Contents
Physics Topics can be both theoretical and experimental, with scientists using a range of tools and techniques to understand the phenomena they investigate.
What is Breakdown of a Diode? What are the Uses of Light Emitting Diode?
Zener Diode
When an ordinary semiconductor diode is reverse-biased, a very small saturated reverse current flows across the junction due to the flow of a few thermally-generated minority carriers (electrons in p -region and holes in n -region). This current is not at all dependent on the applied reverse bias voltage. But, if this reverse bias voltage exceeds a definite value, the reverse current increases abruptly. This situation is known as breakdown of semiconductor diode. As a result, power consumed by the diode ie., rate of production of heat increases rapidly which can damage the diode.
The tolerance of some specially prepared semiconductor diodes is increased in such a way that at reverse bias, even due to flow of high reverse current, the diode is not damaged. This type of diode has important use for practical purposes and is generally known as Zener diode.
Explanation of Zener Effect:
i) If the reverse-bias voltage across a p – n junction diode is very high, the minority charge carriers are accelerated. Due to their high speed, they knock out more electrons from the covalent bonds. Such collisions produce electron-hole pairs. Newly generated carriers in turn, may gain sufficient energy to disrupt more covalent bonds and produce more electron-hole pairs. This phenomenon is cumulative and soon an avalanche of charge carriers is produced causing a flow of large current. Breakdown occuring in this manner is called avalanche breakdown and the diode is called avalanche diode.
ii) If both regions of a semiconductor diode i.e., a p – n junction diode are heavily doped, the thickness of depletion layer decreases to a large extent. Then if a very small reverse bias is applied, a very strong electric field is created between the two ends of the depletion layer. This electric field breaks up the covalent bonds of the semiconductor crystal directly and a huge number of charge carriers are set free within the crystal.
Thus, due to a comparatively small reverse-bias diode-breakdown occurs i.e., at a constant breakdown voltage of small magnitude, the diode reaches a state when a large reverse current flows. Thus breakdown occurs in this manner is called Zener breakdown and the diode is called Zener diode.
In case of any semiconductor diode of this type, avalanche effect and Zener effect occur simultaneously at reverse-bias. Generally, for a near about 6 V breakdown voltage, avalanche effect and Zener effect become equivalent to each other and with respect to temperature no special change of breakdown voltage takes place. So, diodes having breakdown voltage around 6V, are very suitable to use at different temperatures.
Avalanche effect or Zener effect—whichever may be the principal effect, these types of diodes are simply called Zener diode in practical cases.
Characteristic Curve: The ampere-volt (I – V) characteristic curve of a forward-biased Zener diode [Fig.] is similar to that of an ordinary semiconductor diode. But, when the reverse-bias voltage reaches a particular value VZ, the reverse current suddenly potential increases to a large value. This part of the characteristic curve is represented by AB, almost a vertical line.
In an ideal Zener diode, the increase of voltage with the increase of current is zero. In practical cases, this increase is within 1% to 5%.
Rating of a Zener diode: In every Zener diode a reference voltage and a reference power are mentioned. This voltage rating VZ [Fig.] indicates the reverse-bias voltage, at which the reverse current increases abruptly, but no change of the terminal voltage of the Zener diode takes place. The meaning of power rating or watt rating PZ is that, due to increase of current through the diode, if the value of power consumed exceeds the value of PZ, the diode will be damaged.
So, maximum safe reverse current through the diode, Imax = \(\frac{P_Z}{V_Z}\); the point P in Fig. indicates the value.
For example, in case of the rating 47V – 1 W of a Zener diode,
Imax = \(\frac{1 \mathrm{~W}}{4.7 \mathrm{~V}}\) = 0.21 A = 210 mA
In the circuit for a Zener diode, a regulative resistance is so selected that the value of reverse diode current never exceeds Imax.
Circuit symbol: Circuit symbol of a Zener diode is shown below [Fig.].
