P-N Junction in Semiconductors

P-N JUNCTION

P-N JUNCTION The p-type and n-type semiconductors are not used separately for practical purposes because the overall charge of p-type and n-type semiconductors is electrically neutral. However, when p-type and n-type semiconductor materials are joined together, they behave differently.

Remember that although n-type semiconductor has large number of free electrons, but these free electrons are provided by the pentavalent electrically neutral. Thus, the n-type semiconductor is electrically neutral. atoms that are Similarly, in p-type semiconductor holes are provided by the trivalent atoms that are electrically neutral. Therefore, overall charge of p-type semiconductor is also neutral. P-N junction is a fundamental building block of many other semiconductor electronic transistors, solar cells, light emitting diodes (LED), and integrated circuits. devices such as:

BASIC STRUCTURE OF THE P-N JUNCTION Figure (1) schematically shows the p-n junction. It is important to realize that the entire semiconductor single-crystal material, in which one region is doped with acceptor impurity atoms to form the p-region and the adjacent region is doped with donor atoms to form the n-region. The interface separating the n- and p- regions is referred to as the metallurgical junction. is a Figure (1)

For simplicity, we will consider a step junction in which 1the doping concentration is uniform in each region (the p and n layers are uniformly doped by acceptor density Na, and donor density Nd, respectively), 2 and there is an abrupt change in doping at the junction. Initially, at the metallurgical junction, there is a very large density gradient in both electrons and holes concentrations. Majority carrier electrons in the n region will begin diffusing into the p region, and majority carrier holes in the p region will begin diffusing into the n region as shown in Figure (2).

N-region P-region Figure (2)

If we assume there are no external connections to the semiconductor, then this diffusion process cannot continue indefinitely. As electrons diffuse from the n region, positively charged donor atoms are left behind. Similarly, as holes diffuse from the p region, they uncover negatively charged acceptor atoms. The net positive and negative charges in the n and p regions induce an electric field in the region near the metallurgical junction, in the direction from the positive charge region (n region) to the negative charge region (p region). The net positively and negatively charged regions are shown in Figure 3. These two regions are referred to as the space charge region(depletion region).

Figure (3)

Density gradients still exist in the majority carrier concentrations at each edge of the space charge region. We can think of a density gradient as producing a diffusion force that acts on the majority carriers. These diffusion forces, acting on the electrons and holes at the edges of the space charge region, are shown in the figure (3). The electric field in the space charge region produces another force on the electrons and holes, which is in the opposite direction to the diffusion force for each type of particle. In thermal equilibrium, the diffusion force and the E-field force exactly balance each other.

Energy Band Diagram and Depletion Layer of a P-N Junction Let us construct a rough energy band diagram for a P-N junction at equilibrium or zero bias voltage. We first draw a horizontal line for EFin Figure (4)a, because there is only one Fermi level at equilibrium. Figure (4)

Figure (4)b shows that far from the junction, we simply have an N-type semiconductor on one side (with Ec close to EF), and a P-type semiconductor on the other side (with Ev close to EF). Figure (4)

Finally, in Figure (4)c , we draw an arbitrary (for now) smooth curve to link the Ec from the n layer to the p layer. Ev of course follows Ec, being below Ec by a constant Eg. Figure (4)

Figure (4)c shows that a P-N junction can be divided into three layers: the neutral N layer, the neutral P layer, and a depletion layer in the middle. In the middle layer, EF is close to neither Ev nor Ec. Therefore, both the concentrations are quite small. For mathematical simplicity, it is assumed that: electron and hole n 0 and p 0 in the depletion layer The term depletion layer means that the layer is depleted of electrons and holes.

Depletion region Generally, depletion refersto reduction or decrease in quantity of something. In semiconductor physics, the depletion region refers to a region where flow of charge carriers are decreased over a given time and finally results in empty mobile charge carriers or full of immobile charge carriers. The depletion region is also called as depletion zone, depletion layer, space charge region, or space charge layer.

Built-in Potential Barrier If we assume that no voltage is applied across the p-n junction, and then the junction is in thermal equilibrium, the Fermi energy level is constant throughout the entire system. Figure (5) shows the energy-band diagram for the p-n junction in thermal equilibrium.

