Selecting semiconductors for RF/microwave applications involves choosing from among a large array of devices fabricated on different substrates. As the industry hopes to combat the ever-increasing congestion of lower-frequency spectrum with signal bandwidths reaching into the millimeter-wave (mmWave) range, semiconductor materials and devices will be pushed into new regions to achieve more gain and power at higher frequencies.
Bipolar junction transistors (BJTs) fabricated on silicon (Si) substrate wafers—the oldest transistors used in microwave circuits—are still used in both low- and high-power applications. Si bipolar transistors are simple devices that can operate with a single positive voltage supply but provide reasonable gain through RF and lower microwave frequencies.
As with many other high-frequency transistor configurations, BJTs consist of several semiconductor diode junctions, fabricated from layers of p-type and n-type semiconductor materials. The p and the n refer to “positive” and “negative” and denote whether a material has an excess of holes or electrons, respectively. They also indicate how the current will flow in a semiconductor device formed of the materials.
A BJT has three terminals—the base, collector, and emitter—with the voltage between two of the terminals, such as the base and the emitter, serving to control the flow of current from the other terminal, such as the emitter. The input current to a Si BJT controls the flow of output current. Si BJTs can be fabricated as pnp devices, with an n-type semiconductor layer between two p-type layers, or as npn devices, with a p-type semiconductor layer between two high-electron-content n-type layers.