Semiconductor Device

P-N Junction

When a P-type semiconductor is suitably joined to an N-type semiconductor, a common junction is produced which is known as a P-N junction. Practically, the two regions are created side by side in the same piece of semiconductor. P-N junction is a fundamental building block of many other semiconductor electronic devices such as diodes, transistors, solar cells, light-emitting diodes (LED), and integrated circuits.

P-N Junction Diode (Semiconductor Diode)

When a pure form of semiconductor is doped with P-type material at one end and N-type material at another end, the resulting semiconductor material is called a P-N junction diode. It consists of two electrodes i.e. anode and cathode. The anode refers to the P-type region and the cathode refers to the N-type region. It acts like a one-way conductor. The arrowhead shown in the circuit symbol points to the direction of the current flow when it is forward-biased.

Depletion layer and barrier potential in P-N junction diode

In the P-N junction diode, the holes are the majority of charge carriers in the P-region and electrons in the N-region. As soon as a P N junction is formed, the charge carriers diffuse from a region of high concentration to a region of low concentration. So, the holes diffuse from P to the N region and electrons from N to the P region. The holes and electrons recombine and terminate their existence at the junction as shown in the figure. The recombination of mobile holes and free electrons produce the narrow region at the junction is called the depletion layer. The depletion layer is the charge empty region. But it contains fixed positive and negative ions called dipoles. The thickness of the depletion layer is of the order of a few microns. The potential developed across the P-N junction due to the migration of electrons into the P-region and holes into the N-region is called barrier potential(VB). The barrier potential is about 0.3 V for germanium and 0.7 V for silicon at room temperature. The barrier potential of a P- N junction depends on the type of semiconductor material, amount of doping, and temperature.

Biasing of P-N junction Diode
The process of providing potential difference across the P-N junction diode is called biasing of the diode. There are two types of biasing. They are:

1] Forward biasing
2] Reverse biasing


1] Forward Biasing

A diode is said to be forward-biased if its P-side is connected to the positive terminal and its N-side to the negative terminal of a battery (source of potential difference). A forward-biased diode is shown in the figure. In this biasing, the applied forward potential acts against the field due to the potential barrier, and the depletion layer becomes thin.

Figure: Forward biasing of P-N diode

When the diode is forward-biased:
i) The width of the depletion layer decreases.
ii) The barrier potential decreases (V − VB).
iii) The flow of current inside the diode is due to the majority of charge carriers.
iv) The diode offers very low resistance called forward resistance.
v) The diode acts as a closed switch.

2] Reverse Biasing

A diode is said to be reverse-biased if its P-side is connected to the negative terminal and its N-side is connected to the positive terminal of the external voltage source. A reverse-biased diode is shown in the figure below. In this biasing, the applied reverse voltage acts in the same direction of the field due to the potential barrier.

Figure: Reverse biasing of P-N diode

When the diode is reverse-biased,
1] The width of the depletion layer increases.
2] The barrier potential increases (V + VB).
3] The flow of negligible current inside the diode is due to minority charge carriers.
4] The diode offers very high resistance called reverse resistance.
5] The diode acts as an open switch.

Characteristics of a Junction Diode

The graphical relationship between current and potential differences across the junction diode is called the characteristics of a diode. It is also known as the I-V characteristics of the junction diode. These are of two types.

Forward Bias Characteristics

It is the graph showing the variation of forward current with forward voltage. The figure shows the circuit arrangement of the forward characteristics of a diode. The voltmeter (V) measures the potential difference across the diode and the milli-ammeter (mA) measures the current flow through it. The forward voltage 𝑉𝐹 across the diode is varied with the help of rheostat 𝑅ℎ and the corresponding diode current 𝐼𝐹 is noted. A graph of 𝑉𝐹 versus 𝐼𝐹 is shown in the figure. This graph represents the forward characteristics of the diode.

When the applied voltage is small in forward biasing, a small current flows through the diode. As the voltage is increased, the current through the diode is also increased. The forward voltage at which the diode current increases rapidly or sharply is called knee voltage 𝑉𝐾. The knee voltage is equal to the barrier potential of the diode which is about 0.7 V for silicon and 0.3 V for germanium.

Reverse Bias Characteristics

It is the graph showing the variation of reverse current with reverse voltage. The circuit diagram for the reverse characteristics of the diode is shown in the figure. The voltmeter (V) measures the reverse voltage and the micro-ammeter (μA) measures the reverse current flowing through the diode. The reverse voltage across the diode 𝑉𝑅 is varied with the help of rheostat 𝑅ℎ and the corresponding reverse current 𝐼𝑅 is noted by the micro-ammeter. A graph of 𝐼𝑅 versus 𝑉𝑅is shown in the figure, which represents the reverse characteristics of the diode.

When the diode is reverse biased, the majority charge carriers are blocked, and only a small current flows through the diode due to minority carriers. As the high reverse voltage is applied, the diode current begins to increase sharply. The reverse voltage at which the diode current starts to increase sharply is called break-down voltage 𝑉𝑏. The breakdown voltage depends on the density of dopants and the thickness of the depletion region.

Rectification and Rectifier
The process of converting ac into dc is called rectification. The device used for rectification is called a rectifier. When the P-N junction diode is forward-biased, it offers low resistance, and current flows through it. But when reverse biased, it offers high resistance, and no current flows through it. This unidirectional current conducting property of a junction diode is
used in rectification. There are two types of rectifiers.

1] Half Wave Rectifier
A device that rectifies only half a cycle of ac into dc is called half wave rectifier. The half-wave rectifier is shown in the figure. It consists of a transformer, a diode D and a load resistance 𝑅𝐿. The primary coil of a transformer is connected with ac supply and the secondary coil is connected to the load resistance through the diode.

