Maharashtra Board Class 12 Physics Chapter 16 Semiconductor Devices PDF Download

16. Semiconductor Devices

Can you recall?

1. What is a p-n junction diode?

2. What is breakdown voltage and knee voltage?

3. What is a forward and reverse biased diode?

16.1 Introduction

In XI Std. we have studied a p-n junction diode. When the diode is forward biased, it behaves as a closed switch and current flows in the diode circuit. When the diode is reverse biased, it behaves as an open switch and no current flows in the diode circuit. This switching action of a diode allows it to be used as a rectifier.

Generation of AC at a power station is more cost effective than producing DC power. The transmission of AC power is also more economic than transmitting DC power. This AC voltage varies sinusoidally. In India, it is 230 V and has a frequency of 50 Hz. There are many electronic gadgets such as a TV, or a mobile charger which require a DC supply. Therefore, it is necessary to convert AC voltage into a DC voltage. The AC mains voltage is rectified by using junction diodes to obtain a DC voltage. In this chapter, we will study the use of diode as a rectifier and also different types of rectifiers. We will also study filters which remove the AC component from the rectified voltage and voltage regulators which provide the required DC voltage.

Working of a simple rectifier circuit is shown in Fig. 16.1. The AC mains supply is connected to the primary of a transformer and its secondary is connected to a rectifier circuit. The AC voltage shown as a sinusoidal wave from the secondary of the transformer is converted into a DC voltage by a diode rectifier. This is shown next as a pulsating wave (b). The output of the rectifier contains some AC component. This AC component in the DC output of a rectifier is called ripple and is shown at the output of the rectifier. It is removed by using a filter circuit. The output of the filter circuit is almost a pure DC. (It can still contain some ripple). The voltage regulator circuit shown after the filter restricts the output voltage to the desired value. The output voltage at this stage is a across pure DC (d).

16.2 p-n Junction Diode as a Rectifier

An AC voltage varies sinusoidally, i.e. its value and direction changes in one cycle. A rectifier converts this bidirectional voltage or current into a unidirectional voltage or current. The conversion of AC voltage into a DC voltage is called rectification. An electronic circuit which rectifies AC voltage is called rectifier. There are two types of rectifier circuits, 1) half wave rectifier and 2) full wave rectifier.

16.2.1 Half Wave Rectifier

A simple half wave rectifier circuit using only one diode is shown in Fig. 16.2.

The secondary coil AB of a transformer is connected in series with a diode D and the load resistance RL. The use of transformer has two advantages. First, it allows us to step up or step down the AC input voltage as per the requirement of the circuit, and second it isolates the rectifier circuit from the mains supply to reduce the risk of electric shock. The AC voltage across the secondary coil AB changes its polarities after every half cycle. When the positive half cycle begins, the voltage at the point A is at higher potential with respect to that at the point B, therefore, the diode (D) is forward biased. It conducts (works as a closed switch) and current flows through the circuit. When the negative half cycle begins, the potential at the point A is lower with respect to that at the point B and the diode is reverse biased, therefore, it does not conduct (works as an open switch). No current passes through the circuit. Hence, the diode conducts only in the positive half cycles of the AC input. It blocks the current during the negative half cycles. The waveform for input and output voltages are shown in Fig. 16.3. In this way, current always flows through the load RL in the same direction for alternate positive half cycles.

Hence a DC output voltage obtained across RL is in the form of alternate pulses.

Teacher's Note

In India, we use 230V AC at 50 Hz in our homes. This AC voltage needs to be changed to DC for our mobile chargers and electronic devices. A rectifier diode does this job by allowing current to flow only in one direction.

Exam Trick

Remember: In a half wave rectifier, only half of the AC cycle is used. Just like using only one half of a football field, you get less output. That is why it is called "half" wave rectifier.

Points to Remember

A diode acts like a closed switch when forward biased and allows current to flow.
A diode acts like an open switch when reverse biased and blocks current.
In a half wave rectifier, only positive half cycles produce output.
The output of a rectifier has ripple, which is unwanted AC component.
A filter circuit removes the ripple from the rectifier output.

16.2.2 Full Wave Rectifier

As discussed in the previous section, the output of a half wave rectifier is available only in alternate positive half cycles of the AC input. All negative half cycles are lost and the efficiency of a half wave rectifier is very poor. Therefore, a rectifier circuit using two diodes is more useful.

