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Note for Analog Electronic Circuits - AEC by RAVIKUMAR KUMAR

  • Analog Electronic Circuits - AEC
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[ PAGE – 1. 1 ] CHAPTER - 1 -------------------------------------- [P-N JUNCTION DIODE ] --------------------------------  DEFINITION: When a p-type semiconductor is suitably joined to n-type semiconductor, the contact surface is called p-n Junction.  FORMATION OF PN JUNCTION  In actual practice, the characteristic properties of PN junction will not be apparent if a ptype block is just brought in contact with n-type block.  It is fabricated by special techniques and one common method of making PN junction is called Alloying.  In this method, a small block of indium (trivalent impurity) is placed on an n-type germanium slab as shown in Fig (i). The system is then heated to a temperature of about 500ºC. The indium and some of the germanium melt to form a small puddle of molten germanium-indium mixture as shown in Fig (ii). The temperature is then lowered and puddle begins to solidify.  Under proper conditions, the atoms of indium impurity will be suitably adjusted in the germanium slab to form a single crystal.  The addition of indium overcomes the excess of electrons in the n-type germanium to such an extent that it creates a p-type region.  As the process goes on, the remaining molten mixture becomes increasingly rich in indium. When all germanium has been re-deposited, the remaining material appears as indium but- ton which is frozen on to the outer surface of the crystallized portion as shown in Fig (iii).  PROPERTIES OF PN JUNCTION  To explain PN junction, consider two types of materials: 1) P-Type-P-type semiconductor having –ive acceptor ions and +ive charged holes. 2) N-Type -N-type semiconductor having +ive donor ions and –ive free electrons.  P-type has high concentration of holes & N-type has high concentration of electrons.  The tendency for the free electron to diffuse over p-side and holes to n-side process is called Diffusion.

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[ PAGE – 1. 2 ]  When a free electron move across the junction from n-type to p-type, positive donor ions are removed by the force of electrons. Hence positive charge is built on the n-side of the junction. Similarly negative charge establish on p-side of the junction.  When sufficient no of donor and accepter ions gathered at the junction, further diffusion is prevented.  Since +ive charge on n-side repel holes to cross from p-side to n-side, similarly –ive charge on p-side repel free electrons to cross from n-type to p-type.  Thus a barrier is set up against further movement of charge carriers is hole or electrons.  This barrier is called as Potential Barrier/ Junction Barrier (V0) and is of the order 0.1 to 0.3 volt. This prevents the respective majority carriers for crossing the barrier region. This region is known as Depletion Layer.  The term depletion is due to the fact that near the junction, the region is depleted (i.e. emptied) of charge carries (free electrons and holes) due to diffusion across the junction. It may be noted that depletion layer is formed very quickly and is very thin compared to the n region and the p-region.  Once pn junction is formed and depletion layer created, the diffusion of free electrons stops. In other words, the depletion region acts as a barrier to the further movement of free electrons across the junction.  The positive and negative charges set up an electric field as shown in fig below.  The electric field is a barrier to the free electrons in the n-region.  There exists a potential difference across the depletion layer and is called barrier potential (V0). The barrier potential of a p-n junction depends upon several factors including the type of semiconductor material, the amount of doping and temperature.  The typical barrier potential is approximately: - For Si, V0 = 0.7 V, For Ge, V0 = 0.3 V.  PN JUNCTION UNDER FORWARD BIASING  When external D.C. voltage applied to the junction is in such a direction that it cancels the potential barrier, thus permitting current flow, it is called Forward Biasing.  To apply forward bias, connect positive terminal of the battery to p-type and negative terminal to n-type as shown in fig below.

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[ PAGE – 1. 3 ]  The applied forward potential establishes an electric field which acts against the field due to potential barrier. Therefore, the resultant field is weakened and the barrier height is reduced at the junction.  As potential barrier voltage is very small (0.1 to 0.3 V), therefore, a small forward voltage is sufficient to completely eliminate the barrier.  Once the potential barrier is eliminated by the forward voltage, junction resistance becomes almost zero and a low resistance path is established for the entire circuit.  Therefore, current flows in the circuit. This is called Forward Current.  With forward bias to PN junction, the following points are worth noting : (i) The potential barrier is reduced and at some forward voltage (0.1 to 0.3 V), it is eliminated altogether. (ii) The junction offers low resistance (called forward resistance, Rf) to current flow. (iii) Current flows in the circuit due to the establishment of low resistance path. The magnitude of current depends upon the applied forward voltage.  CURRENT FLOW IN A FORWARD BIASED PN JUNCTION: It is concluded that in n-type region, current is carried by free electrons whereas in p-type region, it is carried by holes. However, in the external connecting wires, the current is carried by free electrons.  PN JUNCTION UNDER REVERSE BIASING  When the external D.C. voltage applied to the junction is in such a direction that potential barrier is increased, it is called Reverse Biasing.  To apply reverse bias, connect negative terminal of the battery to p-type and positive terminal to n-type.

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[ PAGE – 1. 4 ]  It is clear that applied reverse voltage establishes an electric field which acts in the same direction as the field due to potential barrier.  Therefore, the resultant field at the junction is strengthened and the barrier height is increased as shown in fig below .  The increased potential barrier prevents the flow of charge carriers across the junction.  Thus, a high resistance path is established for the entire circuit and hence the current does not flow.  With reverse bias to PN junction, the following points are worth noting: (i) The potential barrier is increased. (ii)The junction offers very high resistance (Reverse Resistance Rr) to current flow. (iii)No current flows in the circuit due to the establishment of high resistance path.  VOLT-AMPERE CHARACTERISTICS OF PN JUNCTION: Volt-ampere or V-I characteristic of a pn junction (also called a crystal or semiconductor diode) is the curve between voltage across the junction and the circuit current.  Usually, voltage is taken along x-axis and current along y-axis. Fig. shows the circuit arrangement for determining the V-I characteristics of a pn junction.  The characteristics can be studied under three heads namely: 1) Zero external voltage 2) Forward Bias 3) Reverse Bias.  ZERO EXTERNAL VOLTAGE:  When the external voltage is zero, i.e. circuit is open at K; the potential barrier at the junction does not permit current flow. Therefore, the circuit current is zero as indicated by point O in Fig.

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