|Amplifier Teaching Aid (DED Philippinen, 86 p.)|
|Lesson 1 - Semiconductor Review|
An unbiased diode has a depletion layer at the PN-junction. The ions in this deplation layer produce a barrier potential. At room temperature, this barrier potential is approximately 0.7V for a silicon diode.
Fig. 1-2: Unbiased diode
When an external voltage opposes the barrier potential, the diode is forward-biased. If the applied voltage is greater than the barrier potential, the current is large. In other words, current flows easily in a forward-biased diode.
When an external voltage aids the barrier potential, the diode is reverse biased. The width of the depletion layer increases when the reverse voltage increases. The current is approximately zero. The reverse biased diode acts like an open switch.
Too much reverse voltage will produce either avalanche or zener effect. Then, the large breakdown current destroys the diode.
Fig. 1-3: Forward biased diode
What happens to an electron in this circuit?
1. After leaving the source terminal, it enters the right end of the crystal.
2. It travels through the N-region as a free electron.
3. At the junction it recombines with a hole and becomes a valence electron.
4. It travels through the P-region as a valence electron.
5. After leaving the left end of the crystal, it flows into the positive source terminal.