Semiconductors – PN Junction Question and Answers

What is Fermi level?

The maximum energy that an electron in a metal has at the absolute zero temperature is called the Fermi level of energy.

What is the basis for classifying a material as a conductor, semiconductor, or a dielectric? Define what is the conductivity of perfect dielectric?

Conductors possess high conductivity whereas the characteristic property of insulating materials (or dielectrics) is poor conductivity. Semiconductors occupy an intermediate position between conductors and insulators. Though there is no rigid line separating the conductors from semiconductors and semiconductors from insulators, but still according to resistivity the materials of resistivity of the order from 10-8 to 10-3 , 10-13 to 106 and 106 to 1018 ohm-meters may be classified as conductors, semiconductors and dielectrics respectively.

Another classification is based on temperature coefficient of resistivity. Metals have positive temperature coefficient of resistivity. Semiconductors have small negative temperature coefficient of resistivity and insulators have large negative temperature coefficient of resistivity.

Differentiate semiconductors, conductors and insulators on the basis of band gap.

The distinction between conductors, insulators and semiconductors is largely concerned with the relative width of the forbidden energy gaps in their energy band structures. There is a wide forbidden gap (more than 5eV) for insulators, narrow forbidden gap (about 1eV) in case of semiconductors and no forbidden gap in case of conductors.

Define what is the importance of valence shell and valence electrons?

The outermost shell of an atom is called valence shell and the electrons in this shell are called valence electrons. Formation of energy bands occur owing to overlapping of energy levels of these valence electrons in valence shells. With the decrease in interatomic distance between the atoms in a crystal, the energy levels of electrons in outermost shells of atoms overlap to form energy bands.

Is a hole a fundamental particle in an atom?

Hole is not a fundamental particle in an atom. Holes may be thought of as positive particles, and as such they move through an electric field in a direction opposite to that of electrons.

Define a hole in a semiconductor.

When an energy is supplied to a semiconductor a valence electron is lifted to a higher energy level. The departing electron leaves a vacancy in the valence band. The vacancy is called a hole. Thus, a vacancy left in the valence band because of lifting of an electron from the valence band to conduction band is known as a hole.

Define what is hole current?

The movement of the hole (positively charged vacancy in the valence band) from positive terminal of the supply to negative terminal through semiconductor constitutes hole current.

Define what is intrinsic semiconductor ?

An intrinsic semiconductor is one which is made of the semiconductor material in the extremely pure form (impurity content not exceeding one part in 100 million parts of semiconductors).

Why silicon and germanium are the two widely used semiconductor materials?

Because the energy required to release an electron from their valence band (i.e. to break their covalent bonds ) is very small (1.12eV for Si and 0.72eV for Ge).

Which of the two semiconductor materials Si or Ge has larger conductivity at room temperature? Why?

Since energy required in transferring electrons from valence band to conduction band is more in case of Si than that in case of germanium , the conductivity of Ge will be more than that of Si at room temperature.

Why does a pure semiconductor behave like an insulator at absolute zero temperature?

For a pure semiconductor at a temperature of absolute zero (-273.15oC)the valence band is usually full and there are may be no electron in the conduction band and it is difficult to provide additional energy required for lifting electron from valence band to conduction band by applying electric field. Hence the conductivity of a pure semiconductor at absolute zero temperature is zero and it behaves like an insulator.

Define what is the main factor for controlling the thermal generation and recombination?

Temperature, because with the increase in the temperature, concentrations of free electrons and holes increase and the rate of recombination is proportional to the product of concentration of free electrons and holes and also the rate of production of electron-hole pairs (thermal generation) increases with the rise in temperature.

Define mean life of a carrier.

The amount of time between the creation and disappearance of a free electron is called the life time. It varies from a few nanoseconds to several microseconds depending how perfect the crystal is and other factors.

In which bands do the movement of electrons and holes take place?

Free electrons move in valence band while holes in valence band.

