difference between n-channel FET vs p-channel FET
Construction of N- Channel MOSFET. Let us consider an N-channel MOSFET to understand its working. A lightly doped P-type substrate is taken into which two heavily doped N-type regions are diffused, which act as source and drain. Between these two N+ regions, there occurs diffusion to form an Nchannel, connecting drain and source. Semilogarithmic I/V plot of intrinsic, p- and n-type NWs. The calculated specific resistivity values are shown next to the respective curves. The transfer characteristic of the intrinsic NW integrated into a back-gated Schottky-barrier NW-FET and a SEM. This is in contrast to P-Channel JFETs, whose channel is composed primarily of holes, which constitute the current flow. A N-Channel JFET is composed of a gate, a source and a drain terminal. It is made with an N-type silicon channel that contains 2 P-type silicon terminals placed on either side. The gate lead is connected to the p-type terminals, while the drain and source leads are connected to either ends of the.
This page on n-channel FET vs p-channel FET mentions difference between n-channel FET and p-channel FET.
The resistor on the gate of the N-channel MOSFET is used to bleed-off the electric charge from the gate and turn off the MOSFET. The resistor can be 5K-10K. The voltage difference between the gate and source will turn on the MOSFET, but must not exceed a value in the spec sheet known as Vgs. To do so will damage the device. N-type depletion regions close the channel for a p-channel JFET. So by setting the gate-source voltage to some pre-determined fixed negative value, we can cause the JFET to conduct current through its channel at a certain value between zero amperes and I DSS respectively.
Following section compares both FET types with respect to their features, symbol and construction.
n-channel FET
As shown in figure-1, n-channel FET is constructed using a bar of N-type material into which a pair ofP-type regions are diffused. Fig-1 also mentions circuit symbol of n-channel FET.
Following are the features of N-channel Junction FET(JFET):
• Current carriers are electrons in n-channel FET.
• Mobility of electrons is large.
• Input noise is low.
• Trans-conductance is large.
p-channel FET
Similar to n-channel FET, p-channel FET is constucted using a bar of P-type material into which a pair ofN-type regions are diffused. Fig-2 mentions circuit symbol of p-channel FET.
Following are the features of P-channel Junction FET(JFET):
• Current carriers are holes in p-channel FET.
• Mobility of holes is poor.
• Input noise is large.
• Trans-conductance is small.
From the comparison between n-channel FET and p-channel FET it isimperative that n-channel FET serves better than p-channel FET.
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An extrinsic semiconductor is one that has been doped; during manufacture of the semiconductor crystal a trace element or chemical called a doping agent has been incorporated chemically into the crystal, for the purpose of giving it different electrical properties than the pure semiconductor crystal, which is called an intrinsic semiconductor. In an extrinsic semiconductor it is these foreign dopant atoms in the crystal lattice that mainly provide the charge carriers which carry electric current through the crystal. The doping agents used are of two types, resulting in two types of extrinsic semiconductor. An electron donor dopant is an atom which, when incorporated in the crystal, releases a mobile conduction electron into the crystal lattice. An extrinsic semiconductor which has been doped with electron donor atoms is called an n-type semiconductor, because the majority of charge carriers in the crystal are negative electrons. An electron acceptor dopant is an atom which accepts an electron from the lattice, creating a vacancy where an electron should be called a hole which can move through the crystal like a positively charged particle. An extrinsic semiconductor which has been doped with electron acceptor atoms is called a p-type semiconductor, because the majority of charge carriers in the crystal are positive holes.
Doping is the key to the extraordinarily wide range of electrical behavior that semiconductors can exhibit, and extrinsic semiconductors are used to make semiconductor electronic devices such as diodes, transistors, integrated circuits, semiconductor lasers, LEDs, and photovoltaic cells. Sophisticated semiconductor fabrication processes like photolithography can implant different dopant elements in different regions of the same semiconductor crystal wafer, creating semiconductor devices on the wafer's surface. For example a common type of transistor, the n-p-n bipolar transistor, consists of an extrinsic semiconductor crystal with two regions of n-type semiconductor, separated by a region of p-type semiconductor, with metal contacts attached to each part.
Conduction in semiconductors[edit]
A solid substance can conduct electric current only if it contains charged particles, electrons, which are free to move about and not attached to atoms. In a metal conductor, it is the metal atoms that provide the electrons; typically each metal atom releases one of its outer orbital electrons to become a conduction electron which can move about throughout the crystal, and carry electric current. Therefore the number of conduction electrons in a metal is equal to the number of atoms, a very large number, making metals good conductors.
Unlike in metals, the atoms that make up the bulk semiconductor crystal do not provide the electrons which are responsible for conduction. In semiconductors, electrical conduction is due to the mobile charge carriers, electrons or holes which are provided by impurities or dopant atoms in the crystal. In an extrinsic semiconductor, the concentration of doping atoms in the crystal largely determines the density of charge carriers, which determines its electrical conductivity, as well as a great many other electrical properties. This is the key to semiconductors' versatility; their conductivity can be manipulated over many orders of magnitude by doping.
