Heterojunction Bipolar Transistor

The first SiGe device to go into production was the HBT in 1999. The performance of a Si bipolar (BJT) can be enhanced by the addition of SiGe to the base region of the transistor. Two schemes exist to enhance performance, a rectangular Ge base profile (HBT) and a linearly graded base. The linear base provides a built in electric field which accelerates electrons across the base region. The band structure for the HBT and the linear grade transistors are shown on the right.

The demonstrated cutoff frequencies (fT - the frequency where the gain for the transistor equals 1) for HBTs has now reached 360 GHz and this is predicted to increase as technology progresses. There is a downside to this, however, in that the breakdown voltage between the collector and the emitter also decreases as fT is increased.

Freddy R Vallenilla R

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EES SEC 2