1. It's complicated, complicated, complicated, and needs further study. 2. If the Hall voltage is negative, it is an n-type semiconductor; If the Hall voltage is positive, it is a P-type semiconductor. 3. Choose materials with high mobility and high resistivity. Semiconductors are ideal materials for manufacturing Hall elements. It has high mobility and moderate resistivity. However, for metal conductors, the mobility and resistivity are very low, while for poor conductors, although the resistivity is high, the mobility is moderate and the Hall coefficient is very small, so it is impossible to manufacture Hall elements. Finally: love the ball.