Definition
A planar transistor is the traditional, flat MOSFET in which the source, gate, and drain all sit on a single two-dimensional plane at the surface of the silicon wafer. The gate controls a horizontal channel from above through a thin oxide. For roughly four decades this simple, easy-to-manufacture structure was the workhorse of every integrated circuit, including the early generations of Bitcoin mining chips.
Why flat eventually failed
As planar transistors shrank below about 28nm, the gate could only touch the channel from one side, and it lost the ability to fully shut off current. Electrons began leaking through even when the device was supposed to be "off," wasting power as heat and limiting how low the supply voltage could go. For a mining ASIC, where idle leakage is pure inefficiency, this leakage wall set a hard floor on joules per terahash.
The move to three dimensions
The fix was to give the gate more sides of the channel to grip. Raising the channel into a fin produced the FinFET, and later fully surrounding it produced the gate-all-around transistor. Both deliver the steeper, cleaner switching that flat devices lost, which is why the leading edge abandoned planar designs around 2012. Planar transistors remain in wide use on mature, low-cost nodes for power management, analog, and microcontrollers.
The planar transistor was superseded by the FinFET and then the GAAFET; its decline marked the point where Dennard scaling broke down.
In Simple Terms
A planar transistor is the traditional, flat MOSFET in which the source, gate, and drain all sit on a single two-dimensional plane at the surface…