Current Components in P-N Diode

CURRENT COMPONENTS IN P-N DIODE

When a PN diode is forward biased, the depletion region is very small and large forward current flows. The holes from p side are injected to N side and electrons from N-side are injected to P side. Thus current carried by electrons carried by electrons in P-side. Thus current carried by electrons in P- side, I_np is due to minority carriers and it decreases exponentially with respect to distance measured from the junction.

Similarly the current due to holes diffusing from p-side to n-side is due to minority carriers and decreases exponentially with respect to distance measured from junction. This current is represented as I_pn. Assume the doping concentrations of two regions are not equal i.e., acceptor concentration (N_A) is very greater than donor concentration (N_D). Hence hole diffusion current is also very much higher than the electron diffusion current.

The distance is denoted by x, then at the junction x = 0, the electrons constituting current in p-side is I_np (0) and the holes crossing the junction from p-side to n-side is I_pn(0). Thus, the current at the junction is the total current I flowing through the circuit and is given as

On p-side the current component is due to holes on p-side which are majority charge carriers. It is denoted by I_pp(x). The total current in p-side is written as

Where I_np(x) = current due to electrons in p-side. Similarly, the electron on n- side, which are majority carriers, constitute current component I_nn(x). Thus the total current is the sum of current components due to electrons in n-side and holes in n- side.

Where I_pn(x) - current due to holes in n-side.

Fig.1.20 shows the current components as a function of x. The current I_pp decreases towards the junction and current I_nn also decreases towards the junction. I_pn and I_np decreases towards the junction. I_pn and I_np decreases exponentially with respect to distance from the junction.

RESISTANCE OF PN DIODE

The dynamic resistance is the reciprocal of the slope of V-I characteristics of diode. This can be written as

We know that, the current is given by

Differentiate with respect to V

We know that

Substitute (2) in (1)

TEMPERATURE DEPENDENCE OF PN DIODE

As mentioned earlier the reverse saturation current I_o depends on temperature and V_T is also temperature dependence. Thus the diode current I is also temperature dependent.

This dependence of I_o on temperature T is given by

When temperature is increased, the diode current will also increase. So, the voltage should be reduced to keep diode current constant.

dV/dT decreases with increase in temperature. Hence the sign is negative.

The reverse saturation current I_o increases by 11% per degree rise in temperature for Germanium and silicon.

Thus the reverse saturation current approximately doubles for every 10°C rise in temperature.

At temperature T₂,