Hello Friends! Hope you’re well today. I welcome you on board. In this post, I’ll detail the Introduction to Semiconductors.A semiconductor is a material whose properties stand between conductor and insulator. The flow of current in a semiconductor is better than the insulator and pooper than the conductor.The properties of semiconductors can be alerted by adding impurities, this process is known as doping. When semiconductor materials are doped with trivalent atoms like boron, it behaves like an acceptor while they are doped with pentavalent atoms like Arsenic, Antimony, it behaves like a donor.When the temperature increases the resistivity of semiconductor material falls which is opposite in the case of conductor i.e. resistivity increases with the increase of temperature.In this post, I’ll walk you through the complete Introduction to Semiconductors covering semiconductor types, structure, working, and material used.Let’s dive in.
Introduction to Semiconductor
- A semiconductor is a material whose properties lie between conductor and insulator. Silicon, germanium, and gallium arsenide are the common semiconductors used in many applications. Silicon is the best semiconductor used in many electronic circuits.
- Silicon carries four electrons in the valence shells, hence forming the bond with the neighboring silicon atom, giving it a very stable full orbital eight electron structure where each atom shares one electron.
- Gallium arsenide stands the second best semiconductor material used in solar cells, laser diodes, and microwave frequency integrated circuits.
- Doping is used to customize the electrical properties of semiconductors. Both donor and acceptor elements are added in the semiconductor materials to turn them into N-type and P-type semiconductors.
- In doping, these elements are added as an impurity to alter the semiconductor electrical properties. Don’t get confused with the donors and acceptors we’ll touch them briefly later in this post.
- Semiconductors are not good insulators nor good conductors because they carry very few electrons that can roam around freely inside the material. Semiconductor materials are closely packed together in the form of crystal, forming a crystal lattice.
- It’s important to note that in their pure form semiconductors are barely used because they are not good conductors nor good insulators. They are, however, useful when impurities are added in the semiconductor materials.
- Before we go further and define the types of semiconductors, let’s first study the working of conductors and insulators.
- Conductors are the materials that support the flow of current and carry resistance in micro ohms. The ability of electrons to move freely inside the conductor materials allows them to support the flow of electrical current.
- Copper and aluminum act like a good conductor. The electrons available in the outermost shell are not tightly attracted by the parent nucleus the reason these electrons move freely and allow the current to flow inside the conductor.
- The electronic drift formed in the conductor material is responsible for the flow of current.
- This drift is formed when the voltage is applied across the conductor material which allows the electrons in the outermost shell to leave their parent atom and roam around to construct the electronic drift. The movement of these electrons depends on the applied voltage.
- Copper, silver aluminum and carbon are the good conductors that carry very few electrons in the outermost shell and while voltage is applied across the material, these electrons easily leave the valence shell which are then combined by the nearby atoms, thus creating a ‘domino effect’ that produces the electrical current.
- The energy is lost during the flow of current that generates heat the reason you’ll notice conductors become hot with the flow of current.
- Insulators are the materials that resist the flow of current. They are the opposite of conductors. These materials don’t have electrons that can move freely inside the material.
- The electrons in the outermost shell are strongly attracted by the positively charged parent nucleus. Quartz, marble, and PVC plastics are common examples of insulators.
- Insulators are the key component of many electronic circuits because without them these circuits would short circuit.
- Insulators carry resistance millions of ohms and are not affected by the normal change in temperature unless they are brought under the influence of very high temperatures where they turn to conductors.
- Insulators materials like glass or porcelain are added in the transmission cables to avoid any short circuit. While printed circuit boards are composed of epoxy glass resin.
Types of Semiconductors
There are two main types of semiconductors:1: Intrinsic Semiconductors2: Extrinsic Semiconductors
1: Intrinsic Semiconductors
- The semiconductors in their pure form are known as intrinsic semiconductors. Simply put, when the number of holes inside the material is equal to the number of electrons, the material behaves like an intrinsic semiconductor.
- Pure silicon and germanium are the examples of intrinsic semiconductors which come with forbidden energy gaps of 1.1 eV and 0.72 eV and respectively.
