14.8 SPECIAL PURPOSE p-n JUNCTION DIODES

. SPECIAL PURPOSE p-n JUNCTION DIODES In the section, we shall discuss some devices which are basically junction diodes but are developed for different applications. . .

Zener diode It is a special purpose semiconductor diode, named after its inventor C. Zener. It is designed to operate under reverse bias in the breakdown region and used as a voltage regulator. The symbol for Zener diode is shown in Fig.

. (a). Zener diode is fabricated by heavily doping both p-, and n- sides of the junction. Due to this, depletion region formed is very thin (< – m) and the electric field of the junction is extremely high (~ × V/m) even for a small reverse bias voltage of about 5V.

The I-V characteristics of a Zener diode is shown in Fig. . (b). It is seen that when the applied reverse bias voltage(V) reaches the breakdown voltage (Vz) of the Zener diode, there is a large change in the current.

Note that after the breakdown voltage Vz, a large change in the current can be produced by almost insignificant change in the reverse bias voltage. In other words, Zener voltage remains constant, even though current through the Zener diode varies over a wide range. This property of the Zener diode is used for regulating supply voltages so that they are constant. Let us understand how reverse current suddenly increases at the breakdown voltage.

We know that reverse current is due to the flow of electrons (minority carriers) from p n and holes from n p. As the reverse bias voltage is increased, the electric field at the junction becomes significant. When the reverse bias voltage V = Vz, then the electric field strength is high enough to pull valence electrons from the host atoms on the p-side which are accelerated to n-side. These electrons account for high current observed at the breakdown.

The emission of electrons from the host atoms due to the high electric field is known as internal field emission or field ionisation. The electric field required for field ionisation is of the order of V/m. FIGURE . (a) A full-wave rectifier with capacitor filter, (b) Input and output voltage of rectifier in (a).

FIGURE . Zener diode, (a) symbol, (b) I-V characteristics. EXAMPLE . FIGURE .

Zener diode as DC voltage regulator (to be corrected). Zener diode as a voltage regulator We know that when the ac input voltage of a rectifier fluctuates, its rectified output also fluctuates. To get a constant dc voltage from the dc unregulated output of a rectifier, we use a Zener diode. The circuit diagram of a voltage regulator using a Zener diode is shown in Fig.

. . The unregulated dc voltage (filtered output of a rectifier) is connected to the Zener diode through a series resistance Rs such that the Zener diode is reverse biased. If the input voltage increases, the current through Rs and Zener diode also increases.

This increases the voltage drop across Rs without any change in the voltage across the Zener diode. This is because in the breakdown region, Zener voltage remains constant even though the current through the Zener diode changes. Similarly, if the input voltage decreases, the current through Rs and Zener diode also decreases. The voltage drop across Rs decreases without any change in the voltage across the Zener diode.

Thus any increase/ decrease in the input voltage results in, increase/ decrease of the voltage drop across Rs without any change in voltage across the Zener diode. Thus the Zener diode acts as a voltage regulator. We have to select the Zener diode according to the required output voltage and accordingly the series resistance Rs. Example .

In a Zener regulated power supply a Zener diode with VZ = . V is used for regulation. The load current is to be . mA and the unregulated input is .

V. What should be the value of series resistor RS? Solution The value of RS should be such that the current through the Zener diode is much larger than the load current. This is to have good load regulation.

Choose Zener current as five times the load current, i.e., IZ = mA. The total current through RS is, therefore, mA. The voltage drop across RS is . – .

= . V. This gives RS = .0V/( × – ) A = <. The nearest value of carbon resistor is <.

So, a series resistor of < is appropriate. Note that slight variation in the value of the resistor does not matter, what is important is that the current IZ should be sufficiently larger than IL. . .

Optoelectronic junction devices We have seen so far, how a semiconductor diode behaves under applied electrical inputs. In this section, we learn about semiconductor diodes in which carriers are generated by photons (photo-excitation). All these devices are called optoelectronic devices. We shall study the functioning of the following optoelectronic devices: (i) Photodiodes used for detecting optical signal (photodetectors).

(ii) Light emitting diodes (LED) which convert electrical energy into light. (iii) Photovoltaic devices which convert optical radiation into electricity (solar cells).