Electric switches

PN Junction diode:

In this article we will learn what is forward bias and reverse bias? what is diffusion current and drift current? what is avalanche breakdown?

Before going to this question, know about PN junction semiconductor and how it's formed?

       It's a combination of P type semiconductor with N type semiconductor. It's formed when a P type semiconductor is joined a N type semiconductor. P type semiconductors have holes, which is positively charged and N type semiconductor have electrons, which is negatively charged. Where P type and N type joints and form a junction, this junction is known as PN-junction. The electrons near to the junction is jump from N to P and holes near to the junction is jump from P to N. This phenomena creates a space charge region as shown in below figure.

       Now, the electrons are available at P region and holes are available at N region and the electric field is created in this space charge region because of the movement of the holes and electrons. The direction of electric field is N type region to P type region. Some electrons moves back from P to N in the space region and some holes moves back from N type to P type space region. This continues to happen till equilibrium is reached at this region. This movement of holes and electrons in space region gives rise to a current and this current is known as diffusion current.

let's start with first question, what is forward bias and reverse bias? 

        In forward bias, the positive terminal of battery is connected towards the P region of the PN junction and the negative terminal of the battery is connected towards the N region of the PN junction. The positive terminal repulse the holes towards the junction in the P region and the negative terminal repulse the electrons towards the junction in N region. This results in the junction to shrink. Due to this depletion region or space charge region narrows down. 

        If the voltage in forward bias, is able a specified range, this electrons in the N regions drifts through the junction and migrates to the P region and the holes in the P region drifts through the junction and migrates to the N region. Now, because of this movements of electrons and holes, current starts following through the circuit and this current is known as the drift current.

        In reverse bias, the negative terminal of battery will be connected towards the P type semiconductor and the positive terminal of the battery will be connected to the n type of semiconductor. Here, holes is attracted to the negative terminal of battery and electrons are attracted to the positive terminal of battery. This results in increase the depletion region. Now in this condition, PN junction behaves like an insulator and it will not allow any current to flow in the circuit. But in particular condition, if the battery voltage is above a reverse bias breakdown voltage. The electrons and holes breakdown thorough the PN junction and cross, resulting in the current to flow through the circuit. This breakdown is called as avalanche breakdown. In this process, the current following through the PN junction is very high and ultimately the PN junction gets damaged due to overheating caused by the excess amount of current. 

This is how PN junction semiconductor works.

VI characteristic of PN junction diode:

It is a relationship between current through the diode and applied voltage. It is represented as a graph. X axis represents as a voltage and Y axis represents current through the diode. Right side of this graph is forward voltage region and diode is in forward biased. Left side is reverse voltage region and diode is in reverse biased.

A diode has non-linear characteristic. When voltage is positive, current exponentially increases until reaches value Vd, which is depends on built-in electric field of depletion region. Vd is 0.7V for silicon and 0.3V for germanium diodes. For voltage greater than Vd, current rapidly increases and is limited only by resistance in circuit.

When voltage is negative, very small amount of current will flow through the diode, this current is known as leakage current or reverse saturation current. Reverse saturation current is few micro ampere and is practically negligible. But after reverse breakdown, current is increases.

So, working of diode is divided into three regions;

1) Forward region

2) Reverse region

3) Breakdown region

SCR:

In this article, we will discuss about what is SCR? Working of SCR.

Symbol:

Structure:

SCR is known as Silicon Controlled Rectifier. SCR has three terminals; anode, cathode and gate. It is unidirectional device, which means it allows flow of current in only one direction. SCR is a bipolar device, which means it can block positive and negative voltage. It four layers (p,n-,p,n+) and three junction (J1, J2, J3). In layer positive (+) and negative (-) is denoted for heavily and lightly doped like n+ is heavily doped and n- is lightly doped and with sign is denoted as normally doped. 

Working of SCR:

SCR is a semi controlled device means it can control only ON state of SCR but we cannot control the OFF state of SCR. ON state can be controlled by gate current. 

SCR works in three modes;

1. Reverse blocking mode
2. Forward blocking mode
3. Forward conducting mode

1) Reverse blocking mode:

In reverse blocking mode, the negative terminal of supply connected with anode terminal of SCR and the positive terminal of the supply is connected with cathode terminal of SCR. Now for junction J1, as shown in below figure, p terminal is connected with negative of battery and n terminal of SCR is connected with positive terminal of battery, So, junction J1 is in reverse bias. Same this way, J2 is forward bias and J3 is in reverse bias. It means that, whatever negative voltage is given by supply that will drop in J1 and J3. But here, J3 is heavily doped and any semi conductor device which is heavily doped has lower voltage withstand capability and it will break quickly when applying some amount of voltage. So, if you increase supply voltage more, than J3 will break and start conducting. In this condition entire voltage drop will occur across junction J1. 

