The ESD Handbook

20ESD in Silicon Germanium

20.1 Heterojunctions Bipolar Transistors

In today’s radio frequency (RF) technologies, the silicon (Si) homo-junction bipolar transistor has been dominated by new hetero-junction bipolar technologies from Silicon Germanium (SiGe), Gallium Arsenide (GaAs), to Indium Phosphide (InP) [1–74]. The hetero-junction bipolar transistor (HBT) has outperformed the homo-junction bipolar junction transistor (BJT). With the introduction of the Silicon Germanium HBT [24–63], and presently the Silicon Germanium Carbon HBT, these devices became the natural evolutionary step beyond the silicon BJT.

From the early days of the invention of the silicon bipolar transistor, as early as the 1950s and 1960s, the question of the limitations of the transistor speed was being addressed by early device researchers such as Early, Johnson, Webster, Kirk [1–9]. From the Johnson Limit relationships, it is clear that there was a trade-off between the maximum power and the speed of the transistor [6]. Additionally, the problem of scaling the base width, and at the same time having a low base series resistance was a natural constraint to the scaling of the BJT. An independent variable was needed that allowed continuous scaling of the base region. The solution to faster transistors was the ability to modify the energy bandgap to improve device performance.

The work by H. Kroemer on the hetero-junction opened a new door on achieving higher speeds than anticipated with silicon homo-junctions ...