Contents
From the study of subatomic particles to the laws of motion, Physics Topics offer insights into the workings of the world around us.
What is a p-n Junction Diode? What is Rectification?
Definition: By opposite kind of doping, if one part of a semi-conducting crystal be made n-type and the other part p-type then that crystal is called a p-n Junction.
In Fig., a p-n junction is shown. There is a junction ¡n between p -type and n -type parts. Remember that, by joining two different p -type and n -type crystals, a p-n junction is not formed because in that case, the crystals would not be joined uniformly and so the junction would not act properly.
To construct a p-n junction diode, a single crystal is doped with p -type in one half and n – type in other half. The junction thus developed is not so perfect.
Though it is possible to make the junction thin up to the order of 10-6m so far. Here we will take the junction as an ideal one to avoid complexity. It means we consider a perfect junction between p and n -parts.
Circuit symbol: The symbol by which a p-n junction is denoted in an electrical circuit is shown in Fig.
The base of the triangle indicates p-end and the line drawn at the vertex parallel to the base indicates n -end.
Depletion region or depletion layer: As soon as a p-n junction is formed, electrons and holes start to diffuse through the junction.
At first we take a look about the condition at the junction before diffusion starts. In that time, the free electrons in n -part move disorderly, whereas the donor ions remain fixed in their own positions [Fig.]. As the free electrons remain confined within the n -part, the net charge of this part is zero. On the other hand, the free holes in p -part move disorderly, whereas the acceptor ions remain fixed. Just like n -part, net charge of p – part is also zero.
Due to lack of free electron in p-part, the free electrons start to move from n to p-part. Similarly, due to lack of free holes in n-part, the free holes start to move from p to n-part, i.e., diffusion starts. In this way, the electrons just entering the p-part neutralise the holes of the acceptor ions near the junction. This makes the acceptor ions negative in charge, i.e., net charge in p-part becomes negative[Fig.]. Similarly, the holes just enetering the n-part neutralise the electrons of the donor ions near the junction. This makes the donor ions positive in charge, i.e., net charge in n-part becomes positive.
Because of diffusion, the amount of positive and negative charges in n -part and p -part respectively increases rapidly. At a certain moment the amount of these charges becomes so high that no electron or hole can cross the region junction any more. In other word, net dif-fusion in this state, comes to zero.
In this condition up to a certain distance from either side of the plane no free charge does exist. The region on either side of the junction plane containing no free charge is known as depletion region (or depletion layer) [Fig.]
Inevitably, depletion region contains negative and positive ions in p -part and n -part respectively. Hence due to higher potential in n -type and ower potential in p -type, a potential barrier developes at p-n junction plane [Fig.]. Neither the majority carriers nor the electron and hole can overcome this barrier.
However, the movement of minority carriers continues even after the development of depletion region. Since the depletion layer in n -region is at positive potential, the minority carrier electrons of the depletion layer in the p -region get attracted towards that direction. Again, the minority carrier holes of the depletion layer In the n -region get attracted towards the depletion layer in p -region having negative potential.
Application of forward and reverse bias to p-n junction: Biasing is the connection of electrical components like diode, transistor, etc. (see section 1.4) with an external source of electricity (like a battery).
It has already been discussed that, even before any biasing, i.e., before application of any external voltage across a p-n junction, the n -end attains a positive potential and the p -end a negative potential. This is essentially a reverse bias on the junction, called the natural reverse bias. As a result, a depletion layer is naturally formed around the position of the n and p -regions.
In practice, the transformation of this depletion layer on application of external bias, i.e., actual external voltages on a p-n junction, determines the working principle of such a junction.
Application of reverse bias: Reverse bias is applied to a p-n junction by connecting the n -end of the p-n junction with the positive terminal of the external source and the p -end with the negative terminal [Fig.]. If reverse bias is applied by means of an external battery B to a p-n junction, the thickness of the depletion region increases.
The majority carriers cannot cross the junction and hence no current is obtained in the external circuit. But due to the motion of minority carriers, a small current is obtained whose value in case of germanium is approximately 10-6 A and in case of silicon, it is only about 10-9 A. This current is called the reverse saturation current of a diode. In most cases, this current is neglected.
Application of forward bias : To apply forward bias to p-n junction, its p-end is connected with the positive terminal of the external source of electricity and n-end with the negative terminal [Fig.]. A part of the forward bias applied by means of an external battery B, is used to decrease the value of potential barrier. To do so applied voltage is to be increased.
Very soon, the voltage thus applied reaches a particular value, when the depletion region vanishes. If the forward bias is increased more the holes present in the p-region and electrons in the n-region can cross the junction easily. This is due to the applied positive potential at the p-end and negative potential at the n-end which help the holes and electrons, respectively to pass through the junction. As a result, a current flows through the
external circuit. According to the conventional rule, the direction of current flow in the external circuit is just opposite to the direction of flow of electrons, i.e., the direction along which the holes flow.