Semiconductor doping[edit]
Semiconductor doping is the process that changes an intrinsic semiconductor to an extrinsic semiconductor. During doping, impurity atoms are introduced to an intrinsic semiconductor. Impurity atoms are atoms of a different element than the atoms of the intrinsic semiconductor. Impurity atoms act as either donors or acceptors to the intrinsic semiconductor, changing the electron and hole concentrations of the semiconductor. Impurity atoms are classified as either donor or acceptor atoms based on the effect they have on the intrinsic semiconductor.
Donor impurity atoms have more valence electrons than the atoms they replace in the intrinsic semiconductor lattice. Donor impurities 'donate' their extra valence electrons to a semiconductor's conduction band, providing excess electrons to the intrinsic semiconductor. Excess electrons increase the electron carrier concentration (n0) of the semiconductor, making it n-type.
Acceptor impurity atoms have fewer valence electrons than the atoms they replace in the intrinsic semiconductor lattice. They 'accept' electrons from the semiconductor's valence band. This provides excess holes to the intrinsic semiconductor. Excess holes increase the hole carrier concentration (p0) of the semiconductor, creating a p-type semiconductor.
Semiconductors and dopant atoms are defined by the column of the periodic table in which they fall. The column definition of the semiconductor determines how many valence electrons its atoms have and whether dopant atoms act as the semiconductor's donors or acceptors.
Group IV semiconductors use group V atoms as donors and group III atoms as acceptors.
Group III–V semiconductors, the compound semiconductors, use group VI atoms as donors and group II atoms as acceptors. Group III–V semiconductors can also use group IV atoms as either donors or acceptors. When a group IV atom replaces the group III element in the semiconductor lattice, the group IV atom acts as a donor. Conversely, when a group IV atom replaces the group V element, the group IV atom acts as an acceptor. Group IV atoms can act as both donors and acceptors; therefore, they are known as amphoteric impurities.
| Intrinsic semiconductor | Donor atoms (n-Type Semiconductor) | Acceptor atoms (p-Type Semiconductor) | |
|---|---|---|---|
| Group IV semiconductors | Silicon, Germanium | Phosphorus, Arsenic, Antimony | Boron, Aluminium, Gallium |
| Group III–V semiconductors | Aluminum phosphide, Aluminum arsenide, Gallium arsenide, Gallium nitride | Selenium, Tellurium, Silicon, Germanium | Beryllium, Zinc, Cadmium, Silicon, Germanium |
The two types of semiconductor[edit]
N-type semiconductors[edit]
N-type semiconductors are created by doping an intrinsic semiconductor with an electron donor element during manufacture. The term n-type comes from the negative charge of the electron. In n-type semiconductors, electrons are the majority carriers and holes are the minority carriers. A common dopant for n-type silicon is phosphorus or arsenic. In an n-type semiconductor, the Fermi level is greater than that of the intrinsic semiconductor and lies closer to the conduction band than the valence band.
Examples - Phosphorus , Arsenic , Antimony , etc.
P-type semiconductors[edit]
P-type semiconductors are created by doping an intrinsic semiconductor with an electron acceptor element during manufacture. The term p-type refers to the positive charge of a hole. As opposed to n-type semiconductors, p-type semiconductors have a larger hole concentration than electron concentration. In p-type semiconductors, holes are the majority carriers and electrons are the minority carriers. A common p-type dopant for silicon is boron or gallium. For p-type semiconductors the Fermi level is below the intrinsic semiconductor and lies closer to the valence band than the conduction band.
Examples - Boron , Aluminium , Gallium , etc.
Use of extrinsic semiconductors[edit]
Extrinsic semiconductors are components of many common electrical devices. A semiconductor diode (devices that allow current in only one direction) consists of p-type and n-type semiconductors placed in junction with one another. Currently, most semiconductor diodes use doped silicon or germanium.
Transistors (devices that enable current switching) also make use of extrinsic semiconductors. Bipolar junction transistors (BJT), which amplify current, are one type of transistor. The most common BJTs are NPN and PNP type. NPN transistors have two layers of n-type semiconductors sandwiching a p-type semiconductor. PNP transistors have two layers of p-type semiconductors sandwiching an n-type semiconductor.
Field-effect transistors (FET) are another type of transistor which amplify current implementing extrinsic semiconductors. As opposed to BJTs, they are called unipolar because they involve single carrier type operation – either N-channel or P-channel. FETs are broken into two families, junction gate FET (JFET), which are three terminal semiconductors, and insulated gate FET (IGFET), which are four terminal semiconductors.
Other devices implementing the extrinsic semiconductor:
See also[edit]
References[edit]
- Neamen, Donald A. (2003). Semiconductor Physics and Devices: Basic Principles (3rd ed.). McGraw-Hill Higher Education. ISBN0-07-232107-5.
External links[edit]
N Channel Fet