Epitaxial growth
In epitaxial structures for GaN HEMTs, what is the main purpose of adding carbon or iron to the GaN buffer layer beneath the channel?
- ATo make it emit light
- BTo make the buffer highly resistive, suppressing off-state leakage and short-channel effects
- CTo make the buffer low-resistance so current flows easily
Show answer and explanation
Answer: B. To make the buffer highly resistive, suppressing off-state leakage and short-channel effects
GaN tends to become n-type because of unintentionally incorporated donors, and current flowing through the buffer makes the device hard to turn off. Adding carbon or iron lets deep levels capture electrons, making the buffer highly resistive. However, trapped charge can cause a temporary drop in current (current collapse), so the doping level must be tuned.
- A. It is not intended to produce light.
- B. Correct. It makes the buffer resistive to suppress leakage.
- C. The purpose is the opposite: to make the buffer highly resistive.