Semiconductor diode: If forward bias is applied to a p-n junction [Fig.(a)], current flows through it; but when reverse bias is called [Fig.(b)], current flow is negligible. So, the p-n junction acts as a valve, i.e., the current through it is unidirectional. p-n junction is also called a semiconductor diode. It can be used as a rectifier just like a vacuum diode, although their properties are not exactly identical.
Characteristics of p-n junction diode: The variation of current with potential difference applied to a p-n junction diode in its forward and reverse biased condition, is shown in Fig.
It is known as I-V characteristics or simply characteristic curve of a p-n junction.
Some properties of characteristic curve:
i) Due to presence of minority carriers, a reverse saturation current exists in reverse bias[Fig.]
ii) To neutralise the reverse saturation current, a minimum forward biasing is essential [OA in Fig.]
iii) With increase in potential difference in forward bias, current increases rapidly (AB part in that curve).
iv) The characteristic curve of p-n junction is not linear. i.e., V and I are not proportional to each other. Hence it is a non-ohmic electrical component. Since ΔI is the change of current due to change of potential difference ΔV, the ratio \(\frac{\Delta V}{\Delta I}\) is called dynamic resistance of the junction. The value of this dynamic resistance rp is different in the different portions of characteristic curve.
p-n junction rectifier: The arrangement which converts an alternating waveform into unidirectional waveform, e.g., an alternating current into unidirectional current, is called a rectifier. A p-n junction diode is used for rectification of alternating current.
Half-wave rectification: The required circuit diagram half-wave rectification, the input as well as the output wave-forms are shown in Fig. For the positive half-cycle of alternating current, the p-n junction gets forward biased and for the negative half-cycle, it gets reverse biased. So, only for the positive half-cycle of the input alternating voltage, output voltage and current are obtained which is evidently unidirectional. Since, only one half-cycle of the input wave can be rectified by this arrangement, so it is called half-wave rectification.
Each wave-crest in the dc output is called a ripple. In a half-wave rectifier, the number of wave-crests in the alternating input becomes equal to the number of ripples in the dc output. Hence, if the frequency of alternating input be 50 Hz, the frequency of the ripples will also be 50 Hz.
Full-wave rectification: The required circuit diagram for full-wave rectification and the input-output waveforms are shown in Fig. A full wave can be rectified by using two p-n junctions.
For one half-cycle of the alternating current, the diode D1 gets forward biased but the diode D2 gets reverse biased. As a result, current flows only through the diode D1 in this case. For the next half-cycle, the diode D2 gets forward biased but the diode D1 gets reverse biased. As a result, current flows only through the diode D2. Note that, for both the half-cycle of a complete cycle, the current through the load resistance is unidirectional. Since, both the half-cycles of the input wave can be rectified by this arrangement, so it is called full-wave rectification. In this case, the number of ripples becomes double the number of wave-crests of the alternating input. Hence, if the frequency of the alternating input be 50 Hz, the frequency of the ripples will be 100 Hz.
Advantage of silicon over germanium for use as rectifiers: Due to the presence of minority carriers, a very small current passes through the junction in the reverse bias. Hence, a p-n junction is not completely free from error as a rectifier. The value of this reverse current is approximately 10-6 A for germanium and only 10-9A for silicon. Clearly, this reverse current can easily be neglected for silicon. Hence, silicon is more useful than germanium as a rectifier.
Numerical Examples
Example 1.
The potential barrier of a p-n junction diode is 0.4 V. If the thickness of the depletion region be 4.0 × 10-7 m, what will be the electric field Intensity in this region? An electron from the n -region moves towards the p-n junction with velocity 6 × 105 m ᐧ s-1. What will be the velocity of that electron with which it enters the p -region?
Solution:
Electric field intensity, E = \(\frac{V}{d}\)
Here, V = value of potential barrier = 0.4 V
and d = thickness of depletion region = 4 × 10-7m.
∴ E = \(\frac{0.4}{4 \times 10^{-7}}\) = 106V ᐧ m-1
Let an electron enters to the depletion region from n -region with velocity v1 and come out from the depletion region with velocity v2. Due to this, increase in potential energy is eV.
∴ According to the principle of conservation of energy,
\(\frac{1}{2} m v_1^2\) = eV + \(\frac{1}{2} m v_2^2\)
or, \(\frac{1}{2}\)(9 × 10-31) × (6 × 105)2 = 1.6 × 10-19 × 0.4 + \(\frac{1}{2}\) × (9.1 × 10-31) ᐧ \(v_2^2\)
or, 1.64 × 10-19 = 0.64 × 10-19 + 4.55 × 10-31 ᐧ \(v_2^2\)
or, \(v_2^2\) = \(\frac{1 \times 10^{-19}}{4.55 \times 10^{-31}}\) = 22 × 1010
or, v2 = 4.7 × 105 m ᐧ s-1