Current Voltage Characteristics Of A Diode
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Current Voltage Characteristics Of A Diode

Assume that the diode in figure 9.1 requires a minimum current of 1 mA to be above the knee of its i-v characteristic. Let the offset voltage of the diode, Vγ = 0.7 V.
(a) What is an i-v characteristic of a diode?
(b) What is the knee of the i-v characteristic?
(c) What value of the resistance R, will establish a current of 5 mA in the circuit?
(d) Given the value of R determined in (c), calculate the minimum voltage that the circuit must provide in order to maintain diode current above the knee.

The strings: S7P2A21 Problems (a) and (b) are identity problems - S7P2A21 (identity-physical)
Problem (c) is a force problem - S7P3A31 (force - pull)
Problem (d) is a force problem - S7P3A32 (force - push)

The math:
Pj Problem of Primary Interest (problem (d)) is of type force (force- push).

(a) i-v characteristic curve of a diode is the graph of the Diode Equation:
iD = I0[e(qvD/kT) - 1] (Diode equation)
Io is the reverse saturation current
Where q is the charge of one electron
VD is the voltage across the pn junction of a diode
k is Boltzman's constant = 1.381 X 10-23 J/K
T is the temperature of the material in degree kelvins. kT/q is a constant at a given temperature = 25 mV at room temperature.

(b) A diode has an offset voltage (contact potential). A forward bias diode will not conduct current when the voltage across it is less or equal to the offset voltage. The part of a diode's i-v characteristic where the diode starts to conduct current is the knee of the diode.

(c) V = 5 V, I = 5 mA = 0.005 A, offset voltage = 0.7 V
So, R = (V - offset voltage)/I = (5 - 0.7)/0.005 = 860 Ω

(d) Vmin > 0.001(860) + 0.7 = 1.56 V.

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