During the positive half cycle of input voltage 𝑉𝑖𝑛, the diode is forward biased and it conducts. The current flows through the circuit and output voltage 𝑉0 is dropped across 𝑅𝐿.

During the negative half cycle of 𝑉𝑖𝑛, the diode is reverse biased and no voltage is dropped across 𝑅𝐿. The current flows during the positive half cycle and blocks during the negative half cycle of input voltage. Hence, a half wave of input ac is rectified into pulsating dc voltage as shown in the figure.

2] Full Wave Rectifier
A device that converts the full cycle of ac into dc is called a full wave rectifier. There are two types of full-wave rectifiers. They are:

i) Center Tapped Full Wave Rectifier and
ii) Bridge Rectifier

i) Center Tapped Full Wave Rectifier

The circuit diagram for the full wave rectifier is shown in the figure. It consists of a transformer, two diodes, and load resistance. The diodes D1 and D2 are connected to the center-tapped secondary coil of a transformer through the load resistance RL. The diodes are connected in such a way that they conduct during alternate half cycles of the input or supply voltage.

During the positive half cycle of input voltage Vin, the diode 𝐷1 is forward biased and D2 is reverse biased. So, D1 conducts and D2 is off.

During the negative half cycle, D2 is on and D1 is off. In both half cycles, unidirectional current flows through RL. The half-wave is Rectified by D1 and the next half-wave is rectified by D2. Thus the combination of D1 and D2 works as a full-wave rectifier. The input and output of the center are tapped.

ii) Bridge Rectifier
A full wave rectifier in the form of a bridge is called a bridge rectifier. It converts the full cycle of ac into dc. The full wave bridge rectifier circuit is shown in the figure. It consists of four diodes, and, a transformer, and a load resistance. The diodes are connected in such a way that two diodes conduct during the positive half cycle and the other two diodes conduct during the negative half cycle.
During the positive half cycle, terminal A of the secondary coil is positive and B is negative. The diodes become forward-biased while the diodes are reverse-biased. So, the current flows along ACDEFGB producing drop across.
During the negative half cycle, the secondary terminal B becomes positive and A is negative. The diodes and are forward biased where as and are reverse biased and the current flows along BGDEFCA and produces a drop across. In both half cycles, unidirectional current flows through. In this way, the bridge rectifier acts as a full wave rectifier. The input and output of the bridge rectifier are shown in the figure.

Fig: a) Circuit diagram of bridge rectifier b) Input and output voltage of bridge rectifier

Filter Circuit

Generally, a rectifier is needed to produce pure dc. However, the output of the rectifier is pulsating dc with both ac and dc components. The ac component in the output is undesirable called the ripple factor and must be removed from the output of the rectifier. A circuit that removes ac component in the output of the rectifier but allows the dc component to reach the load is called a filter circuit. The filter circuits convert pulsating dc into steady dc from the output of the rectifier. The filter circuit is connected between the rectifier and the load as shown in the figure. It consists of passive elements like a capacitor, inductor, or their combination. The most common filter circuits are as follows.

a) Capacitor filter
b) Inductor filter
c) LC filter
d) π- filter (CLC filter)

a) Capacitor filter

The circuit diagram for the capacitor filter is shown in the figure. It consists of a capacitor C connected across the rectifier output and load resistance. The capacitor offers high resistance for dc and low resistance for ac components. Thus most of the ac components pass through the capacitor through the ground whereas the dc component drops across the load. It is found that very little ripple is left in the output. It produces a dc output voltage equal to the peak value of the rectified voltage. This type of filter is widely used in power supplies.

Fig: Capacitor filter

Note: Reactance is a form of opposition that electronic components exhibit to the passage of ac because of capacitance or inductance. It is denoted by X.

b) Inductor filter

The circuit diagram for the inductor filter is shown in the figure. It consists of an inductor of inductance L connected in series with the load resistance. The inductor offers high resistance to ac but very low resistance to dc. The inductor blocks most of the ac components and very little ripple factor is left in the output.

c) LC filter (choke input filter)

The LC filter is shown in the figure.

It consists of an inductor of inductance L is connected in series with the output of the rectifier and the capacitor of capacitance C is connected parallel to the load resistance. In this filter, the inductive reactance is much greater than the capacitive reactance. The inductor offers high resistance to ac and low resistance to dc components. Therefore, it blocks ac and allows dc to pass through it. On the other hand, the capacitor offers high resistance to dc and low resistance to ac components. Therefore a capacitor bypasses the ac which is not blocked by the inductor. Thus the output of this filter has very little ripple and good regulation. The LC filter is used as a switching regulator in computers, monitors, and different variety of equipment

d) π-filter
The circuit diagram for the π-filter is shown in the figure. It consists of a filter capacitor C1 connected across the rectifier output, an inductor L in series, and another capacitor C2 connected across the load resistance. The filtering action of each component of this filter circuit is described below.
Capacitor C1
It offers low resistance to ac component and infinite resistance to the dc component of the rectifier output. So, the capacitor C1 bypasses an appreciable amount of ac to the ground whereas the dc component flows towards the inductor.
Inductor L
The inductor offers very high resistance to ac and very low resistance to dc components of rectifier output. Therefore, it blocks ac and allows dc to pass through it.
Capacitor C2
Similarly, the capacitor C2 bypasses the ac component of the rectifier output which is not blocked by the inductor, and due to high resistance, the dc component flows towards load resistance.
In this way, the π-filter blocks ac components to the output and the output nearly contains dc components. The π-filter circuit is better than other filter circuits. This filter is used in low-current equipment.