Figure (5)

The conduction and valance band energies must bend as we go through the space charge region, since the relative position of the conduction and valence bands with respect to the Fermi energy changes between p and nregions. Electrons in the conduction band of the n region see a potential barrier in trying to move into the conduction band of the p region. This potential barrier is referred to as the built-in potential barrier and is denoted by bior Vbi.

bi = ? ? ?? . ?? ??? ??? Vbi = ? ? ?? . ?? ??? Or ??? ?? . ?? ??? = Vt . ?? Where: Vt =KT/e and is defined as the thermal voltage. Boltzmann s constant is 8.62 10 5 [eV/K] or 1.38*10-23 [J/K] q = e = 1.6*10 19 C The built-in potential is determined by Na and Nd , The larger the Na or Nd is, the larger the bi is.

At this time, we should note an important point concerning notation. Previously in the discussion of a semiconductor material, Ndand Nadenoted donor and acceptor impurity concentrations in the same region, there by forming a compensated semiconductor. From this point, Ndand Nawill denote the net donor and acceptor concentrations in the individual n and p regions, respectively. If the p region, for example, is a compensated material, then Na difference between the actual acceptor and donor impurity concentrations. The parameter Ndis defined in a similar manner for the n region. will represent the

Depletion-Layer Width Where sis the semiconductor permittivity. For silicon, is equal to 12 times the permittivity of free space(8.85*10-12 farad/meter). For germanium, is equal to 16 times the permittivity of free space.

Example Solution :

Zero Biased Junction Diode When a diode is connected in a Zero Bias condition, no external potential energy is applied to the P-N junction. However, if the diodes terminals are shorted together, a few holes (majority carriers) in the P-type material with enough energy to overcome the potential barrier will move across the junction against this barrier potential. This is known as the Forward Current and is referenced as IF. Likewise, holes generated in the N-type material (minority carriers), move across the junction in the opposite direction. This is known as the Reverse Current and is referenced as IR.

This transfer of electrons and holes back and forth across the P-N junction is known as diffusion, as shown below:

The potential barrier that now exists discourages the diffusion of any more majority carriers across the junction. However, the potential barrier helps minority carriers (few free electrons in the P-region and few holes in the N-region) to drift across the junction. Then an Equilibrium or balance will be established when the majority carriers are equal and both moving in opposite directions, so that the net result is zero current flowing in the circuit. When this occurs the junction is said to be in a state of Dynamic Equilibrium.

Forward Bias Junction Diode When a diode is connected in a Forward Bias condition, a negative voltage is applied to the N-type material and a positive voltage is applied to the P-type material (i.e the p side connected to the more positive potential than the n side.). P-region N-region

The application of a forward biasing voltage on the junction diode results in the depletion layer becoming very thin and narrow which represents a low impedance path through the junction thereby allowing high currents to flow.

If this external voltage becomes greater than the value of the potential barrier (approx. 0.7 volts for silicon and 0.3 volts for germanium) the potential barriers would be overcomed and current will start to flow. This is because the negative voltage pushes electrons towards the junction giving them the energy to cross over and combine with the holes being pushed in the opposite direction towards the junction by the positive voltage. This results in a characteristics curve of zero current flowing up to this voltage point, called the knee on the static curves and then a high current flow through the diode with little increase in the external voltage as shown below.

As the forward voltage across the diode is increased further, the exponentially. At very high forward voltages, the forward current saturates, and heating effects may cause the diode to break. The point at which this sudden increase in current takes place is represented on the static I-V characteristics curve above as the knee point. current increases

Reverse Bias Junction Diode When a diode is connected in a Reverse Bias condition, a positive voltage is applied to the N- type material and a negative voltage is applied to the P-type material (with the n side is connected to the more positive potential.)

The positive voltage applied to the N-type material attracts electrons towards the positive electrode and away from the junction, while the holes in the P-type end are also attracted away from the junction towards the negative electrode. Here, the potential barrier to the diffusion current and the space charge width are increased. Since the potential barrier is now large, the overall result is a small net current flowing from n side to p side, which is called the reverse saturation current (Isor Io). Increasing the reverse voltage across the junction further causes no change to the current until, at large reverse voltages, Zener and avalanche breakdown processes cause large reverse currents to flow.

Finally, if the reverse bias voltage Vr applied to the diode is increased to a sufficiently high enough value, it will cause the P-N junction to overheat and fail due to the avalanche effect around the junction. This may cause the diode to become shorted and will result in the flow of maximum circuit current and this shown as a step downward slope in the reverse static characteristics curve.

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