Classification Based on Transistor Structure
Point Contact Transistors
These were the very-first Germanium transistors, which worked based on the basis of a difficult and unreliable electrical forming process, causing them fail quite often. Further, they had a common-base current gain α greater than 1 and exhibited negative resistance.
Bipolar Junction Transistor (BJT)
BJTs are the transistors with three terminals (Emitter, Base and Collector) and hence possess two-junctions viz., Base-Emitter junction and Collector-Base junction. These are current-controlled devices whose conduction current relies on both majority as well as minority charge carriers (hence bipolar). Further, these can be either (i) npn transistor with majority charge carriers as electrons or (ii) pnp transistor with majority charge carriers as holes, depending on their doping nature. Apart from these, many other types of BJTs found are
Heterojunction Bipolar Transistor:
These are suitable for high frequency applications and have their emitter and base regions made of differing semiconductor materials.
Schottky Transistor or Schottky-Clamped Transistors:
These devices use a Schottky barrier to avoid the transistor from being saturated.
These are specially designed transistors operating in avalanche breakdown region (where the operating voltage will be greater than the break-down voltage) and possess very high switching speeds.
These are the transistors where two individual transistors are cascaded in such a way that the resulting device possesses a very high current gain.
Multiple Emitter Transistor:
This kind of transistor is specially designed to realize the NAND logical operation.
Multiple Base Transistor:
It is used to amplify very-low level signals present in noisy environments by adding the signal constructively, but the random noise stochastically.
These are formed by diffusing the semiconductor material with the necessary dopants.
Field Effect Transistor (FET)
These are the voltage-controlled transistors which are tri-terminal wherein the Gate terminal controls the flow of current between the Source and the Drain terminals. These are also called unipolar devices as their conduction current is only due to the majority charge carriers according to which they can be either n-channel (the majority charge carriers are electrons) FETs or p-channel (the majority charge carriers are holes) FETs. Further, FETs can be classified as
Junction FETs (JFETs):
JFETs can either pn JFETs or Metal Semiconductor FETs (MESFETs) depending on whether they have pn or a Schottky barrier junction.
Metal Oxide Semiconductor FETs (MOSFETs) or Insulated Gate FETs (IGFETs):
These devices have an insulating layer beneath their Gate terminal which results in very high input impedance. These can be either of depletion- or enhancement-mode in nature depending on whether they have a pre-existing channel or not which inturn influence their behaviour in the presence and absence of gate voltage.
Dual Gate MOSFET (DGMOSFET):
These are particularly useful in RF applications and have two serial gate controls.
High Electron Mobility Transistor (HEMT) or Heterostructure FET (HFET):
These are characterized by the presence of a hetero-junction which comprises of different materials on either side of the junction and are used in very high microwave frequency applications. Other variations of these devices include Metamorphic HEMT, Pseudomorphic HEMT (PHEMT), Induced HEMT, Heterostructure Insulated Gate FET (HIGFET) and Modulation Doped FET (MODFET).
These are double gate transistors whose effective channel width is decided upon by the thin silicon ‘fin’ forming the body of the device.
Vertical MOS (VMOS):
This device is structurally similar to MOS device except that they have V-groove in them, increasing their complexity and cost.
These are trench-structure based FETs almost similar to VMOS except the fact that they have a ‘U’-shaped groove instead of ‘V’-shaped groove.
FETs based on this technology possess a vertical structure with the Source and the Drain terminals at their top and bottom, respectively.
Metal Nitride Oxide Semiconductor (MNOS):
This kind of transistor is an add-on to MOS technology and uses nitride oxide as the insulating layer.
Fast Reverse or Fast Recovery Epitaxial Diode FETs (FREDFETs):
These are ultra-fast FETs with fast turn-off ability for the body diode.
Depletion FETs (DEPFETs):
These FETs are formed on fully depleted substrates.
Tunnel FET (TFET):
These work on the principle of quantum tunneling and are widely used in low energy electronics including digital circuits.
Ion-Sensitive FET (ISFET):
This FET uses the ion concentration in the solution to regulate the amount of current flow through it. These devices are widely used in the field of biomedicine and environmental monitoring.
Biologically Sensitive FET (BioFET):
In these FETs, biological molecules which bind to the gate terminal change its charge distribution and alter the channel-conductance. There are many variations among these devices like EnFETs, ImmunoFETs, GenFETs, DNAFETs, CPFETs, BeetleFETs, etc.
Nanoparticle Organic Memory FET (NOMFET):
This FET mimics the behaviour of inter-neuron signal transmission and is used in the field of artificial intelligence.
Organic FETs (OFETs):
These FETs possess a structure based on the concept of thin film transistors and use organic semiconductors for their channel. These are used extensively in the field of biodegradable electronics.
These are the FETs with their die area consisting of hexagonal shaped basic cells which inturn reduce the die size while increasing the channel density.
Carbon Nanotube FET (CNTFET):
This FET comprises a channel made of carbon nanotubes (single or an array) instead of bulk silicon.
Graphene Nanoribbon FET (GNRFET):
These FETs use graphene nanoribbons as a material for their channel.
Vertical Slit FET (VeSFET):
These are two-gate FETs with a vertical silicon slit, nothing but a narrow passage of silicon between two larger silicon regions.
Quantum FETs (QFETs):
These transistors are characterized by a very high speed of operation and work on the principle of quantum tunneling.
Inverted-T FET (ITFET):
These have a part of the device vertically extending from the horizontal plane.
Thin-Film Transitor (TFT):
They have thin films of active semiconductor, insulator and metal deposited over a non-conducting substrate like glass.
These are used in high speed integrated circuits and work by using the electromagnetic forces.
Electrolyte Oxide Semiconductor FET (EOSFET):
These have the metal-part of standard MOSFETs replaced by electrolyte solution and are used to detect neuronal activity.
Classification Based on their Function
Small Signal Transistors:
These type of transistors are particularly used to amplify the low level signals (rarely for switching) and can be either npn or pnp in nature.
Small Switching Transistors:
These are extensively used for switching purposes although they can be employed for amplification process. These transistors are available in both npn as well as pnp forms.
These are used as power amplifiers in high power applications and can be npn or pnp or Darlington transistors.
High Frequency Transistors:
These are also known as RF Transistors and are used in high speed switching devices wherein the small signals operate at high frequencies.
These are two-terminal light-sensitive devices are nothing but the standard transistors which have photosensitive region as a substitute for the base region.
These transistors are exclusively used as switches and are not suitable for amplification.
Biomedical and Environmental Transistors:
These are used in the field of biomedicine and environmental sensing.
In addition to these, there also exists an Insulated-Gate Bipolar Transistor (IGBT) which combines the features of both BJTs as well as FETs as it uses an insulated gate to control a bipolar power transistor acting as a switch. There are also devices which comprise two tunneling junctions enclosing a gate-controlled island called Single Electron Transistors (SETs).
Certain transistors like those of Junctionless Nanowire Transistor (JNT) even lack the presence of gate junction which leads to denser and cheaper microchips. Lastly, it is to be noted that these are just a countable number of transistor types amongst the ample types present in the market.