Zener diode as a voltage regulator: Zener diode is used to obtain constant voltage across a load resistance connected to a fluctuating dc voltage source. Zener diode and load resistance is to be connected in parallel.
Working principle: A fluctuating dc voltage source (unregulated voltage source) is connected to a Zener diode through a resistance Rs in series such that Zener diode is reverse biased [Fig.]. If the input voltage increases current through Rs and through Zener diode also increases.
Due to this, voltage drop across Rs increases, but the voltage drop across Zener diode remains constant as it operates in breakdown region. Breakdown voltage of Zener diode does not change by changing current through it. On the other hand, if the input volt age is decreased, current through R and Zener also decreases, but voltage drop across Zener remains same.
As soon as the reverse bias voltage of a Zener diode reaches VZ, in spite of increasing the current through it indefinitely, the terminal voltage of the diode remains constant at VZ. Hence, the terminal voltage of a load resistance RL connected parallel to the Zener diode also remains at VZ, in spite of any change of current through it.
Conversely it may be said that, to maintain a constant potential difference across a load resistance, a Zener diode of equal voltage rating is to be connected in reverse-bias parallel to the load resistance. This is called voltage regulation across a load resistance.
The voltage across a Zener diode thus serves as a reference so the diode is referred to as a reference diode.
Selection of Rs : If (VZ – PZ) be the rating of the Zener diode, then the maximum safe current through it is Imax = PZ/PV. The resistance RS is so chosen in the circuit that it restricts the Zener current below Imax even for the maximum value of the unsteady input voltage.
Load regulation: It is the capability to maintain a constant voltage (or current) level on the output channel of a power supply despite changes in the load resistance. More simply, load regulation is a measure of the ability of an output channel to remain constant for given changes in the load.
In the circuit of Fig., a milliammeter is connected to mea-sure current (IL) flowing through load resistance RL and a volt meter to measure potential difference across RL. Fig.(a) shows the changed circuit. Keeping the supply voltage Vi constant, RL is changed step by step and in each step IL and VL are recorded. Now a graph of VL – IL is drawn [Fig.(b)].
Evidently, the part AB indicates the regulated voltage. If the Zener diode behaves ideally, the line AB would become horizontal. Actually, point B exists a bit lower. From the portion BC it is understood that, if the magnitude of IL is very high, i.e., the Zener current is very low, VL becomes uncontrollable. As current IL is maximum at the poìnt B of the regulated zone so, the ratio \(\frac{V_L}{I_L}\) at B indicates the minimum value of the load resistance RL. In spite of fluctuation of load resistance above that minimum value through a long range, the potential difference across the load resistance VL remains almost constant.
In Fig.(b), if voltage at the point A is VNL (NL means no load or zero current) and voltage at the point B is VL, then percentage regulation = \(\frac{V_{N L}-V_L}{V_{N L}}\) × 100%.
In Ideal Zener diode, this percentage regulation is zero and in actual practice this value lies within 1% to 5%.
Light Emitting Diode or LED
If a specially made semiconductor diode or p – n junction in forward-bias emits light spontaneously, it is known as light emitting diode or LED.
Silicon (Si) or Germanium (Ge) diode is unsuitable as LED. For LED, semiconductor crystals of Gallium arsenide (GaAs), Gaffium Phosphide (GaP), Silicon Carbide (SiC) etc. are used. Colour of light emitted from LED depends on the band gap of the semiconductor crystal and strength of doping.
Working principal: When a p-n junction diode is forward-biased, both the electrons and the holes move towards the junction. As they cross the junction, recombination of a few electrons and holes takes place and energy is released at the junction in the form of light. Actually, photons are emitted from the p – n junction. The colour of the emitted light depends on the energy of the photons.
From the principle of conservation of momentum, it is found that a photon can be emitted only when an electron and a hole combine with equal and opposite momentum. This condition is fulfilled in some crystals like GaP or SIC but not in Ge or Si. In the later, the released energy of electron-hole pair is converted to heat energy which only makes the crystal heated. For this, GaP or SiC like crystals rather than Ge or Si are generally used to construct LED.
