Semiconductor Electronics Materials Devices And Simple Circuits

In a p-n junction diode, the behavior between the p and n ends can be understood by analyzing its resistance characteristics. When the resistance is high, the diode is in its "off" state, functioning as an open circuit or a switch that is not conducting.
In the context of this question, the resistance measurement indicates whether or not current can flow through the diode. A high resistance implies that the diode blocks current flow, akin to an open switch, while low resistance means it allows current, acting like a closed switch.

Thus, when the resistance is high, the p-n junction diode effectively acts as a switch in its "off" position, preventing current from passing through.
Therefore, the correct answer to the question is that the diode acts as a switch when the resistance between the p and n ends is high.

The dynamic resistance of a diode is defined as the rate of change of voltage with respect to the current. It is given by: From the graph, at V, we can estimate the current and the change in voltage and current near this point. For instance, if the current is approximately 20 mA at V and the slope of the curve near this voltage is estimated, we can calculate . For example, if the current changes by 10 mA for a voltage change of 0.2 V, the dynamic resistance is: Thus, the dynamic resistance at V is approximately 20 .

- Circuit Arrangement for Studying V-I Characteristics of a p-n Junction Diode: The circuit consists of a power supply connected in series with the p-n junction diode and a variable resistor. A voltmeter is connected across the diode to measure the voltage, and an ammeter is connected in series to measure the current.

- Shape of the Characteristics of a Diode: The V-I characteristics of a diode show a sharp increase in current once the forward voltage exceeds a certain threshold (approximately 0.7V for silicon). In reverse bias, the current remains very small until breakdown occurs at a high reverse voltage.

- Two Information from the Characteristics: 1. The threshold voltage (or cut-off voltage) at which the diode starts to conduct in the forward direction. 2. The reverse saturation current, which is the small current that flows when the diode is reverse biased and no significant conduction occurs. Thus, the V-I characteristics provide information about the diode's forward threshold and its behavior under reverse bias.

In a p-n junction diode that is forward biased, the direction of the applied voltage allows current to flow through the diode. Let's discuss the movement of the charge carriers (electrons and holes) that contribute to the current when the p-n junction is forward biased.

1. Forward Biasing of the P-N Junction:

When the p-n junction is forward biased, the p-type semiconductor is connected to the positive terminal of the battery, and the n-type semiconductor is connected to the negative terminal of the battery. This results in the following effects:

• The applied voltage reduces the potential barrier at the junction, making it easier for charge carriers to cross the junction.
• The electrons in the n-type region (which are the majority carriers in the n-type semiconductor) are repelled by the negative terminal of the battery and are pushed toward the p-n junction.
• The holes in the p-type region (which are the majority carriers in the p-type semiconductor) are repelled by the positive terminal of the battery and are pushed toward the p-n junction.

2. Movement of Charge Carriers:

The forward bias causes the following movements of charge carriers:

• Electrons (Minority carriers in the p-type region): The electrons in the n-type region, which are majority carriers, move towards the p-type region due to the reduced potential barrier. These electrons cross the junction and move into the p-type material, where they recombine with holes. This movement of electrons from the n-type side to the p-type side is what contributes to the current flow in the circuit.
• Holes (Minority carriers in the n-type region): The holes in the p-type region, which are majority carriers, move towards the n-type region. These holes cross the junction and move into the n-type material, where they recombine with electrons. The movement of holes from the p-type side to the n-type side also contributes to the current flow.

3. Current Flow in the Circuit:

As electrons flow from the n-type material into the p-type material, they move through the external circuit towards the positive terminal of the battery, creating a current in the external circuit. Similarly, holes move from the p-type material to the n-type material and also contribute to the current. Thus, the movement of both electrons and holes results in a continuous current flow in the circuit.

4. Conclusion:

In a forward-biased p-n junction, the current is produced by the movement of electrons from the n-type material to the p-type material, and the movement of holes from the p-type material to the n-type material. The flow of these charge carriers (electrons and holes) across the junction leads to a continuous current in the external circuit.

Step 1: The circuit. The total voltage across the series combination of the diode and the resistor is equal to the supply voltage . The current through the diode is 15 mA.

Step 2: Applying Kirchhoff's Voltage Law. The voltage across the resistor is given by Ohm's Law: The voltage across the diode is the threshold voltage, .

Step 3: Total supply voltage. The total voltage is the sum of the voltage across the resistor and the diode: Thus, the value of is 15.7 V.

To determine the truth of the assertion and the reason provided, we need to evaluate them based on semiconductor physics.

Assertion (A): We cannot form a p-n junction diode by taking a slab of a p-type semiconductor and physically joining it to another slab of an n-type semiconductor.

Explanation: A p-n junction diode is formed by doping a single crystal of semiconductor material such that one side becomes p-type and the other becomes n-type. This ensures a continuous lattice structure and a seamless junction. Simply joining two slabs physically would introduce interface defects and create a lack of a continuous crystal lattice, leading to high recombination of charge carriers at the interface and ineffective diode operation. Thus, the assertion is true under practical conditions required for effective diode function, but a simple physical joining does not form a functional diode due to discontinuities.

Reason (R): In a p-type semiconductor, while in an n-type semiconductor .

Explanation: This reason is technically incorrect. In a p-type semiconductor, the majority carriers are holes ( ), and in an n-type semiconductor, the majority carriers are electrons ( ). Therefore, is true for p-type, and is true for n-type, which is the opposite of the given reason.

The correct answer: Assertion (A) is false and Reason (R) is also false.