Unipolar Transistor
A unipolar transistor is a transistor in which current is carried by only one type of charge carrier, electrons or holes, flowing through a single channel of the same doping type from source to drain. The family includes the junction field-effect transistor and the MOSFET, and the term is the natural opposite of bipolar, the word used for transistors whose action depends on both electrons and holes and on carriers injected across a junction. William Shockley gave the field-effect transistor the name unipolar in his 1952 paper on a unipolar field-effect transistor, published at a time when the point-contact and junction devices were all bipolar and he wanted a label that stressed the difference in physical principle.
Unipolar describes the carriers, not the polarity of the voltage and not the dopant of the whole chip. In an n-channel device only electrons move along the channel, and in a p-channel device only holes do, so the current is a majority-carrier current in the channel. This is a different statement from saying the device needs one supply polarity, and a frequent mistake is to confuse the two: n-channel and p-channel unipolar transistors exist in both depletion and enhancement types, and complementary pairs of them make up CMOS logic. A MOSFET might puzzle beginners because its inversion layer is formed from carriers that are minority carriers in the body, but once that layer forms it is the only carrier type conducting, so the device still counts as unipolar.
The practical differences from bipolar behaviour follow directly from using one carrier type. A bipolar transistor must inject minority carriers into its base, and those carriers linger as stored charge; a unipolar transistor has no such stored charge, so it can switch off without that delay, which is one reason power MOSFETs dominate high-frequency switching converters. The gate of a unipolar device controls the channel through an electric field and draws practically no steady current, which gives a very high input resistance compared with the base current demanded by a bipolar transistor. Because mobility in the channel falls as temperature rises, the current in many unipolar devices decreases as they heat, which tends to spread current evenly and resist the thermal runaway that threatens bipolar power devices.
The idea long preceded the practical device, which is part of why Shockley's 1952 paper mattered. Julius Lilienfeld had patented field-effect amplifier structures in the 1920s, but surface states in the semiconductor prevented the field from penetrating the material, so no useful amplifier followed. Shockley's contribution in 1952 was a theory for a field-effect structure whose channel is controlled by a reverse-biased junction rather than by an insulated surface, avoiding the surface problem, and a Bell Labs team including George Dacey and Ian Ross demonstrated a working unipolar device in 1953. Insulated-gate unipolar devices only arrived once surface passivation improved, with the first working MOSFET demonstrated by Mohamed Atalla and Dawon Kahng at Bell Labs in late 1959 and presented in 1960.
Unipolar devices also carry limitations that keep bipolar transistors in service. For a given area, a bipolar transistor typically provides higher transconductance, which is why it still excels in some analog, radio-frequency and low-noise-voltage applications, and its predictable base-emitter voltage makes it a reliable building block for references and temperature sensors. Unipolar devices, by contrast, have larger part-to-part variation in threshold or pinch-off voltage and, in MOS types, are vulnerable to static damage of the thin gate insulator. Hybrid devices such as the insulated-gate bipolar transistor deliberately combine a MOS gate with bipolar conduction to obtain high-voltage, high-current capability at moderate switching speed, demonstrating that the two families are complements rather than competitors.
Today the unipolar principle is the foundation of nearly all digital electronics. Every processor, memory and phone chip is built mostly from MOSFETs, and power conversion in laptops, vehicles and solar inverters relies on unipolar silicon and, increasingly, silicon carbide and gallium nitride devices, which are valued because their majority-carrier conduction avoids the reverse-recovery losses of bipolar junctions. Unipolar diodes, notably the Schottky diode, apply the same principle to rectification. The word unipolar itself is now used less often as a name for the family than as a technical contrast, since the field-effect label and the specific names JFET and MOSFET have taken over.
Read unipolar transistor as the class of transistors in which one kind of charge carrier does all the work, steered by a field instead of injected across a junction.