PN Junction Diodes

PN JUNCTION DIODES

PN Junction Diode

It is a two-terminal device consisting of a PN junction formed either in Ge (or) Si crystal. It's circuit symbol is shown in Figure 3.26 (a). The P and N regions are referred to as anode and cathode respectively.

In Figure 3.26 (b) shows the symbol of PN junction diode. The commercially available diode, usually have some notations to identify the P and N terminal (or leads). The standard notation consists of type numbers proceeded by IN, such as IN240 and IN1250. Here 240 and 1250 correspond to colour band.

The diode shown in figure 3.27 has a colour band located near one of the ends. The end, which is near the colour band, is identified as cathode (K). And other end, obviously, is the anode (A).

Characteristic of a PN Junction

It is very important to known now a device responds (or behaves). When it is connected in an electrical circuit. This information is obtained an electrical of a graph known as its volt-ampere or V-I characteristics.

It is a graph between the voltage applied across the terminals of a device and the current that flows through it. Figure 3.28 shows the V-I characteristics of a typical PN junction diode with respect to break down voltage (V_BR). It may be noted that the compete graph can be divided into two parts namely

(i) Forward characteristics

(ii) Reverse characteristics

Forward Characteristics

Figure 3.29 shows the circuit arrangement for obtaining the forward characteristics of a diode. In this circuit, the diode is connected to DC through a potentiometer (P) and a resistance (R).

The potentiometer helps in varying the voltage applied across the diode. The resistance (R) is included in the circuit, so as to limit the current through the diode.

The positive terminal of the voltage source connected to the anode of a diode and negative terminal to the cathode. Hence the diode is forward-biased. Let us gradually increase the voltage in small steps of about 0.1V and record the corresponding values of diode current. Plot a graph with voltage across diode on X-axis and diode current on Y-axis. We shall obtain a curve OAB as shown in figure 3.29 (b). the curve OAB is called the forward characteristic of a silicon PN junction diode.

The voltage, at which the diode starts conducting, is called a knee voltage, barrier voltage cut-in voltage (or) threshold voltage. The knee voltage is designated either by V_K, V_B (or) V_r. Its value is equal to 0.7 V for silicon and 0.3 V for germanium.

Reverse Characteristics

The circuit arrangement for obtaining the reverse characteristic of a diode is as shown in fig.3.30(a). here change made the diode terminal interchange.

It may be noted that negative terminal of the voltage source is connected to the anode of a diode and positive terminal to the cathode. Hence, the diode is reverse biased. The applied reverse voltage is gradually increased above zero in suitable steps and the values of diode current are recorded at each step. Plot the graph with reverse voltage on X-axis and reverse current on Y-axis. The curve OCD as shown in Figure 3.30 (b). The curve OCD is called reverse characteristic of the diode. The reverse characteristics indicated that. When the applied voltage is below the break down voltage (V_BR). The diode current is small and remains constant. This value of current is called reverse saturation current (I_o). It is of the order of nanoamps for silicon diode and micro amps for germanium diode.

When the reverse voltage is increased to a sufficient large value, the diode reverse current increases as rapidly as shown by the curve CD in the Figure 3.30 (b). The applied reverse voltage, at which this happens is known as break down voltage (V_BR) of a diode.

Diode Current Equation

The mathematical equation, which describes the forward and reverse characteristics of a semiconductor diode is called the diode current equation

Let I = Forward diode current

I_o = Reverse diode current

V = External voltage.

η = constant,

whose value = 1 for geranium

Whose value = 2 for silicon

V_T = volt - equivalent of temperature

The current equation for a forward biased diode is given by the relation

………(1)

We know that room temperature V, is equal to 26 mV, ther equation (1) becomes

Diode current at (or) below the knee of the curve,

For germanium (η = 1)

= I_o (e^40V -1)

and for silicon (η = 2)

= I_o (e^20v -1)

Example: 1

Find the diode current for the germanium diode for the forward bias voltage of 0.22V at room temperature of 25°C with reverse saturation current equation 1 mA take η = 1.

Solution:

V_F = 0.22 V ,T = 25° ,C = 298°K , I_o = 1 mA, η = 1

V_T = T /11600 = 298/11600 = 25.7 mV

I = (1 × 10⁻³) (e^8.56 - 1)

I = 5.22 Amps

Example: 2

Determine the current flowing, when 0.1 V forward biased is applied at room temperature. The current at room temperature is 2 × 10^-7 Amps

Solution:

1_o = 2×10^-7, V_F = 0.1 volts.

I = I_o (e^40V_F -1)

= 2 × 10⁻⁷ (e^40×0.1 -1)

I = 10.7 μΑ

Applications of PN Junction Diode

(i) As rectifiers (or) power diodes in DC power supply

(ii) As signal diodes in communication circuit

(iii) As zener diode in voltage stabilizing circuit

(iv) As varactor diode in radio and TV receivers

(v) As a switch in logic circuits used in computers.