Basic Electronics Zener Diode Introduction:- A normal P-N junction diode is usually fabricated by adjusting the P-type and N-type semiconductors on a single semiconductor crystal. The characteristics of a junction diode demonstrate that it is designed largely for operating in the forward direction. Applying a large amount of forward bias causes greater forward current with a small value of forward voltage. However, reverse biasing the diode do not cause conduction of current till high values of reverse voltage are reached. If the reverse voltage is large enough, breakdown occurs and a reverse current starts to flow. Ordinary Junction diodes are generally damaged when this breakdown occurs. The flow of current in zener diodes is controlled by the minority charge carriers under the reverse bias condition, so they can also be referred to as break down diodes. During specific conditions of fabrication, a special type of diode is formed that will not be ruined when the breakdown voltage is increased, given that the current does not exceed a defined limit to prevent the case of overheating. This type of devices is referred to as zener diodes. Zener diodes allow current to flow in the forward direction in the similar manner as an ideal diode, and also it permits current to flow in the reverse direction when the voltage is above a certain value known as the breakdown voltage. This voltage can also be referred as zener knee voltage or zener voltage. Zener Diode I-V Characteristics Curve In the forward bias condition, the zener diode behaves like an ideal diode within specified current and power limits, but it differs in reverse bias condition where the zener diode has very steep avalanche characteristic at the breakdown voltage in reverse bias condition. Zener operates mainly in the reverse bias mode by connecting anode to the negative terminal of the power supply. Zener diodes are categorised and rated by the voltage at which they will turn on or start to conduct the reverse bias current. Generally these zener diodes are used to regulate the voltage. In reverse bias condition after the break down zener diode provides a constant output voltage even if we increase the input voltage. There are specifically two separate mechanisms that might cause a breakdown in a zener diode: Avalanche Breakdown It is predominant above approximately 5.5 volts. This mechanism is also referred to as impact ionisation or avalanche multiplication. For reverse conduction it is necessary to visualise the phenomenon of avalanche breakdown. This process begins when a large negative bias is applied to the PN junction, sufficient energy is imparted to thermally generated minority charge carriers in the semiconductors. As a result the free carriers acquire required kinetic energy to break the covalent bonds and create an electric field through collisions with crystal particles. The charge carriers created in collision contribute to the reverse current, well beyond the normal reverse saturation current and may also possess enough energy to participate through collisions, creating an additional electric field and the avalanche effect by impact ionization, once a sufficiently high reverse bias is provided this process of conduction takes place very much like an avalanche: a single electron can ionise several others. Zener Breakdown It is predominant below approximately 5.5 volts. This mechanism is also referred to as a high field emission mechanism. The phenomenon of zener breakdown is related to the concept of avalanche breakdown. Zener breakdown is achieved by heavily doped regions in the neighbourhood of ohmic contact. It is the second method of disturbing the covalent bonds of the crystal atoms and increasing the reverse bias zener diode current, to be sustained at a much lower specific voltage than normal diode. The reverse bias voltage known as zener voltage, where this mechanism occurs is determined by the diode doping concentration and it occurs when the depletion layer field width is sufficiently enough to disrupting the covalent bonds and cause number of free charge carriers due to electric field generation to swell. Zener Diode as Voltage Regulator I-V characteristics of zener diode make it suitable for application such as a voltage regulator. A voltage stabilizer is a combination of elements that are designed to ensure the output voltage of a supply fairly remains constant. Excess voltage protection is done by using zener diodes because there will be reverse current due to minority charge carriers starts flowing through the diode after the reverse bias voltage exceeds a certain value. Keeping the zener diode in parallel with a variable load resistance RL, ensures a constant output voltage even though the load current and the supply voltage varies. In practical circuits the simplest form of current source is a resistor. The key in using the zener diode as voltage regulator is that as long as the zener diode is reverse biased, the flow of current greater than a few micro amperes must be accompanied by a voltage greater than the Zener voltage. This type of arrangement of the circuit provides safety for equipment connected to terminals. This arrangement of regulator circuit is referred to as a shunt regulator in which the regulating element is placed in parallel with the load. The input voltage to the system is a few volts and as long as it is more than the desired output voltage, a stable voltage will be produced across the zener diode. As the input voltage increases, current through the zener diode increases, but the drop in voltage remains constant which is the necessary feature required for zener diodes. Therefore, reverse current in the circuit has increased, voltage drop across the resistor increases by an amount equal to the difference between the applied input voltage and the zener knee voltage of the zener diode. The output voltage of regulator system is fixed as the zener knee voltage of the zener diode and can be used in power devices requiring a fixed voltage of firm value. The zener diode will continue in regulating the voltage till the zener diode current falls below the minimum Iz min value in the reverse breakdown region. The source resistance Rs is connected in series with zener diode to limit the flow of current through the diode with voltage source connected across the combination. The cathode terminal of zener diode is connected to the positive terminal of the voltage source so that the zener diode is biased in reverse condition and will be operating in breakdown region. When the load is not connected across the zener diode, no load current will be conducted and all the current due to the circuit will pass through the zener diode dissipating maximum amount of power that causes overheating of the diode and damages permanently. Selecting the appropriate values of series resistance Rs is also important because it also causes greater diode current, so that maximum power dissipation of the diode should not be exceeded under no load or at high impedance condition. Whenever a load is connected in parallel with zener diode, voltage across the load is same as the zener diode voltage. However the source voltage must be greater than the zener voltage and the upper limit of zener current depends on the power rating of the zener diode; otherwise the zener voltage will simply follow the applied input voltage. Bipolar Junction Transistor The Bipolar Transistor basic construction consists of two PN-junctions producing three connecting terminals with each terminal being given a name to identify it from the other two. These three terminals are known and labelled as the Emitter ( E ), the Base ( B ) and the Collector ( C ) respectively. Bipolar Transistors are current regulating devices that control the amount of current flowing through them from the Emitter to the Collector terminals in proportion to the amount of biasing voltage applied to their base terminal, thus acting like a current-controlled switch. As a small current flowing into the base terminal controls a much larger collector current forming the basis of transistor action. The principle of operation of the two transistor types PNP and NPN, is exactly the same the only difference being in their biasing and the polarity of the power supply for each type. Bipolar Transistor Construction:- The construction and circuit symbols for both the PNP and NPN bipolar transistor are given above with the arrow in the circuit symbol always showing the direction of “conventional current flow” between the base terminal and its emitter terminal. The direction of the arrow always points from the positive P- type region to the negative N-type region for both transistor types, exactly the same as for the standard diode symbol. Bipolar Transistor Configurations As the Bipolar Transistor is a three terminal device, there are basically three possible ways to connect it within an electronic circuit with one terminal being common to both the input and output. Each method of connection responding differently to its input signal within a circuit as the static characteristics of the transistor vary with each circuit arrangement. • Common Base Configuration – has Voltage Gain but no Current Gain. • Common Emitter Configuration – has both Current and Voltage Gain. • Common Collector Configuration – has Current Gain but no Voltage Gain. The Common Base (CB) Configuration As its name suggests, in the Common Base or grounded base configuration, the BASE connection is common to both the input signal AND the output signal. The input signal is applied between the transistors base and the emitter terminals, while the corresponding output signal is taken from between the base and the collector terminals as shown. The base terminal is grounded or can be connected to some fixed reference voltage point.