Reverse blocking mode of SCR is same as diode. When supply voltage is increased to breakdown voltage, infinite current will flow through the SCR and in this condition junction J1 is break and start conducting. But if this voltage is increased to break over voltage, SCR may damage.

2) Forward blocking mode:

In forward blocking mode, positive terminal of supply is connected with anode terminal of SCR and negative terminal of supply is connected with cathode terminal if SCR. It means Vak>0 and Ig=0. In this mode, junction J1 is in forward bias, J2 is in reverse bias and J3 is in forward bias. Here, in this mode also, current following through the SCR is zero, beacuse junction J2 is in reverse bias and entire voltage drop will occur in junction J2. So, in this mode, if you supply positive supply voltage than also SCR will not allow the current to flow. But if you increase positive voltage junction J2 will break, this voltage is denoted by Vbo (Voltage break over). When Vin > Vbo, Junction J2 will break and start conducting and the current will flow through the SCR. But this is not a safe for SCR, if you will do the same for 4 to 5 times, SCR may get damage. So, this is not safe way to conducting SCR. 

3) Forward conducting mode:

In forward conducting mode, connection is same as forward blocking mode, but in this mode, we are supplying gate current. If you supply gate current to the junction J2, it will neutralise the charge stored across the depletion region and junction J2 will in forward bias, even when supply voltage is less than break over voltage. So, in this mode, current will flow through the SCR and all junctions remains in forward bias. 

Now, if the gate current is increase, more the value of gate current, fastest the charge neutralisation. Once the SCR enters in conduction, it means once the junction J2 is in forward bias and start conducting by supping gate current, even after this if you will remove the gate current than also SCR start conducting. 

So, in SCR we can only control the ON state of SCR by gate current, once it enters in to ON state you cannot control the OFF state, that's why this device is known as semi controlled device. To control the OFF state of the SCR, we have to use separate commutation circuit or change the input supply, so it will convert into reverse blocking mode. So, turn OFF state is depends upon the supply voltage and the commutation circuit.

Latching current: The latching current is define as the minimum amount of the anode current is required to turn ON the SCR and the latching current is denoted as IL.

Holding current: To turn OFF SCR, we need to decrease the anode current up to some desired value and this value is known as holding current. The holding current is denotes as IH. It means holding current is defined as the minimum amount of anode current below which SCR turned OFF.

So, for ON state of SCR, IA > IL,

and for OFF state pf SCR, IA < IH

Here we can derive that the latching current is always grater than the holding current, IL> IH

BJT:

In this article, we will discuss about the bipolar junction transistor, which is also known as BJT. The BJT is a solid-state device and used for amplifying, controlling and generating electrical signals.

What is the meaning of “bipolar”? Bipolar word is used because of the current through the BJT is by both the carriers; the electrons and the holes. The junction stands for, inside the BJT, there is PN junction and Transistor is stands for transfer of resistance.

Construction:

The transistor has three terminals; emitter (E), base (B) and collectors (C).

There are two types of transistors;

              1)      NPN transistor

              2)      PNP transistor

      Symbol:

1)  NPN transistor: In this type of BJT, three regions are used; one P region and two n region. P region is sandwich between two N regions. Here, emitter and collector are connected with N type of region and base is connected with P type region. The base terminal is always lightly doped and thin area. The collector terminal is highly doped and wide area.

As shown in above figure, there are three types of current; emitter current (IE), base current (IB), and collector current (IC). As we discuss earlier, base is lightly doped and thin, value of base current is very low (in terms of micro-amps). The value of collector current and emitter current is nearly equal. By the above figure, we can say that, the collector current is addition of the base current and the emitter current.

2)  PNP transistor: In this type of BJT, two P type of region and one n type region is used. The collector and emitter terminal of transistor is connected as P reigon and the base region is connected as N type region.

As shown in above figure, the emitter current is addition of the base current and the collector current.

Configuration:

Transistor has three terminals. With respect to these terminals, transistor can use with common emitter, common base and common collector configurations. The basic two configurations are common emitter and common base. By this configuration, we can define current gain of transistor. Current gain is defined as ratio of output current and input current.

For common emitter configuration, the output current is IC and the input current is IB. so, gain is IC/IB and it is denoted as α.

For common base configuration, the input current is IE and the output current is IC. So, gain is IC/IE and it is denoted as β.

Modes of operation;

The transistor will work on three regions;

                1) Active

a.       Forward active

b.       Reverse active

                2) Saturation

                3) Cut-off

Mode	NPN	PNP	Feature	Usage
Cut-off	Reverse	Reverse	No current	Switch
Forward active	Forward	Reverse	Voltage controlled current	Amplifier
Saturation	Forward	Forward	Low resistance	Switch
Reverse active	Reverse	Forward	Voltage controlled current	Usually not used