Answer: No, the diffusion capacitance is due to the excess carrier due to the holes (for example) diffuse to n region, creating charges that give rise to the capacitance. This condition takes place under forward bias. On the other hand, the junction capacitance is due to the reverse bias resulting in the formation of the depletion region. Under forward bias, we still have a little bit of depletion region to form a lower value of junction capacitance.
2. What are the excess charges accumulated at the junction due to diffusion process?
Answer: It should be the minority carrier (through diffusion) in the region of
the dominant carrier (doped side). Therefore, Qp for hole concentration
in the n region; whereas, Qn is for electron concentration in the p
region. This will give rise to diffusion capacitance as stated in (1) above.
3. Why the capacitance formula is Q/V_T but not Q/V where V=potential difference between anode and cathode V_T=thermal voltage?
Answer: The capacitance should be the ratio of the change in CHARGE Q to the change in voltage V (derivative) or precisely can be represented by dQ/dV. This is actually discussed in Lecture 6 when we derived the diffusion capacitance at the pn junction under forward bias. Hence, the formula that you presented is not precise (you can but you are really talking about instantaneous capacitance which in most cases we are not interested in) and has to be corrected by introducing the derivative or change. Since the diode current has exponential dependency on V/V_T, when you take the derivative you will end up with the diffusion capacitance Cd = I * tau_T / V_T as we derived in class.
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