The TE performance of narrow-gap semiconductors and semimetals is more sensitive to the value of R . zT degrades when R is close to 1 (Figure ). However, R >> 1 (Figure ) or 1/R >> 1 both ensure that the bipolar conduction e↵ects on the TE performance is suppressed, leading to larger zT values. In summary, (i) Semiconductors with large E g (wider gap) will not su↵er from bipolar conduction e↵ects and therefore, exhibit higher maximum zT .