Semiconductor Devices

P-N Junction Diode

It is an electronic device which is formed after joining p-type and n-type semiconductors. Its electrical symbol is:

When p-n junction is formed, free electron near the junction of p-type into holes and a layer is formed near the junction called depletion layer. When electron diffuse into holes +ve and −ve ions develop a potential called barrier potential. This potential prevents further diffusion of electrons into holes.

Biasing of Diode

The process of connecting diode to the battery is called biasing of diode.

1) Forward Biasing

• P-side of diode is connected to +ve terminal of battery.
• Depletion layer decreases and barrier potential decreases.
• Diode acts as closed switch.
• Current flows through the diode.

2) Reverse Biasing

• P-side of diode is connected to −ve terminal of battery.
• Depletion layer increases and barrier potential increases.
• Diode acts as open switch.
• Current does not flow through the diode.

Characteristics of Diode

The graphical relation between voltage and current passing through the diode is called characteristics of diode.

i) Forward Characteristic

ii) Reverse Characteristic

Rectifier

It is an electrical device that converts AC to DC.

Diode as Rectifier

1. Half-wave rectifier
2. Full-wave rectifier

1) Half-Wave Rectifier

The circuit arrangement is as shown in figure. It consists of AC input, a transformer whose primary is connected to input and secondary is connected to load resistor through diode.

During +ve half cycle diode becomes forward biased, current flows through the diode and output is obtained across load resistor. During −ve half cycle diode becomes reverse biased, current doesn’t flow through the diode and output is not obtained. In this way only +ve half cycle is obtained as output and diode works as half-wave rectifier.

2) Full-Wave Rectifier

The circuit arrangement is as shown in fig. It consists of AC input, a transformer, two diodes connected to centre tapping through load resistor.

During +ve half cycle diode D₁ becomes forward biased and diode D₂ becomes reverse biased. So current pass through D₁ and output is obtained. During −ve half cycle diode D₁ becomes reverse biased and D₂ becomes forward biased. So current pass through D₂ and output is obtained. In this way both half cycle are obtained as output and diode works as full wave rectifier.

Zener Diode

It is heavily doped PN junction diode that works in reverse bias condition without being damaged.

Electrical Symbol of Zener Diode

Zener Diode as Voltage Regulator

When input voltage increases beyond certain limit, the current through the circuit increases sharply causing increase in voltage drop across R_s. Thus voltage across zener diode remains constant and also the output voltage remains constant.

When the input voltage decreases, then current through the circuit decreases causing decrease in voltage across R_s. As a result output voltage remains constant.

Logic Gates

The electronic circuit which makes logic decision is called logic gate.

Logic gate is an electronic circuit which can take one or more input but output will be one. Logic gates are made up of diode and transistor. A logic gate is used to allow and denied a digital signal.

Types of Logic Gate

1. AND gate
2. OR gate
3. NOT gate
4. NAND gate
5. XOR gate
6. XNOR gate
7. NOR gate

1) AND Gate

AND gate whose output is high i.e. 1 when both the inputs are 1. If a single input is 0, the output 0 will be obtained.

OR Gate

The gate whose output is high i.e. 1 if any or all inputs are 1 is called OR gate. It has two or more inputs but only one output.

NOT Gate

The gate whose output is high i.e. 1 when the input is low i.e. 0 is called NOT gate. It is also known as inverter because the output is always the complement of input. It has only one input and only one output.

Logic Symbol / Transistor Realization

Universal Gate

1) NAND Gate

The combination of AND gate and NOT gate is called NAND gate. This gate is designed in such a way that the output of AND gate is connected to the input of NOT gate. It has two or more inputs but only one output. In this gate the output is low if all the inputs are high.

Operation

1. When both A and B are low, Y is high.
2. When A is high and B is low, Y is high.
3. When A is low and B is high, Y is high.
4. When both A and B are high, Y is low.

NAND Gate as Universal Gate

NAND gate is universal gate. The repeated use of this gate can produce all the three basic gates i.e. OR, AND and NOT gate. This gate serve as a building block in digital circuits.

1) NOT Gate from NAND Gate

NOT gate can be made from NAND gate by connecting its two inputs together as shown in fig. The output is A̅ which can be proved to be equal to A̅ by using DeMorgan’s theorem. The truth table also verifies this fact.

2) AND Gate from NAND Gates

Two NAND gates are used to produce AND gate as shown in fig. The output of first NAND gate is A·B̅? [source notation] which is given to the second NAND gate acting as inverter.

The final output is inverted and is given by Y = A·B which is the logic function of AND gate. This fact is also verified by the truth table.

3) OR Gate from NAND Gates

Three NAND gates are used to produce OR gate as shown in fig. The first two NAND gates are acted as NOT gates and their outputs are fed to the third NAND gate. The final output is A+B which can be proved by using DeMorgan’s theorem.

2) NOR Gate

The combination of OR gate and NOT gate is called NOR gate. This gate is designed in such a way that the output of OR gate is connected to the input of NOT gate. It has two or more inputs but only one output. In this gate the output is high (1) when all inputs are low (0).

Operation of NOR Gate

1. When A and B both low, Y is high.
2. When A is high and B is high, Y is low.
3. When A is low and B is high, Y is low.
4. When A and B both high, Y is low.

NOR Gate as Universal Gate

The repeated use of NOR can produce all three basic gates like OR, AND and NOT gate. Therefore NOR gate is also called universal gate. This gate acts as building block in digital circuits.

1) NOT Gate from NOR Gate

NOT gate can be produced from NOR gate by connecting its two inputs together as shown in fig. The output is A̅ which can be proved by using DeMorgan’s theorem. The truth table also verifies this fact.

2) OR Gate from NOR Gates

Two NOR gates are used to make OR gate as shown in fig. The output of first NOR gate i.e. \overline{A+B} is given to the second NOR gate which acts as inverter. The final output is inverted and is given by Y = A+B which is the logic function of OR gate. This fact is also verified by truth table.

3) AND Gate from NOR Gates

Three NOR gates are used to make AND gate as shown in fig. The first two NOR gates are operated as NOT gates and their outputs are fed to the third NOR gate. The final output is Y = A·B which can be proved by DeMorgan’s theorem. The truth table also verifies this fact.