Introduction to Semiconductor ,Conductor and Insulator

 

A semiconductor material is one whose electrical properties lie in between insulators and Conductors. 

Solid-state materials are commonly grouped into three classes: insulators, semiconductors, and conductors. (At low temperatures some conductors, semiconductors, and insulators may become superconductors.) 

Examples : germanium and silicon. 

Insulators An insulator is a material that does not conduct electrical current. Most good insulators are compounds rather than single-element materials and have very high resistivities. Valence electrons are tightly bound to the atoms; therefore, there are very few free electrons in an insulator. 

Examples of insulators are rubber, plastics, glass, mica, and quartz.

Conductors A conductor is a material that easily conducts electrical current. Most metals are good conductors. The best conductors are single-element materials, such as copper (Cu), silver (Ag), gold (Au), and aluminum (Al), which are characterized by atoms with only one valence electron very loosely bound to the atom. These loosely bound valence electrons can become free electrons with the addition of a small amount of energy to free them from the atom. 

Types of Semiconductors: 

a. Intrinsic Semiconductors 

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

Example: pure germanium and silicon which have forbidden energy gaps of 0.72 eV and 1.1 eV respectively. The energy gap is so small that even at ordinary room temperature; there are many electrons which possess sufficient energy to jump across the small energy gap between the valence and the conduction bands. 

an intrinsic semiconductor may be defined as one in which the number of conduction electrons is equal to the number of holes. 

Schematic energy band diagram of an intrinsic semiconductor at room temperature is shown in Fig. below.  

   

b. Extrinsic Semiconductors: 

Those intrinsic semiconductors to which some suitable impurity or doping agent or doping has been added in extremely small amounts (about 1 part in 108) are called extrinsic or impurity semiconductors. 

Depending on the type of doping material used, extrinsic semiconductors can be sub-divided into two classes: (i) N-type semiconductors and  (ii) P-type semiconductors. 

 

(i) N-type Extrinsic Semiconductor: 

This type of semiconductor is obtained when a pentavalent material like antimonty (Sb) is added to pure germanium crystal. As shown in Fig. below, each antimony atom forms covalent bonds with the surrounding four germanium atoms with the help of four of its five electrons. The fifth electron is superfluous and is loosely bound to the antimony atom. 

Hence, it can be easily excited from the valence band to the conduction band by the application of electric field or increase in thermal energy. It is seen from the above description that in N-type semiconductors, electrons are the majority carriers while holes constitute the minority carriers.  

 

  

 

 

(ii) P-type Extrinsic Semiconductor: 

 This type of semiconductor is obtained when traces of a trivalent like boron (B) are added to a pure germanium crystal. In this case, the three valence electrons of boron atom form covalent bonds with four surrounding germanium atoms but one bond is left incomplete and gives rise to a hole as shown in Fig. below. 

Thus, boron which is called an acceptor impurity causes as many positive holes in a germanium crystal as there are boron atoms thereby producing a P-type (P for positive) extrinsic semiconductor. 

In this type of semiconductor, conduction is by the movement of holes in the valence band. 

  

 

Majority and Minority Carriers: 

In a piece of pure germanium or silicon, no free charge carriers are available at 0ºK. However, as its temperature is raised to room temperature, some of the covalent bonds are broken by heat energy and as a result, electron-hole pairs are Produced. These are called thermally-generated charge carriers. They are also known as intrinsically-available charge carriers. Ordinarily, their number is quite small. An intrinsic of pure germanium can be converted into a P-type semiconductor by the addition of an acceptor impurity which adds a large number of holes to it. 

Hence, a P-type material contains following charge carriers: 

(a)  Large number of positive holes—most of them being the added impurity holes with only a very small number of thermally generated ones. 

(b) A very small number of thermally-generated electrons (the companions of the thermally generated holes mentioned above). 

Obviously, in a P-type material, the number of holes (both added and thermallygenerated) is much more than that of electrons. Hence, in such a material, holes constitute majority carriers and electrons form minority carriers 

Similarly, in an N-type material, the number of electrons (both added and thermally-generated) is much larger than the number of thermally-generated holes. 

  

P-N Junction Diode  

Diodes are made from a single piece of Semiconductor material which has a positive “P-region” at one end and a negative “N-region” at the other, and which has a resistivity value somewhere between that of a conductor and an insulator

●It is two terminal devices consisting of a P-N junction formed either in Ge or Si crystal. The P and N type regions are referred to as anode and cathode respectively.

  

●A P-N junction diode is a one way device offering low resistance when forward biased and behaving almost as an insulator when reverse biased. Hence such diodes are mostly used as rectifiers for converting alternating current into direct current. 

 

V/I Characteristic  

  

Forward characteristic 

When the diode is forward biased and applied voltage is increased from zero hardly any current flows through the device in the beginning. It is so because the external voltage is being opposed by the internal barrier voltage VB whose value is 0.7 V for Si and 0.3 V for Ge. As soon as VB is neutralized, current through the diode increases rapidly with increasing applied battery voltage. It is found that as little a voltage as 1.0 V produces a forward current of about 50 mA. 

 

 Reverse characteristic  

When the diode is reverse biased majority carriers are blocked and only a small current (due to minority carriers) flows through the diode. As the reverse voltage is increased from zero, the reverse current very quickly reaches its maximum or saturation value I0 which is also known as leakage current. It is of order of nano ampers (nA) for Si and micro ampers (µA) for Ge

KEY POINT

 Atom  The smallest particle of an element that possesses the unique characteristics of that element.

Barrier potential  The amount of energy required to produce full conduction across the pn junc-

tion in forward bias.

Conductor  A material that easily conducts electrical current.

Crystal  A solid material in which the atoms are arranged in a symmetrical pattern.

Doping  The process of imparting impurities to an intrinsic semiconductive material in order to 

control its conduction characteristics.

Electron  The basic particle of negative electrical charge.

Free electron An electron that has acquired enough energy to break away from the valence band 

of the parent atom; also called a conduction electron.

Hole  The absence of an electron in the valence band of an atom.

Insulator : A material that does not normally conduct current.

Ionization The removal or addition of an electron from or to a neutral atom so that the resulting 

atom (called an ion) has a net positive or negative charge.

Metallic bond A type of chemical bond found in metal solids in which fixed positive ion cores are 

held together in a lattice by mobile electrons.

Orbital Subshell in the quantum model of an atom.

PN junction :The boundary between two different types of semiconductive materials.

Proton : The basic particle of positive charge.

Semiconductor : A material that lies between conductors and insulators in its conductive proper-

ties. Silicon, germanium, and carbon are examples.

Shell An energy band in which electrons orbit the nucleus of an atom.

Silicon A semiconductive material.

Valence  : Related to the outer shell of an atom.

Bulk resistance (rB)  

It is the sum of the resistance values of the P and N type semiconductor materials of which the diode is made of Usually, it is very small, 

  

It is the resistance offered by the diode well above the knee voltage when current resistance is large. 

Obviously, this resistance is offered in the forward direction. 

 

Junction resistance (rj)  

It is value for forward biased junction depends on the magnitude of forward dc current. 

Dynamic or ac resistance  

                                rac  or rd = rB + rj 

For large values of forward current, rj is negligible. Hence, rac = rB for small values of  IF, rB is negligible as compared to rj  

                                rac  = rj