } (\alpha): \quad \alpha = \frac{I_C}{I_E} \\[6pt]\text{Common collector current gain } (\gamma): \quad \gamma = \frac{I_E}{I_B} \\[10pt]\textbf{Where,} \\[4pt]I_C = \text{Collector current} \\[4pt]I_B = \text{Base current} \\[4pt]I_E = \text{Emitter current}$$​​

In a common collector configuration, the output voltage is taken from the collector, and the input is applied to the emitter. The voltage gain in a common collector configuration is slightly less than one. While the voltage gain is modest, this configuration is known for other favorable characteristics like high input impedance and low output impedance.

• The ripple factor (γ) for a full-wave rectifier using a capacitor filter is given by:

​$$\gamma = \frac{1}{4\sqrt{3}R_LCf}$$​​

Where:

• C = capacitance
• f = input frequency
• $$R_L$$​​ = load resistance

To reduce the AC ripples, the capacitance should be increased and the input frequency should also be increased.

This is because the ripple factor (γ) is inversely proportional to both the capacitance and the frequency.

As the capacitance C increases and the frequency f increases, the ripple factor decreases, thus reducing the AC ripples.

When a diode is forward biased, increasing the voltage reduces the resistance of the depletion region, allowing more current to flow. However, there's a limit to how much current a diode can handle.
 If the forward voltage exceeds the diode's rated maximum forward voltage, excessive current can flow through the diode, causing it to overheat and potentially damage the junction, leading to a "burn out" condition.

 The operating point is also known as the quiescent point in transistor operation, typically defined as the point where the transistor is biased in the absence of any input signal.
 The correct term for the operating point of a transistor is the quiescent point, also referred to as the Q-point. It represents the DC operating point where the transistor is in its steady-state condition before any AC signal is applied.

Voltage shunt: In this configuration, a portion of the output voltage (collector voltage) is fed back to the input (base) through a resistor, creating a feedback loop. 
Negative feedback: This feedback is negative because it opposes the input signal, stabilizing the circuit and reducing distortion. 
Shunt: The feedback is considered "shunt" because the feedback voltage is applied in parallel with the base resistor. 

From the energy band diagram and the expressions derived, the built-in potential can be calculated as:

​$$V_{bi} = V_T \ln \left( \frac{N_A N_D}{n_i^2} \right)$$​​

The built-in potential is affected by both temperature and doping concentration. As temperature increases, the intrinsic carrier concentration $$n_i$$​ increases, which leads to a change in the built-in potential. Similarly, changes in doping concentration $$N_A \text{ or } N_D$$​ also influence the potential.

where:

​$$V_T = \frac{kT}{q}$$​​

(Thermal voltage)

• $$N_A$$​  and $$N_D$$​​ are the doping concentrations for the p and n regions, respectively.
• $$n_i$$​ is the intrinsic carrier concentration.

​

AC forward resistance: This refers to the resistance a diode offers to a small change in current when it is forward biased. This is the ratio of the change in forward voltage to the corresponding change in current. 
DC forward resistance: This refers to the ratio of the DC voltage applied to a diode to the resulting DC current. While related, it is not the same as AC forward resistance. 
AC reverse resistance: When a diode is reverse biased, its resistance is very high. This is because the diode is essentially blocked from conducting current in the reverse direction. Therefore, there is no meaningful "AC reverse resistance" in the same way as with forward bias. 
DC reverse resistance: Similar to AC reverse resistance, this is not a commonly used term as the diode's high resistance under reverse bias makes it essentially infinite for practical purposes. 

$$\textbf{Current Gain Relations in Transistors:} \\[8pt]1. \; \text{Common emitter current gain } (\beta): \\\beta = \dfrac{I_C}{I_B} \\[12pt]2. \; \text{Common base current gain } (\alpha): \\\alpha = \dfrac{I_C}{I_E} \\[12pt]3. \; \text{Common collector current gain } (\gamma): \\\gamma = \dfrac{I_E}{I_B} \\[14pt]\text{Where:} \\\bullet \; I_E = \text{Emitter current} \\\bullet \; I_C = \text{Collector current} \\\bullet \; I_B = \text{Base current} \\[12pt]\textbf{Calculation:} \\[6pt]\text{Given:} \\\bullet \; I_E = 4 \, A \\\bullet \; \alpha = 0.75 \\[10pt]I_C = \alpha \times I_E = 0.75 \times 4 = 3 \, A \\[12pt]\text{Now, using the relation:} \\I_E = I_C + I_B \;\;\Rightarrow\;\; 4 = 3 + I_B \;\;\Rightarrow\;\; I_B = 1 \, A$$​​

In a metal oxide semiconductor field-effect transistor (MOSFET), the metal oxide layer, typically made of silicon dioxide (SiO₂), acts as a dielectric material. It separates the gate from the semiconductor and helps in controlling the flow of current through the channel when a voltage is applied to the gate. The dielectric layer prevents direct current flow between the gate and the channel while allowing the electric field to influence the channel.

Explanation: When a p-n junction diode is heated, the temperature increase causes several changes in its behavior, impacting both the forward and reverse bias characteristics. 
Increased carrier concentration: Higher temperature leads to more free charge carriers (electrons and holes) in the semiconductor material, which increases the conductivity. 
Reduced barrier potential: The barrier potential at the p-n junction decreases with rising temperature, allowing more current to flow across the junction. 
Forward bias current increase: Due to the higher carrier concentration and reduced barrier potential, the forward current in the diode increases significantly with temperature. 
Reverse leakage current increase: Although the reverse current is normally small, it also increases with temperature.