P-n Junction and diode

p-n JUNCTION

The total charge on each side of a PN Junction must be equal and opposite to maintain a neutral charge condition around the junction. If the depletion layer region has a distance D, it therefore must therefore penetrate into the silicon by a distance of Dp for the positive side, and a distance of Dn for the negative side giving a relationship between the two of:

Dp*NA = Dn*ND 

in order to maintain charge neutrality also called equilibrium.

SEMICONDUCTOR DIODE

A Semiconductor Diode is a p-n junction that has metallic contacts at both of its ends in order to apply external voltage. It is a p-n junction diode. It has the ability to conduct current in only one direction.

p-n junction diode under forward bias

When an external voltage V is applied across a semiconductor diode such That p-side is connected to the positive terminal of the battery and n-side To the negative terminal.

p-n junction diode under reverse bias

When an external voltage (V ) is applied across the diode such That n-side is positive and p-side is negative, it is said to be Reverse biased.

APPLICATION OF JUNCTION DIODE AS A RECTIFIER

In a half-wave rectifier, one half of each a.c input cycle is rectified. When the p-n junction diode is forward biased, it gives little resistance and when it is reversing biased it provides high resistance. During one-half cycles, the diode is forward biased when the input voltage is applied and in the opposite half cycle, it is reverse biased. In the Full-wave rectifier circuits are used for producing an output voltage or output current which is purely DC. The main advantage of a full-wave rectifier over half-wave rectifier is that such as the average output voltage is higher in full-wave rectifier, there is less ripple produced in full-wave rectifier

SPECIAL PURPOSE p-n JUNCTION DIODES

Zener diode

It is a special purpose diode, named after the inventor C. Zener It is designed to operate under reverse bias in the breakdown region


The symbol for Zener diode is

 Fabrication – It has heavily doped p- side and n-side. Due to this, the depletion region formed is very thin about < 10-6. . Hence, the field at the junction is extremely high about -5 * 106 V/m for a small reverse bias voltage of 5 Volts

Light emitting diode

It is a heavily doped p-n junction which under forward bias emits Spontaneous radiation. The diode is encapsulated with a ttransparen Cover so that emitted light can come out.The V-I characteristics of a LED is similar to that of a Si junction Diode. But the threshold voltages are much higher and slightly different For each colour. The reverse breakdown voltages of LEDs are very low, Typically around 5V. So care should be taken that high reverse voltages Do not appear across them.

Solar cell

A solar cell is basically a p-n junction which Generates emf when solar radiation falls on the p-n junction. It works on the same principle (photovoltaic effect) as the photodiode, except that No external bias is applied and the junction area Is kept much larger for solar radiation to be Incident because we are interested in more power.

JUNCTION TRANSISTOR

n-p-n transistor: Here two segments of n-type semiconductor (emitter and collector) are separated by a segment of p-type Semiconductor (base).

P-n-p transistor: Here two segments of p-type semiconductor (termed as emitter and collector) are separated by a segment of n-type semiconductor (termed as base).

The schematic representations of an n-p-n and a p-n-p Configuration.

DIGITAL ELECTRONICS AND LOGIC GATES

AND Gate

This gate works in the same way as the logical operator "AND". The AND gate is a circuit that performs the AND operation of the inputs. This gate has a minimum of 2 input values and an

Logic Design

Truth Table

OR Gate

This gate works in the same way as the logical operator "OR". The OR gate is a circuit which performs the OR operation of the inputs.

Logic Design

Truth Table

NOT Gate

The NOT gate is also called an inverter. This gate gives the inverse value of the input value as a result.

Logic Design

Truth Table

NAND Gate

The NAND gate is the combination of AND gate and NOT gate. This gate gives the same result as a NOT-AND operation.

Logic Design

Truth Table

NOR Gate

The NOR gate is the combination of an OR gate and NOT gate. This gate gives the same result as the NOT-OR operation.

Logic Design

Truth Table

INTEGRATED CIRCUITS

(i)  The concept of fabricating an entire circuit on a small single chip of semiconductor is called Integrated circuit; The chip dimensions are as small as 1mm * 1 mm or even less than that

(ii)  The integrated chip can be categorised into (a) linear or analog IC (b) digital IC

(iii) Linear or Analog IC – The signals change continuously over a range of values between maximum and minimum; The output varies linearly as the input ; The most useful linear IC is an operational amplifier

(iv)  DigitaIC – These have two values – high and low; These contain logical gates. – Depending upon the number of circuit components or logic gates used, they can be classified into SSI, MSI, LSI or VLSI