If we classify the materials on basis of their electrical property we can broadly classify them into three main categories namely Conductors, Insulators and Semiconductors. In this blog we will be discussing about semiconductors, doping and the method of doping.
Though the exact invention year of semiconductor characteristic of material is not known but one of the characteristics of semiconductors was discovered as far back as 1821, but the mechanism of the diode that is the foundation of modern semiconductors was first invented in 1874. Furthermore, in 1947, transistors were developed to control the flow of electricity. Semiconductor has conductivity level in between conductor and insulator, simply means lesser conductivity then conductors and better conductivity then insulators. If we see this with respect to the value of conductivity then we can say that solid has conductivity ranging from 10-20 ohm-1m-1 to 107 ohm-1m-1 and if we look for semiconductors it is around 10-7 ohm-1m-1 to 10-3 ohm-1m-1. The beauty of semiconductor is that we can change the electrical properties of semiconductor material by adding desired amount of additional impurities and those impurities are called dopants. Because of this we can play with the electrical properties and that has made our life easy as you can see everywhere there are chips, transistors etc. and those are possible only because we can play with the electrical properties of the material.
- Conductivity of Metals- 104 ohm-1m-1 to 107 ohm-1m-1
- Conductivity of Insulators- 10-20 ohm-1m-1 to 10-10 ohm-1m-1
- Conductivity of Semiconductors – 10-6 ohm-1m-1 to 104 ohm-1m-1
Doping is a process of adding desired amount of impurity atoms (Donors or acceptors) in order to change the electrical, optical and structural property of intrinsic semiconductors. Pure semiconductors which are not doped are called intrinsic semiconductors and doped semiconductor materials are called extrinsic semiconductors.
- Carrier concentration- The concentration of the dopant used affects many electrical properties. Most important is the material’s charge carrier concentration. In an intrinsic semiconductor under thermal equilibrium, the concentrations of electrons and holes are equivalent. That is, number of wholes and number of electrons are equal
n= p= ni2
But in extrinsic semiconductor it becomes like
Here n0 is the concentration of conducting electrons, p0 is the conducting hole concentration, and ni is the material’s intrinsic carrier concentration. The intrinsic carrier concentration varies between materials and is dependent on temperature. Silicon’s ni, for example, is roughly 1.08×1010 cm−3 at 300 kelvins, about room temperature.
Doping concentration for silicon semiconductors may range anywhere from 1013 cm−3 to 1018 cm−3. Doping concentration above about 1018 cm−3 is considered degenerate at room temperature. Degenerately doped silicon contains a proportion of impurity to silicon on the order of parts per thousand. This proportion may be reduced to parts per billion in very lightly doped silicon. Typical concentration values fall somewhere in this range and are tailored to produce the desired properties in the device that the semiconductor is intended for.
Types of Dopants
Mainly there are two types of dopants impurities, one is trivalent impurities also called acceptor impurities and one is pentavalent impurities also called donor impurities. When a pure/ intrinsic semiconductor is doped with trivalent impurities these are called P-Type semiconductors and when they are doped with pentavalent impurities they are called N-Type semiconductor.
Acceptors (P-Type Dopants)
- Boron – Boron is a p-type dopant. Its diffusion rate allows easy control of junction depths. Common in CMOS technology. Can be added by diffusion of diborane gas. The only acceptor with sufficient solubility for efficient emitters in transistors and other applications requiring extremely high dopant concentrations. Boron diffuses about as fast as phosphorus.
- Aluminium – Used for deep p-diffusions. Not popular in VLSI and ULSI. Also a common unintentional impurity.
- Gallium – Is a dopant used for long-wavelength infrared photoconduction silicon detectors in the 8–14 μm atmospheric windows. Gallium-doped silicon is also promising for solar cells, due to its long minority carrier lifetime with no lifetime degradation; as such it is gaining importance as a replacement of boron doped substrates for solar cell applications.
- Indium – Is a dopant used for long-wavelength infrared photoconduction silicon detectors in the 3–5 μm atmospheric windows.
Donors (N-Type Dopants)
- Phosphorus- Is an n-type dopant. It diffuses fast, so is usually used for bulk doping, or for well formation. Used in solar cells. Can be added by diffusion of phosphine gas. Bulk doping can be achieved by nuclear transmutation, by irradiation of pure silicon with neutrons in a nuclear reactor. Phosphorus also traps gold atoms, which otherwise quickly diffuse through silicon and act as recombination centres.
- Arsenic – is an n-type dopant. Its slower diffusion allows using it for diffused junctions. Used for buried layers. Has similar atomic radius to silicon, high concentrations can be achieved. Its diffusivity is about a tenth of phosphorus or boron, so it is used where the dopant should stay in place during subsequent thermal processing. Useful for shallow diffusions where well-controlled abrupt boundary is desired. Preferred dopant in VLSI circuits. Preferred dopant in low resistivity ranges.
- Antimony – is an n-type dopant. It has a small diffusion coefficient. Used for buried layers. Has diffusivity similar to arsenic, is used as its alternative. Its diffusion is virtually purely substitution, with no interstitials, so it is free of anomalous effects. For this superior property, it is sometimes used in VLSI instead of arsenic. Heavy doping with antimony is important for power devices. Heavily antimony-doped silicon has lower concentration of oxygen impurities; minimal auto doping effects make it suitable for epitaxial substrates.
- Bismuth – is a promising dopant for long-wavelength infrared photoconduction silicon detectors, a viable n-type alternative to the p-type gallium-doped material.