In a full wave rectifier, current flows through the load in the same direction during both the half cycles of input AC voltage. This is because, the full wave rectifier circuit consists of two diodes conducting alternately. Figure 16.4 shows typical circuit of a full wave rectifier. The circuit consists of a centre tapped transformer and diodes D1 and D2.

The diodes D1 and D2 are connected such that D1 conducts in the positive half cycle and D2 conducts in the negative half cycle of the input voltage. During the positive half cycle of the input voltage, the point A is at a higher potential than that of the point B and the diode D1 conducts. The current through the load resistance RL follows the path APQRC as shown in Fig. 16.4. During the negative half cycle of the input voltage, point B is at higher potential than point A and the diode D2 conducts. The current through the load resistance RL follows the path BPQRC. Thus, the current flowing through the load resistance is in the same direction during both the cycles.

The input and output waveforms of a full wave rectifier are shown in Fig. 16.5. First waveform is input AC. The second wave form shows the output when the diode D1 conducts and the third waveform shows the output when diode D2 conducts. The fourth waveform shows the total output waveform of the full wave rectifier.

The maximum efficiency of a full wave rectifier is 81.2% and the maximum efficiency of a half wave rectifier is 40.6%. It is observed that the maximum efficiency of a full wave rectifier is twice that of half wave rectifier.

A full wave rectifier utilises both half cycles of AC input voltage to produce the DC output

Example 16.1: If the frequency of the input voltage 50 Hz is applied to a (a) half wave rectifier and (b) full wave rectifier, what is the output frequency in both cases?

Solution:

(a) The output frequency is 50 Hz because for one AC input pulsating we get one cycle of DC.

(b) The output frequency is 100Hz because for one input ac cycle we get two cycles of pulsating DC.

Advantages of a full wave rectifier

1) Rectification takes place in both the cycles of the AC input.

2) Efficiency of a full wave rectifier is higher than that of a half wave rectifier.

3) The ripple in a full wave rectifier is less than that in a half wave rectifier.

Teacher's Note

A full wave rectifier is like using both sides of a coin, while a half wave rectifier uses only one side. A full wave rectifier gives more output and is better, just like a complete water tap gives more water than a half-open tap.

Exam Trick

Remember: Full wave = both halves used = better output. Half wave = one half lost = poor output. Full wave efficiency is 81%, half wave is only 41%. The difference is almost double!

Points to Remember

A full wave rectifier uses two diodes and conducts during both half cycles.
The output frequency of a full wave rectifier is twice the input frequency.
Full wave rectifier has higher efficiency than half wave rectifier.
Centre-tapped transformer is needed for full wave rectification.
Full wave rectifier produces less ripple than half wave rectifier.

16.2.3 Ripple Factor

The output of a rectifier consists of a small fraction of an AC component along with DC called the ripple. This ripple is undesirable and is responsible for the fluctuations in the rectifier output. Figure 16.6 (a) shows the ripple in the output of a rectifier.

The effectiveness of a rectifier depends upon the magnitude of the ripple component in its output. A smaller ripple means that the rectifier is more effective.

The ratio of root mean square (rms) value of the AC component to the value of the DC component in the rectifier output is known as the ripple factor, i.e.,

Ripple factor = rms value of AC component / value of DC component

16.2.4 Filter Circuits

The output of a rectifier is in the form of pulses as shown in the fourth waveform in Fig 16.5. The output is unidirectional but the output does not have a steady value. It keeps fluctuating due to the ripple component present in it. A filter circuit is used to remove the ripple from the output of a rectifier.

A filter circuit is a circuit which removes the AC component or the ripple from a rectifier output and allows only the DC component.

Teacher's Note

Think of a filter like a tea strainer. Just as a strainer removes tea leaves and lets only tea pass through, a filter removes the AC ripple and lets only DC voltage pass to the device. This protects your mobile phone and other electronics.

Exam Trick

Remember: Filter = removes ripple = smooth DC output. Like filtering water to remove dirt, we filter rectified voltage to remove ripple. Cleaner output means safer electronics!

Points to Remember

Ripple is the unwanted AC component in the DC output of a rectifier.
A capacitor is the simplest type of filter used in power supplies.
Filters remove ripple and smooth the DC output voltage.
A smaller ripple factor means a more effective rectifier.
Filter circuits are essential for protecting electronic devices from voltage fluctuations.

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