Reverse Bias :

When the diode is reverse biased then the depletion region width increases, majority carriers move away from the junction

and there is no flow if current due to majority carriers but there are thermally produced electron hole pair also. If these

electrons and holes are generated in the vicinity of junction then there is a flow of current. The negative voltage applied to the

diode will tend to attract the holes thus generated and repel the electrons. At the same time, the positive voltage will attract the

electrons towards the battery and repel the holes. The electron in the p-material and hole in the n-material are being forced to

move forward each other and will probably combine. This will cause current to flow in entire circuit. This current is usually very

small (interms of micro ampher to nano amphere). Since this current is due to minority carriers and these number of minority

carriers are fixed at a given temperature therefore, the current is almost constant known as reverse saturation current Ico. In

actual diode, the current is not almost constant but increases slightly with voltage. This is due to surface leakage current. The

surface of diode follows ohmic law (V=IR). The resistance under reverse bias condition is very high 100k to mega ohms.

When the reverse voltage is increased, they and certain value, then breakdown to diode takes place and it conducts heavily.

This is due to avalanche or zener breakdown. Fig4

Fig.4

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Forward bias :

When the diode is forward bias, then majority carriers are pushed towards junction, when they collide and recombination takes

place. Number of majority carriers are fixed in semi-conductor. Therefore as each electron is eliminated at the junction, a new

electron must be introduced, this comes from battery. At the same time, one hole must be created in p-layer. This is formed by

extracting one electron from p-layer. Therefore, there is a flow of carriers and thus flow of current.

Space Charge Capacitance on Transition Capacitive CT :

Reverse bias causes majority carriers to move away from the junction, thereby creating more ions. Hence the thickness of

depletion region increases. This region behaves as the dielectric material used for making capacitors. The p-type and n-type

conducting on each side of dielectric act as the plate. The incremental capacitance CT is defined by

Where, dQ is the increase in charge caused by a change dv in voltage.

i = dQ / dt

i = CT dv / dt

CT is not constant, it depends upon applied voltage, therefore it is defined as dQ/dV. When p-n junction is forward biased,

then also a capacitance is defined called diffusion capacitance CD (rate of change of injected charge with voltage) to take into

account the time delay in moving the charges across the junction by the diffusion process. Therefore, it cannot be identified in

terms of a dielectric and plates as space charge capacitance. It must be considered as a fictitious element that allow us to

predict time delay. If the amount of charge to be moved across the junction is increased, the time delay is greater, it follows

that diffusion capacitance varies directly with the magnitude of forward current.

Real Diode: Small Signal Operation: (Load Line)

Consider the diode circuit shown in Fig.5

This equation involves two unknown and can not be solved. The other equation interms of these two variables, is given by the

static characteristic. The straight line represented by this above equation is known as the load line. The load line pass

through two points, I = 0 , VD = V and VD = 0, I = V / RL. The slope of this line is equal to 1/ RL. The point of inter–section of

straight line and diode characteristic gives the operating point. Fig.6

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V=1V, RL=10ohm. (Fig7).

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Therefore, as the diode voltage varies, diode current also varies, sinusoidally (Fig 8).

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certain applications only a.c. equivalent circuit is required. Since only a.c. response of the circuit is considered d.c. Source

is not shown (Fig 9). The resistance rf represents the dynamic resistance or a.c. resistance of the diode .It is obtained by

taking the ratio of at operating point.