Diode : Introduction, Working, and its VI Characteristics

A diode is a crystalline piece of semiconductor material which is formed by the combination of p type semiconductor and n type semiconductor. (refer diode construction).

As n-type semiconductor has plenty of free electrons to donate and p-type semiconductor has plenty of vacant positions to accept electrons. So, if we excite or apply the voltage to the diode terminal then these free electrons are also excited and ready to move. The movement of these free electrons decides the direction of conventional current flow and the behavior and working of the diode.

Here in this article, we will discuss this phenomenon in detail which will help us to determine the behavior of a PN junction diode.

Diode Working

A p-n junction diode working depends on its Biasing. Biasing means applying voltage at diode’s terminal. Diode works in different manner in different biasing conditions due to which diode is also characterized as unilateral or unidirectional device.

Forward Bias Diode

A p-n junction diode is said to be Forward biased when its positive terminal i.e. anode (p-type semiconductor side) is connected to positive terminal of electrical supply and its negative terminal i.e. cathode (n-type semiconductor side) is connected to negative terminal of supply as shown in given figure.

In forward biased p-n junction diode, electric current is flow from p-type semiconductor to n-type semiconductor, or we can say electric current is entering from p-side of the p-n function diode. resulting the flow of free electrons form n-type semiconductor to p- type semiconductor through depletion region. As n-type semiconductor have plenty of free electrons.

Let us discuss this phenomenon in details, when we apply forward biased to a p-n junction diode then positive potential of battery attracts electrons from p side this will result more holes or vacant positions on p-side. These holes on p-side attracts more electrons from n-side but these electrons require more energy to pass this depletion region as this depletion region have no charge carriers.

So, when a voltage that is greater then, this energy requirement of electrons is applied to diode terminals then electrons from n-side are freely move to p-side and current start conducting in opposite direction. This means diode start conduction and behave like short circuit.

Note :- The minimum forward biased voltage required by the diode, so that it start conduction is called Cut-In Voltage, Knee Voltage or Threshold Voltage

V-I Characteristics of P-N Junction Diode

Above figure represents the V-I Characteristics of a diode during forward bias condition. As we discuss in above paragraph, forward biased diode start conduction and behave like short circuit (means V α I) after a certain applied voltage. We can clearly see that in above, that after knee voltage Vd is proportional to Id.

Reverse Bias Diode

A reverse-biased p-n junction diode refers to a situation where the voltage across the diode is applied in such a way that the voltage applied to the anode is lower than that applied to the cathode. In other words, the negative side of the voltage source is connected to the diode's anode, and the positive side is connected to the cathode.

In reverse bias p-n junction diode, electric current entered from n side of p-n junction diode, means electrons flow from n-side to positive potential of the battery. This will result the number of holes or the vacant position increases on n-side. This uncovered positions or holes form layers near the junction. Similarly, the negative potential of supply attracts more holes from p-side, resulting the number of electrons increase on p-side. These free electrons on p-side form layers at the junction.

So, when we apply voltage to a p-n junction diode then number of electrons (-ve ion) layers on p-side and number of holes (+ve ion) layers on n-side increases. Hence the width of depletion region increases, means the potential barrier increases. That’s why a reverse biased diode does not allow conduction. So, we can say that in reverse bias condition diode act as open circuit or diode is switch off.

However, a small amount (almost negligible) of current flows through the diode due to minority charge carriers that are entering into the depletion region. This current is called reverse saturation current and is denoted by I_s.

Above figure shows V-I characteristics of p-n junction diode during reverse bias condition. As we discussed in above paragraph, that a reverse bias diode is behave like open and does not allow conduction. We can clearly see in above characteristics figure that there is no current (I_d) flow while applying negative voltage. However, a small Reverse Saturation Current is flow due to minority charge carriers.

Avalanche Breakdown in diode

This phenomenon occurs in reversed biased diode.

When a sufficiently high reverse voltage applied to diode terminal then minority charge carriers acquire energy from applied voltage. These charge carriers collide with other atoms and imparts sufficient energy to break their covalent bond. By doing this, these charge carriers form new electron-hole pairs and generate additional charge carriers. This cumulative phenomenon is called avalanche multiplication. It results the flow of large reverse saturation current through diode. Actually, a breakdown occurs called avalanche breakdown or reverse breakdown.

At this condition diode is said to be on avalanche or reverse breakdown region.

Note:- Reverse bias voltage required to derive the diode in reverse breakdown region or avalanche breakdown region is called reverse break down voltage.

V-I Characteristics of P-N Junction Diode

V_D = Applied Voltage

I_D = Diode Current

I_s = Reverse Saturation Current

V_BR = Reverse Breakdown Voltage

V_T = Cut-in Voltage, Threshold Voltage, or knee voltage.

Above we study that, if forward bias voltage is greater than the threshold voltage than diode start conduction means current flow through the diode. In above characteristics graph we clearly see that I_D (Diode Current) rise significantly as applied voltage increased.

And if reverse bias voltage is applied than diode does not allow conduction, however a small amount of current (reverse saturation current) flows through diode. We can see in above vi characteristics graph that in reverse bias region a small current I_s flows through diode up to a certain voltage and if reverse voltage is greater than reverse breakdown voltage than reverse saturation current increase sharply at this condition avalanche breakdown occurs.