Semiconductors

LEDs (Light Emitting Diodes) emit light when electrons recombine with holes, releasing energy equal to the band-gap. The visible spectrum starts from around 1.8 eV (for red light). - Below 1.8 eV: Not enough to emit visible light.
- Above 3 eV: Emission shifts to UV region.
So, 1.8 eV is the minimum band-gap needed to emit visible light.

To find the reverse resistance of the diode, we can use Ohm's law, which is given by: where: - is the change in voltage, - is the change in current.
The reverse potential changes from 10 V to 15 V, so: The reverse current changes from 20 to 40 , so: Now substitute the values into the formula for resistance: Thus, the reverse resistance of the junction diode is .

In this circuit, we have two diodes and , with forward resistance of 250 and infinite backward resistance.
The total resistance in the circuit can be calculated by first considering the diodes' behavior. Since the diodes have infinite backward resistance, they essentially act like open circuits in the reverse direction.
Now, considering the forward resistances of the diodes and the resistors: - The total resistance in the circuit will be the sum of the resistances of the 50 resistor, the 55 diode resistance, and the 175 resistor in series.
Thus, the total resistance is: Now, we can calculate the total current using Ohm's law: where: - is the voltage of the source, - is the total resistance.
Substituting the values: However, this is the total current. The current passing through the 175 resistor can be found using the voltage divider rule. The current passing through the 175 resistor will be the fraction of the total current corresponding to the voltage across the 175 resistor.
The voltage across the 175 resistor is: Finally, the current passing through the 175 resistor is: Thus, the current passing through the 175 resistor is .

We are given the following information: - The ratio of the number density of electrons to holes is , i.e., . - The current due to holes is th of the total current, and the remaining current is due to electrons. The total current can be expressed as: Where: is the current due to electrons, and is the current due to holes. Now, using the relationship for current , where is the number density, is the charge, is the cross-sectional area, and is the drift velocity, we have: Since the ratio of the current is given as , we can write: Substituting , we get: Thus, the ratio of the drift velocity of holes to the drift velocity of electrons is .