Problem 4: (Objective 4: Understand the physics and including diodes.) These que

Question

Problem 4: (Objective 4: Understand the physics and including diodes.) These questions ask you to think through the physical reasons behind the diode operation and the current voltage relationship. (a) In forward bias does drift or diffusion current dominate the diode operation? Why(b) In reverse bias does drift or diffusion current dominate the diode operation? Why (c) At zero bias, what is true about the drill and diffusion currents? (d) In forward bias the diode current increases exponentially with voltage. Explain why this is the case in three or four sentences. (e)In reverse bias (before breakdown) the diode current is constant. Explain why this is the case in two or three sentences.

Explanation / Answer

a) In forward bias, due to the applied bias voltage, electric field across the junction reduces. Drift current is due to this electric field. However, diffusion of minority carriers increases due to the applied volltage (means since positive is connected to p side, it will attract majority carriers from n side which is minority carriers in p side). So diffusion current is dominating .

b) In reverse bias, the applied voltage aids the potential barrier hence the barrier height increases which makes diffusion of carriers nearly impossible (depending on the amount of bias). Hence diffusion current is almost zero.
Drift current exists because of electric field and charges. As long as both are there drift current is there. Hence drift current dominates (though its very small in magnitude).

c) At zero bias, there is diffusion current because of difference in electron hole concentration on both sides. Also there is drift component because of the junction voltage (junction formed due to electrons going from n to p and holes from n to p). Both these currents are equal in magnitude but opposite in direction, hence the total current is zero. So diffusion current = drift current.

d)Under forward bias, the current follows the diode equation which is I = I0*e^(q*V/kT)
hence the current is exponential. Physically, with increasing bias voltage , the barrier for charges to cross to the other side decreases which aids more and more charges to cross and constitute the diffusion current and this growth is exponential.

e) Applied bias affects the barrier height which affects diffusion current. However, drift current depends not on barrier height but on electric field and minority charges. Both dont change with bias (electric field doesn't change since with increase in bias, junction width increases, so voltage per unit width which is electric field is constant). hencein reverse bias, only drift current which is present is constant.

pls rate :)

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