Define what is the mechanism by which conduction takes place inside the semiconductor?

Conduction occurs in any given material when an applied electric field causes electrons to move in a desired direction within the material. This may be due to one or both of two processes, electron motion and hole transfer. In case of former process, free electrons in the conduction band move under the influence of the applied electric field. Hole transfer involves electrons which are still attached to the atoms i.e. those in valence band.

Define what do you mean by drift velocity and mobility of a free electron?

The average velocity of an electron is known as drift velocity whereas mobility of an electron is defined as the drift velocity per unit electric field.

Define mobility of a carrier. Show that the mobility constant of electron is larger than that of a hole.

Mobility is defined as the average particle drift velocity per unit electric field.

The mobility of electrons is more than that of holes because the probability of an electron having the energy required to move to an empty state n the conduction band is much greater than the probability of an electron having the energy required to move to the empty state in valence band. The mobility of electron is about double that of a hole.

Define diffusion current in a semiconductor.

The diffusion of charge carriers is a result of a gradient of carrier concentration (i.e., the difference of carrier concentration from one region to another). In this case concentrations of charge carriers (either electrons or holes ) tend to distribute themselves uniformly throughout the semiconductor crystal. This movement continues until all carriers are evenly distributed throughout the material. This type of movement of charge carriers is called diffusion current.

Define drift current in a semiconductor.

The steady flow of electrons in one direction caused by applied electric field constitutes an electric current, called the drift current.

Define what happens to the conductivity of semiconductor with the rise in temperature? Compare with the conductivity of metals.

With the increase in temperature, the concentration of charge carriers increases resulting in increase in conductivity of semiconductors. The conductivity of metal decreases with the increase in temperature.

Why temperature coefficient of resistance of a semiconductor is negative?

With the increase in temperature, the concentration of charge carriers (electrons and holes) increases. As more charge carriers are made available, the conductivity of a pure semiconductor increases i.e. resistivity of a pure semiconductor decreases with the rise in temperature i.e. semiconductors have negative temperature coefficient of resistance.

Define what is meant by Fermi level in semiconductor? Where does the Fermi level lie in an intrinsic semiconductor?

Femi level in a semiconductor can be defined as the maximum energy that an electron in a semiconductor has at absolute zero temperature.

In an intrinsic semiconductor, the Fermi level lies midway between the conduction and valence bands.

Differentiate between intrinsic semiconductors and intrinsic semiconductors?

An intrinsic semiconductor is one which is made of the semiconductor material in its extremely pure form.

When a small amount of impurity is added to a pure semiconductor crystal during the crystal growth in order to increase its conductivity, the resulting crystal is called extrinsic semiconductor.

Why doping is done in semiconductors?

Intrinsic (or pure ) semiconductor by itself is of little significance as it has little current conduction capability at ordinary room temperature. However, if very small amount of impurity (of the order of one atom per million atoms of pure semiconductor) is added to it in the process of crystallization, the electrical conductivity is increased many times.

Describe the difference between P-type and N-type semiconductor materials.

When a small amount of trivalent impurity (such as boron, gallium, indium or aluminium) is added to a pure semiconductor crystal during crystal growth, the resulting crystal is called a P-type semiconductor.

When a small amount of pentavalent impurity (such as arsenic, antimony, bismuth or phosphrous) is added to a pure semiconductor crystal during crystal growth, the resulting crystal is called the N-type semiconductor.

Define what do you mean by donor and acceptor impurities?

Donor impurities (such as arsenic, antimony, bismuth or phosphorous) when added to a pure semiconductor lattice , form N-type extrinsic semiconductor. The pentavalent impurities are called donor impurities as such impurities donate electrons to the lattice.

Acceptor impurities (such as boron, gallium, indium or aluminium) when added to a semiconductor lattice form P-type extrinsic semiconductor. The trivalent impurities are called acceptor impurities because such impurities accept electrons from the lattice.

