What Is an Epitaxial Transistor?

The epitaxial transistor is the forerunner for many modern semiconductor devices. A standard transistor uses three pieces of semiconductor material fused together directly. Epitaxial transistors are much like a standard transistor, except they have a very thin film layer of pure, uncharged semiconductor material deposited between the transistor sections to insulate them from one another. This greatly improves the device’s speed and performance.

A standard transistor is made up of three pieces of a semiconductive material, such as silicon. The silicon for these pieces is blended with an additive that gives them an electrical charge. For an NPN-type transistor, an industry standard, two of the pieces are negatively charged while the third is positively charged.

To build the transistor, the three pieces of silicon are fused together, with the positively charged piece sandwiched between the two negatively charged pieces. Once these pieces are fused together, an exchange of electrons occurs in the two places where the pieces meet, called junctions. The electron exchange continues in the junctions until a balance between the negative and positive charges is met. Having balanced the electrical charges, these two areas no longer have any charge at all and are called depletion regions.

Depletion regions in a transistor determine many of the device’s operational characteristics, such as how fast the device can change states, called switching, and at what voltages the device will conduct or fail, called its breakdown or avalanche voltage. Because the method of creating depletion regions in standard transistors happens naturally, they are not optimally precise and cannot be controlled to improve or alter their physical structure, beyond changing the strength of the charge initially added to the silicon. For years, germanium transistors had superior switching speeds when compared to silicon transistors simply because the germanium semiconductor tended to naturally form tighter depletion regions.

In 1951, Howard Christensen and Gordon Teal of Bell Labs created a technology we now called epitaxial deposition. This technology, as the name suggests, could deposit a very thin film, or layer, of material on a substrate of an identical material. In 1960, Henry Theurer led the Bell team that perfected use of epitaxial deposition for silicon semiconductors.

This new approach to transistor construction changed semiconductor devices forever. Instead of relying on the natural tendencies of silicon to form a transistor’s depletion regions, the technology could add very thin layers of pure, uncharged silicon that would act as the depletion regions. This process gave designers precise control over the operational characteristics of silicon transistors and, for the first time, cost-effective silicon transistors became superior in all regards to their germanium counterparts.

With the epitaxial deposition process perfected, the Bell team created the first epitaxial transistor, which the company pressed into immediate service in its telephone switching equipment, improving both the speed and reliability of the system. Impressed with the performance of the epitaxial transistor, Fairchild Semiconductors began work on its own epitaxial transistor, the legendary 2N914. It released the device on the market in 1961 and it remained in wide use.

Following Fairchild’s release, other companies, such as Sylvania, Motorola, and Texas Instruments, began work on their own epitaxial transistors, and the Silicon Age of electronics was born. Due to the success of epitaxial deposition in the creation of transistors, and silicon devices in general, engineers sought out other uses for the technology, and it was soon put to work with other materials, such as metal oxides. The direct descendants of the epitaxial transistor exist in nearly every advanced electronic device imaginable: flat screens, digital camera CCDs, cell phones, integrated circuits, computer processors, memory chips, solar cells, and a myriad of other devices that form the foundations of all modern technological systems.