Transistors

A short field guide to the semiconductor device names that show up in amplifier topology claims. The product table should only tag a specific device family when the maker or a reliable service document says so.

Family	Origin	Audio relevance	Gotcha	Today
BJT / bipolar transistor	Bell Labs point-contact transistor, 1947; junction transistor work followed in 1948-1951.	The default solid-state output device for many classic Class A and Class AB amplifiers. Rugged, high-transconductance, and easy to parallel when the thermal design is done well.	BJT does not imply cheap or old. Many serious amplifiers still use modern bipolar output devices.	Current production is broad across many semiconductor makers.
Power MOSFET	MOSFET research matured after Bell Labs work in 1959; power MOSFETs became important to audio in the 1970s.	Popular in audio because the gate is voltage-driven, the overload behavior can be graceful, and high-speed devices made Class D practical.	Vertical power MOSFETs and lateral audio MOSFETs are not the same device family in practice.	Current production is broad, especially for switching and power-control uses.
Lateral MOSFET	Hitachi lateral audio MOSFETs made this family famous in late-1970s hi-fi and pro amplifiers.	Loved by some designers for thermal behavior and tube-like clipping compared with many bipolar stages. Often seen in British and pro-audio MOSFET designs.	A product saying MOSFET may mean lateral audio MOSFET, vertical power MOSFET, or MOSFET driver/regulator parts elsewhere in the circuit.	Specialist audio lateral MOSFETs remain available from makers such as Exicon/ALFET, but the field is much narrower than commodity MOSFETs.
JFET	The field-effect idea predates practical transistors; junction FETs became practical after early transistor work.	Common in low-noise input stages, phono stages, buffer circuits, and discrete op-amp designs rather than as speaker-driving output devices.	A JFET input stage does not make the whole amplifier a FET amplifier.	Still made, but many classic low-noise through-hole parts are discontinued or scarce.
SIT / VFET	Static induction transistor work is associated with Jun-ichi Nishizawa; Sony and Yamaha used VFET/SIT devices in high-end 1970s audio.	The romantic solid-state part: curves closer to a triode than ordinary transistors, used in Sony/Yamaha classics and later Nelson Pass/First Watt SIT amplifiers.	Original Sony/Yamaha VFETs are not normal replacement parts. Failure can make vintage VFET amps hard to restore.	Vintage audio devices are finite; modern SIT audio projects have relied on special production or limited device supply.
IGBT	Commercial power devices emerged in the 1980s.	More common in industrial power conversion than hi-fi output stages, but worth tracking if a manufacturer explicitly uses it.	Do not infer IGBT from "switching amplifier"; most modern Class D output stages use MOSFETs or integrated power stages.	Current production is broad for power electronics.
GaN FET	Gallium-nitride power devices moved from RF/power research into practical fast-switching products in the 2000s-2010s.	Interesting for compact, efficient Class D and switching power supplies because very fast switching can reduce losses and simplify some filtering tradeoffs.	GaN is a switching-device technology, not an automatic sound-quality guarantee.	Current production and adoption are growing.

Research notes

Bell Labs / Nokia Bell Labs transistor history: first transistor demonstration at Bell Labs in 1947.
IEEE and semiconductor-history summaries for MOSFET/JFET/SIT chronology.
Hitachi lateral MOSFET audio history and later Exicon/ALFET audio lateral MOSFET catalogs.
Nelson Pass / First Watt SIT amplifier notes and Sony/Yamaha VFET-era service literature are useful product-level sources before tagging specific products.