Physics M.C.Q (1 Marks)

Concept of holes in the semiconductor:
When an electron is removed from a covalent bond, it leaves a vacancy behind. An electron from a neighbouring atom can move into this vacancy, leaving the neighbour with a vacancy. In this way the vacancy formed is called a hole $($or cotter$)$, and can travel through the material and serve as an additional current carriers.
A hole is considered as a seat of positive charge, having magnitude of charge equal to that of an electron.
Holes acts as a virtual charge, although there is no physical charge on it.
Effective mass of hole is more than an electron.
Mobility of hole is less than an electron.

At least one input required to activate or $−$ gate

A $p-$type semiconductor is formed by doping a pure semiconductor with a $p-$type material. As impurity atoms take the position of the germanium atom in a germanium crystal, three electrons of a $p-$type material form covalent bonds by sharing electrons with three neighbouring germanium atoms. However, the fourth covalent bond is left incomplete, with a want of one electron. This creates a hole. As the atom as a whole is neutral, the $p-$type material is also neutral.

1. In reverse biasing width of depletion layer increases
2. In reverse biasing resistance offered $R_{\text {Reverse }}=10^5 \Omega$.
3. Reverse bias supports the potential barrier and no current flows across the junction due to the diffusion of the majority carriers.

4. Break down voltage: Reverse voltage at which break down of semiconductor occurs. For Ge it is $25V$ and for $Si$ it is $35V.$

A symbol of the diode is represented like this: In this problem first we have to check the polarity of the diodes. $-10$ is the lower voltage in the circuit. Now $p-$side of $p-n$ juction $D_1$ is connected to lower voltage and $n-$side of $D_1$ to higher voltage. Thus $D_1$ is reverse biased. Now, let us analyse $2^{\text {nd }}$ diode of the given circuit. The $p$-side of $p-n$ junction $D_1$ is at higher potential and $n$-side of $D_2$ is at lower potential. Therefore $D_2$ is forward biased.

Hence, current flows through the jucntion from $B$ to $A.$

Intrinsic semiconductors are pure ones. Hence there is no question of impurity. Valence band is the range of energy of electrons which are in the valence shell of the atoms of a substance. Conduction band is the range of energy of electrons which are free and available for conduction.
At room temperature, some electrons of intrinsic semiconductors get excited and reach the conduction band. During this process, the electrons leave empty spaces in the valence shell. These empty spaces are known as holes in a semiconductor. Number of free electrons $(n)$ is equal to the number of holes $(p).$

$a-$ represent $\text{OR}$ gate
$b-$ represent the $\text{AND}$ gate
$c-$ represent, the combination of $\text{OR}$ and $\text{NOT}$ gate, $\text{NOR}$ gate.
$d-$ represent, the combination of $\text{AND}$ and $\text{NOT}$ gate, $\text{NAND}$ gate.

In a $p‒n$ junction, current flows only if it is connected to the battery. If two ends of a $p‒n$ junction are joined by a wire, then there will be diffusion and drift currents in the circuit and they will cancel each other. Hence, no current will flow in the circuit.

$\text{NOT}$ gate is also called invertor circuit since $\text{NOT}$ gate inverts the input order. Hence, statement $-1$ is true statement$-2$ is true statement$-2$ is correct explanation of statement$-1.$

In good conductors, the valance band and the conduction band overlap each other resulting in a zero band gap energy. Whereas, bandgap in insulator is nearly $5\ eV$ and that in semiconductors is nearly $1.1\ eV.$
Hence the given material is a good conductor.

A p-type semiconductor is produced by doping a $14$ group element with a $13$ group element as $14$ group element has $4$ valence electron whereas that of group $13$ has $3$ valence electron. We know that Germanium is a $14$ group element and gallium is a $13$ group element, thus doping germanium with gallium forms a $p-$type semiconductor.

Semiconductors are made of Si, generally. Because $Si$ is the second most abundant element in earth's crust after oxygen and is less expensive than other semiconductors used in intrinsic semiconductors.

Atoms are neutral in the insulator, thus there are no valence electrons in the outer orbit. The electrons are tightly bound with the nucleus hence, at room temperature thermal energy is not enough to push the electrons into conduction band and hence, no electrons are available for conduction.
The energy required for electron to escape out from orbit and to over come the energy gap is thus of the order of $6\ eV.$

Forbidden band gap is the group of energy levels that cannot be occupied by the electrons, these energies lies in between conduction band and valence band. In pure conductors, electrons are loosely bound to the nucleus hence, can detach easily at room temperature also.
A large number of free electrons thus, available are conduction electrons. The energy level of these electrons corresponds to the conduction level, hence valence level and conduction level are overlapped.
Due to this, there is no forbidden band gap present in between conduction band and valence band hence, for metals the forbidden band gap is $0\ eV.$

1. During positive half cycle,

2. During negative half cycle,

3. Output voltage is obtained across the load resistance $RL$. It is not constant but pulsating $($mixture of $ac$ and $dc)$ in nature.
4. Average output in one cycle
5. $\text{r.m.s.}$ output: $\text{I}_\text{rms}=\frac{\text{I}_0}{2},\text{V}_\text{rms}=\frac{\text{V}_0}{2}$

Semi$-$conductors are made of $14^{th}$ Group elements which form covalent bonding.

To increase the amplitude of an $\text{AC}$ signal, transistors are used. $p-n$ junction diodes can be used as rectifiers and in photo$-$diode and $\text{LED.}$