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What is a Planar Transistor?

C.B. Fox
C.B. Fox

The planar transistor was invented by Jean Hoerni in 1959. The design of the planar transistor improved on earlier designs by making them cheaper to make, mass-producible, and better at amplifying electrical input. The planar transistor is built in layers and can have all of its connections in the same plane.

The first layer in a planar transistor is a base of semiconductor material. Many impurities are added to this base that allow it to be a better conductor. A second layer of semiconductor, with fewer impurities, is then put on top of the base. After the second layer is in place, the center of it is etched out, leaving thick edges of the second material around the sides and a thin layer above the base, in the shape of a square bowl.

Man holding computer
Man holding computer

A section of material of the opposite polarity than the initial two layers is then placed in the bowl. Once again, the center of this layer is etched away forming a smaller bowl. A material similar to the first layer of the planar transistor is then added. The second, third and fourth layers are all made flush with the top of the transistor.

The positive and negative components of the planar semiconductor are accessed on the same plane of the device. Metal connectors can be attached to the transistor after the components are in place, allowing the device to receive and emit electricity. The transistor receives input from the first layer and emits output from the fourth. The third layer is used to run a charge into the transistor so that it can amplify input.

Though the design of the device is a bit more complicated than earlier transistors, many planar transistors can be made at the same time. This decreases the amount of time and, subsequently, money needed to produce transistors and has helped paved the way for more affordable electronics. These types of transistors can also boost input to higher levels than earlier models of transistors.

In earlier transistors, the oxide layer that naturally forms on the suface of the semiconductor was removed from the transistor to prevent contamination. This meant that the delicate junctions between the positive and negative sections of the transistor had to be exposed. Constructing the transistor in layers, as Hoerni’s design called for, incorporated the oxide layer as a protective feature for the junctions.

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