- What is a diode?
- Intrinsic and Extrinsic Semiconductor and Doping
- PN Junction
- Biasing of PN-Junction
What is a PN Junction diode?
A diode is a two-terminal semiconductor device that passes current only in one direction. A diode is constructed with a PN junction, and each terminal is connected to one side of the PN junction. Here P and N are semiconductors with P-type or N-type doping. P (positive charge or Holes-carrying) is material and N (Negative charge or Electrons-carrying) is material. We will discuss P-Type and N-Type semiconductors.
A semiconductor is a material that is neither an insulator nor a good conductor in intrinsic form. Atoms of an Element have four valance electrons, the element is a semiconductor. Single element semiconductor materials examples are Germanium, Silicon, and carbon.
in an atom, each electron shell has an energy level that the electrons are confined to that band. An electron in a band acquires enough energy and jumps to the upper electron shell or energy band. The outer shell of an atom is the valance shell. And in terms of energy, it is the valance band. Next to the valance band is the conduction band. The difference in energy levels of valance and conduction band is called the energy gap. If an electron of a valance shell acquires energy more than the energy gap it will jump to the conduction band, or the electron becomes a free electron.
Energy gap differences between insulator, conductor, and semiconductor The Energy gap of the semiconductor is greater than the conductor energy gap. And smaller than the insulator energy gap. Larger energy gap materials require more energy to become electrons to free electrons. Free electrons travel from molecules to molecules or atoms in the applied electric field direction. Insulator materials have a larger energy gap, so they require a high breakdown voltage to start electrons’ conduction.
Intrinsic and Extrinsic Semiconductor and Doping
The Intrinsic semiconductors are pure silicon or germanium. While after adding some impurity to intrinsic semiconductor material is called extrinsic semiconductor. Adding impurity into intrinsic semiconductors is called doping. Doping is done with trivalent or pentavalent impurities. The trivalent or pentavalent impurity selection is used to attain P-Type or N-Type properties in the extrinsic semiconductor.
Silicon and Germanium have four valance electrons. If we add an impurity having five valance electrons into silicon or germanium. Then, a free electron remains in the resultant bonding. So, after this doping material has some electrons in the conduction band, this doping improved the conductivity of the material. Due to the availability of free electrons material can donate electrons. An electron is a negative charge particle. So, this type of impurity is called donor impurity. In N-type, semiconductor material electrons are the majority charge carriers and holes are the minority charge carriers.
If we add trivalent impurity into silicon or germanium. Then an electron is missing in the silicon crystal lattice. This missing electron space is called the hole. Holes accept a free electron. So, the material is the electron acceptor. This type of material is called a P-Type semiconductor. On application of voltage, holes are filled by free electrons coming from the source negative. When an electron is shifted to the next hole, the electron leaves behind a hole also. So, holes flow in the opposite direction of electrons. Or positive to the negative direction of the voltage source. A hole is an absence of an electron in a full valence band not a physical particle as the electron. So, the movement of holes is imaginary. Holes are majority charge carriers in P-type semiconductors and electrons are minority charge carriers.
PN-Junction is the boundary area between two adjoined N-type and P-type semiconductor blocks. At this boundary small portion of both semiconductors is diffused into another. In the diffused area, both types of charge carriers exist with an opposite electric field on both sides. In the diffused region electrons are moved to the P-type boundary layer. This diffusion creates an electric field across the diffusion region. Holes are created at the N-type boundary layer due to the electric field. Between this boundary, almost all charge carriers are depleted and there is an equal electric field on both sides. This situation is called equilibrium. This junction region where charge carriers are almost gone is called the depletion region. The depletion region acts as an insulator due to no charge carriers inside this region.
Vo is junction barrier voltage. VT is the thermal voltage of 26mV at room temperature. In and Np are impurity concentrations and ni is the intrinsic concentration. And “q” is the charge of particle electron.
Biasing of PN-Junction
PN Junction diode can be biased in two ways. One is Forward biased and the second is reverse-biased. In forward biasing anode is connected to the voltage source positive terminal and in the reverse biasing anode is connected to the voltage source negative terminal. In the forward biasing diode or PN junction conduct electric current or react as close circuit switch. In the reverse biasing diode react as open circuit.
In the forward bias, the N-dopped side is connected to the voltage source negative terminal, and the P-dopped side is connected to the positive terminal of the voltage source. Holes are attracted to the negative supply voltage and electrons are attracted to the positive supply voltage. Due to these attractions, as the supply voltage is increased, the depletion region is narrowed. When the supply voltage is greater than the junction barrier voltage. Then both types of charge carriers are crossed the depletion region. Further electrons are injected from the supply negative terminal into the N-type side. From P-type electrons are injected into the supply positive terminal. Thus, the density of holes is increased on the P-type side and N-side also. And injected electrons from supply negative terminals fill holes and flow to supply positive side. Thus, the flow of electrons happens. To sustain this flow, a continuous voltage is needed across the PN junction to keep breaking the junction barrier. This voltage level is called barrier voltage. For silicon diode barrier voltage is 0.7V and 0.3V for Germanium diode. However, these voltage levels can differ when alternative materials and doping are used.
In reverse bias, the negative supply terminal is connected to the diode anode terminal and the supply positive terminal is connected to the diode cathode terminal. In reverse bias, an electric field is applied across the diffusion layer, which forces to expel the majority-carrier electrons in the N-Type side towards the supply positive terminal. The electric field also forces to expel the majority-carrier holes in P-type towards the supply negative terminal. Thus, this electric field is expanding the depletion region. An expanded depletion region means no current flow. The depletion region expands with an increase in reverse voltage.
The depletion region expands over input reverse voltage. At a certain level of reverse voltage, the depletion region expands up to PN junction outer walls. Further increase in reverse voltage causes covalent bonds to break. This effect is called the Avalanche effect and generates many charge carriers, and the diode starts to allow the maximum flow of current. This situation is called the reverse breakdown of the PN junction diode. And voltage level is called the reverse breakdown voltage. In practice, beyond the safe limit high current and the reverse voltage rises the temperature of the diode resulting in permanent damage. However, if reverse current or total power dissipation is controlled under a safe limit, the diode returns to its original condition after removing reverse voltage.
Reverse Leakage Current
Due to a very small amount of minority carriers in PN junction exist so in reversed bias diode exhibit a small amount of current flow. This is called reverse leakage current.