Explain the term doping and its need.

The electrical conductivity of intrinsic semiconductor, which has little current conducting capability at room temperature and so is of little use, can be increased many times by adding very small amount of impurity (of the order of one atom per million atoms of pure semiconductor) to it in the process of crystallization. This process is called doping.

Define what is the effect of temperature on extrinsic semiconductor?

With the increase in temperature of an extrinsic semiconductor, the number of thermally generated carriers is increased resulting in increase in concentration of minority carriers. At temperature exceeding critical temperature the extrinsic semiconductor behaves like an intrinsic semiconductor but with higher conductivity.

Define what are the charge carriers in P-type and N-type semiconductors?

Fee electrons in n-type semiconductors and holes in p-type semiconductors are the charge carriers.

For the same order of doping, why does n-type semiconductor exhibit larger conductivity than p-type semiconductor?

Since the mobility of electrons is higher than that of holes, for same level of doping, n-type semiconductor exhibits larger conductivity.

Define what is the ratio of majority and minority carriers in intrinsic and extrinsic semiconductors?

For intrinsic semiconductor the ratio of majority and minority carriers is Unity.

For extrinsic semiconductor the ratio of majority and minority carriers is Very large.

Define what is a p-n junction?

The contact surface between the layers of p-type and n-type semiconductor pieces placed together so as to form a p-n junction is called the p-n junction.

How do the transition region width and contact potential across a p-n junction vary with the applied bias voltage?

When the p-n junction is forward biased , the transition region width is reduced and the contact potential is also reduced with the increase in applied bias voltage.

When the p-n junction is reverse biased, the transition is widened, and the contact potential is increased and with the increase in applied bias voltage.

Which type of charges present on the two opposite faces of the junction?

Positive charge on n-side and negative charge on p-side of the junction.

Define what types of carriers are present in space charge region?

No mobile carrier is present in the space charge region.

Why is space region called the depletion region?

The region around the junction is completely ionized on formation of p-n junction. As a result, there are no free electrons on the n-side nor the holes on the p-side. Since the region around the junction is depleted of mobile charges, it is called the depletion region.

Why an electric field is produced in a depletion region of a p-n junction?

The separation of positive and negative space charge densities in a p-n junction results in an electric field.

Define what is space charge width?

The space charge region extends into the n and p-regions from the metallurgical junction. The distance is known as the space charge width.

The electric field in the space charge region decreases with forward bias and increases with reverse bias. Why?

Because applied electric field opposes built-in field.

Define what do you understand by reverse saturation current of a diode?

Reverse saturation current of a diode is due to minority carriers and is caused when the diode is reverse biased. Only a very small voltage is reuired to direct all minority carriers across the junction, and when all minority carriers are flowing across, further increase in bias voltage will not cause increase in current. This current is referred to as reverse saturation current.

Define what is the effect of temperature on the reverse current of a p-n junction?

Reverse current of a p-n junction increases with the increase in junction temperature.

Why is silicon preferred to germanium in the manufacturing of semiconductor devices?

Silicon preferred to germanium in the manufacturing of semiconductor devices because such devices have higher peak inverse voltage and current ratings and wider temperature range than germanium ones.

Define peak inverse voltage?

Peak inverse voltage is the maximum voltage that can be applied to the p-n junction without damaging the junction. If the reverse voltage across the junction exceeds its peak inverse voltage(PIV), the junction may get destroyed owing to excessive heat.

Define breakdown voltage.

Breakdown voltage is defined as the reverse voltage at which p-n junction breaks down with sudden rise with reverse current.

Define the limitations in the operation conditions of a p-n junction.

Every p-n junction has limiting values of :

Maximum forward current

Peak inverse voltage (PIV)

Maximum power rating

The p-n junction provides satisfactory performance when operated within these limiting values. The p-n junction diode may get destroyed due to excessive heat if any of these values are exceeded.