Characteristics of a junction diode

Characteristics of a junction diode

Forward characteristics

It is the study of the variation in current through the diode with respect to the applied voltage across the diode when it is forward biased.

The p-n junction diode is forward biased as shown in Figure 9.14(a). An external resistance (R) is used to limit the flow of current through the diode. The voltage across the diode is varied by varying the biasing voltage across the dc power supply. The forward bias voltage and the corresponding forward bias current are noted. A graph is plotted by taking the forward bias voltage (V) along the x-axis and the current (I) through the diode along the y-axis. This graph is called the forward V-I characteristics of the p-n junction diode and is shown in Figure 9.14(b). Three inferences can be brought out from the graph:

(i) At room temperature, a potential difference equal to the barrier potential is required before a reasonable forward current starts flowing across the diode. This voltage is known as threshold voltage or cut-in voltage or knee voltage (V_th). It is approximately 0.3 V for Germanium and 0.7 V for Silicon. The current flow is negligible when the applied voltage is less than the threshold voltage. Beyond the threshold voltage, increase in current is significant even for a small increase in voltage.

(ii) The graph clearly infers that the current flow is not linear and is exponential. Hence it does not obey Ohm’s law.

(iii) The forward resistance (r_f) of the diode is the ratio of the small change in voltage (∆V ) to the small change in current (∆I ), r_f =  âˆ†V/∆I.

(iv) Thus the diode behaves as a conductor when it is forward biased.

However, if the applied voltage is increased beyond a rated value, it will produce an extremely large current which may destroy the junction due to overheating. This is called as the breakdown of the diode and the voltage at which the diode breaks down is called the breakdown voltage. Thus, it is safe to operate a diode well within the threshold voltage and the breakdown voltage.

Reverse characteristics

The circuit to study the reverse characteristics is shown in Figure 9.15(a). In the reverse bias, the p-region of the diode is connected to the negative terminal and n-region to the positive terminal of the dc power supply.

A graph is drawn between the reverse bias voltage and the current across the junction, which is called the reverse characteristics of a p-n junction diode. It is shown in Figure 9.15(b). Under this bias, a very small current in µA, flows across the junction. This is due to the flow of the minority charge carriers called the leakage current or reverse saturation current. Besides, the current is almost independent of the voltage. The reverse bias voltage can be increased only up to the rated value otherwise the diode will enter into the breakdown region.

The forward and reverse charactristics are given in one graph as shown in Figure 9.16.

Ideal diode: It acts like a conductor when it is forward biased. When it is reverse biased, it acts like an insulator. The barrier potential is assumed to be zero and hence it behaves like a resistor.

EXAMPLE 9. 1

An ideal diode and a 5 Ω resistor are connected in series with a 15 V power supply as shown in figure below. Calculate the current that flows through the diode.

Solution

The diode is forward biased and it is an ideal one. Hence, it acts like a closed switch with no barrier voltage. Therefore, current that flows through the diode can be calculated using Ohm’s law.

V = IR

I =V/R = 15/5 = 3 A

EXAMPLE 9. 2

Consider an ideal junction diode. Find the value of current flowing through AB is

Solution

The barrier potential of the diode is neglected as it is an ideal diode.

The value of current flowing through AB can be obtained by using Ohm’s law

I = V/R = 3 - (-7) / 1× 10³ /10³ = 10 =10^-2 A = 10mA