TYPE OF SEMICONDUCTORS -

INTRINSIC SEMICONDUCTORS

An intrinsic semiconductor is one that is in its extremely pure form. Common examples are pure Germanium and pure Silicon. The F.E.G is so small, that even at ordinary temperature there are many electrons jumps from V.B to C.B.

So each electron librated into conduction band a positive hole is created in the V.B. The hole behaves as an apparent free particle with effective +ve charge.

i.e in intrinsic semiconductors, the number of free electrons $\dpi{120} \fn_cm n_e$ is equal to the number of holes $\dpi{120} \fn_cm n_h$ .

i.e $\dpi{120} \fn_cm n_e=n_h=n_i$

Where $\dpi{120} \fn_cm n_i$ is called intrinsic carrier concentration.

When an electric field is applied to intrinsic semiconductors, conduction electrons in C.B move to the anode and holes in V.B move to the cathode.

Remember that the motion of the hole is only a convenient way of describing the actual motion of bond electrons.

hence current consists of the movement of both electrons and holes. i.e total current is $\dpi{120} \fn_cm i=i_e +i_h$

NOTE:-

Apart from the process of generation of conduction electrons and holes, a simultaneous process of recombination occurs in which the electrons recombine with the holes. At equilibrium, the rate of generation is equal to the rate of recombination of charge carriers. The recombination occurs due to an electron colliding with a hole.
The current in the external wire is only due to free electrons.

EXTRINSIC SEMICONDUCTORS

At room temperature, the current conduction properties are very low in the case of intrinsic semiconductors. As such no important electronic devices can be developed using these semiconductors.

To increase electrical conductivity, some impurity has been added. The process for adding impurities is known as DOPING, and the impurity atoms are called dopants. The dopant has to be such that it does not distort the original pure semiconductor lattice. The necessary condition to attain doping is that the sizes of the dopant and the semiconductor atoms should be nearly the same for this, atoms of the 3rd and 5th group of the periodic table are most suitable and the concentration of dopant atoms should be proper i.e few parts per million (ppm).

Generally doping material is either pentavalent (Bi, Sb, As Pb) or trivalent (In, Al, B, Ga). Depending upon the types of impurities, extrinsic semiconductors can be divided into two types.

N-Type semiconductor and P-Type semiconductor

N-Type semiconductor

This type of semiconductor is obtained when a pentavalent material like antimony (Sb) is added to pure germanium or Silicon.

From fig. each antimony atom forms a covalent bond with the four Ge atoms. The fifth electron is loosely bonded to the antimony atom. Hence it can easily be excited from V.B to C.B. . For Ex the energy required is 0.01 eV for Ge and 0.05 eV for Si to separate this electron from its atom.

It may be noted that by giving away its one valance electron, the donor atom become a +ve charged ion. But it can’t take part in conduction because it is tied into the lattice. The rate of recombination of holes would increase due to the increase in the number of electrons. As a result, the number of holes would get reduced further.

In short, we can say that in N-type semiconductors, electrons are the majority carriers while holes constitute the minority carriers.

$\dpi{120} \fn_cm n_e>>n_h$

P-Type semiconductor

This type of semiconductor is obtained when a trivalent material like Boron (B) is added to pure Germanium or Silicon.

In this case, the three valance electrons of the Boron atom form covalent bonds with three Ge atoms but one bond is left incomplete and gives rise to a hole.

Since the neighbouring Ge atom in the lattice wants an electron in place of a hole, an electron in the outer orbit of an atom in the neighbourhood may jump to fill this vacancy, leaving a vacancy or hole at its own site. Thus the hole is available for conduction.

Therefore, the dopant atom of P-Type material can be treated as the core of one -ve charge ion along with its associated hole. But it can’t take part in conduction because it is tied into the lattice. It is obvious that one acceptor atom gives one hole. Here the recombination process will reduce the number( $\dpi{120} \fn_cm n_i$ ) of intrinsically generated electrons to $\dpi{120} \fn_cm n_e$ .

i.e in P-Type semiconductors, the holes are the majority carriers and electrons are minority carriers.

$\dpi{120} \fn_cm n_h>>n_e$

Effect of energy bands of extrinsic semiconductors

In n-type semiconductors, the extra electrons of the dopant atom are nearly free and have an energy level in the band diagram that lies within the forbidden gap just below the conduction band. This extra energy level is created due to the impurity atom which is called donor energy level ( $\dpi{120} \fn_cm E_D$ ). The energy difference between the donor energy level and the bottom of C.B is very small (~ 0.01 eV) and is significantly less than that of the intrinsic material. The result is that at room temperature there are a large number of free electrons in the C.B from the donor impurities but very few (~ $\dpi{120} \fn_cm 10^{-12}$ ) atoms of Si/Ge get ionised.

similarly, in p-type semiconductors, the energy level of trivalent impurity ( called acceptor energy level $\dpi{120} \fn_cm E_A$ ) also lies within the forbidden energy gap, just above (~ 0.01 eV) the V.B. Even room temperature provides enough thermal energy to push the electrons in the V.B to the acceptor energy level ( $\dpi{120} \fn_cm E_A$ ). The result is that at room temperature , there are a large number of holes in the V.B and the conductivity of p- type semiconductors increases significantaly.

The electron and the hole concentration in a semiconductor in thermal equilibrium is given by

$\dpi{120} \fn_cm \left [ n_e.n_h=n_i^{2} \right ]$