- Semiconductor material comes with two bands i.e. valence band, conduction band. The valence band is the band that carries valence electrons (electrons at the outermost shell of an atom that can be shared or transferred to another atom).
- In this band, the material contains the highest range of electron energies. On the other hand, the conduction band comes with a low range of empty electronic states. The gap between these two bands is called the forbidden energy gap.
- This forbidden energy gap is so small in both silicon and germanium in their pure form that even at ordinary room temperature many electrons carry significant energy and can cross the band gap between conduction and valence band.
2: Extrinsic Semiconductors
Semiconductors are called extrinsic semiconductors when impurities are added in them to alter their electrical properties.Extrinsic semiconductors are further divided into two types based on the type of doping material used. There are two main types of extrinsic semiconductors called:a: N-type Extrinsic Semiconductorsb: P-type Extrinsic Semiconductors
a: N-type Extrinsic Semiconductors
- These types of semiconductors are formed when pentavalent impurity element like phosphorous, antimony or arsenic is added into the silicon or germanium arsenide semiconductor.
- These impurity elements are known as pentavalent impurities and carry five electrons in the outermost shell where four are used to form a bond with the neighboring atom, leaving the one-electron free when the voltage is applied.
- Pentavalent impurities contain one free electron to donate, the reason they are called donors.
- Phosphorous and antimony are commonly used pentavalent impurities to customize the electrical properties of silicon semiconductors. These impurity elements contain five electrons in the outermost shell where four form the bond, setting one electron free.
- The resulting material formed in that case carries an excess of free electrons each with a negative charge, the reason they are also called N-type semiconductors.
- The electrons are in excess amount, therefore they are termed as majority carriers in the conductivity, leaving the holes as minority carriers participating in the conductivity process.
- When voltage is applied across the silicon material, it causes the electrons to get free from the silicon outermost shell and that place is occupied by the electrons present in the donor impurity. This process continues and the electrons that leave silicon atom are replaced by the electrons by the donor element.
- This action results in an extra electron every time, leaving more electrons than holes. The reason N-type semiconductors carry a negative charge.
b: P-type Extrinsic Semiconductors
- When we add trivalent impurity like boron or aluminum in the semiconductor material, they will form p-type semiconductors. These trivalent impurity elements carry one three electrons in the outermost shell, therefore they cannot form a complete bond, leaving plenty of holes inside the material.
- In this case, the crystal lattice contains scores of holes and the electrons are significantly missing in the lattice.
- Now, silicon crystal contains a hole, thus electron available in the neighbor atom tries to fill that hole, that in result leaving another hole. That hole is again filled by the neighbor electron, creating another hole.
- This process continues, making the entire lattice positively charged with scores of holes.
- This entire lattice generates the number of holes by creating a shortage of electrons, turning the entire doped crystal into a positive pole.
- As the addition of trivalent impurity produces holes, the trivalent impurity materials are commonly known as acceptors. Because they are always in a position to accept an electron.
- In the P-type extrinsic semiconductors, holes are termed as majority carriers and electrons are known as minority carriers in the conductivity process.
- They are called P-type semiconductors because the acceptor density is always greater than the donor density, leaving the positive charge on the entire crystal lattice.
Let’s summarize what we’ve studied so far.
- A semiconductor is a material that is not a good conductor nor a good insulator and whose electrical properties can be customized with the addition of an impurity element.
- Semiconductors are divided into two main types. The intrinsic semiconductors that are available in pure form (no amount of impurity is added in these semiconductors) and extrinsic semiconductors whose electrical properties can be modified with the addition of impurity elements. These impurity elements define the type of extrinsic semiconductor.
- If the impurity is trivalent material, then the resulting semiconductors formed will be P-type semiconductors, also known as acceptors that carry only three electrons in the valence shell, unable to form complete orbital bond containing eight electrons.
- They are always in a position to accept electrons and in this case, the number of holes is more than the number of electrons. The reason they carry a positive charge.
- On the other hand, when a pentavalent impurity is included in the semiconductor material, they constitute n-type semiconductor material.
- This pentavalent impurity contains five electrons in the outermost shell out of which form electrons form the bond with the neighbor atom leaving one electron free. In this case, the number of electrons is more than the number of holes, and they carry a negative charge.
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