Circuit symbol: With the symbol of ordinary semiconductor diode two arrows directed outwards are drawn. This indicates the circuit symbol of LED [Fig.].
Characteristic curve: Volt-ampere characteristic curve of LED is identical with that of an ordinary semiconductor diode [Fig.]. But when it is forward biased, due to emission of light, a few electron-hole pairs are destroyed. So, the magnitude of current is less than that of an ordinary diode.
But, by the low current, the diode action of LED is not hampered because forward-bias never exceeds 2.5V or 3V and maximum value of current in forward-bias does not exceed 50 mA. If the magnitude of the forward current is increased slowly from 10 mA to 50 mA, the intensity of light emitted from LED continuously increases.
Uses: Power consumed by an LED is very small. In a well-planned circuit, these are not easily damaged and can be used uniformly for a long time. Moreover, LEDs are cheap in price. Hence, LEDs are extensively used in electrical and electronic circuits at present. It is extensively used for fast on-off switching. Besides these, LEDs are used in various electronic circuits, like torchlights, low-power household electric lamps, calculator, digital watches, etc. These diodes are also used in signal lamps.
Photodiode
A photodiode is a special type of reverse-biased semiconductor diode. If light is made to fall on its p – n junction, reverse saturated current increases almost linearly with the intensity of the incident light.
The circuit diagram of a photodiode is shown in Fig.
At reverse bias of a junction diode, naturally a small reverse current flows in the circuit. This is called dark current. Now light is made to fall on p – n junction through a lens. New electron-hole pairs are created in exchange of the energy of the incident photons and hence the reverse current increases. It is found that, the
magnitude of reverse current is proportional to the intensity of the incident light. But, if the energy of the photon of the incident light is not sufficient to create additional electron-hole pairs, the photodiode will not function.
The volt-ampere characteristics of a photodiode is shown in Fig.
Photodiodes are used for identification of sound from sound-tape or sound-track of cinema, determination of intensity of light, light operated switches, electronic counters, CD player, smoke detector, etc.
Circuit symbol: With the symbol of an ordinary semiconductor diode, two arrows directed inward are drawn. This indicates the circuit symbol of photodiode [Fig.].
Solar Cell
A special and very important practical application of photodiode is solar cell. In photodiode, incident solar energy is so converted into electrical energy that, it behaves as a battery. At daytime in presence of sunlight, this solar battery is used as a charger. After-wards, this storage battery is used to operate various electrical appliances. The solar cells are used in artificial satellite or space vehicles to operate various electrical instruments kept inside the satellite or space vehicles. Also solar cells are used in calculators.
Earth surface gets an average of 1000 W solar power per squaremetre in a sunny day. Only 10% of the incident photons can produce electron-hole pair and make a photodiode active. So, approximately 100 W solar power per squaremetre is available for transformation to electrical energy. This available energy is too small compared to approximate expenses. So generally the available solar energy is focused on a small area with the help of a concave mirror.
But due to this process, temperature of the photodiode increases so much that even the much effective silicon crystal loses its efficiency. So in this case, use of semiconductor crystal like Gallium Arsenide (GaAs) is more suitable.
The characteristic curve of photodiode lying in the 4th quadrant (portion AB of Fig.), is very relevant to the action of a solar cell. In this case, potential difference is positive i.e., p – n junction is in forward-bias. But current is negative i.e., reverse current flows through the junction [Fig.]. It is to be noted that, current flows through the p – n junction from negative end to positive end and this flow is identical with the flow of current through a battery.
So the photodiode i.e., the solar diode behaves like a cell or battery. But the forward bias voltage of a cell does not exceed 1 V and the magnitude of reverse current is very small. Hence to increase output power, the internal resistance of a cell is made very small. Output voltage is increased by connecting a large number of cells connected in series and output current is also increased by a large number of such series combinations in parallel. The details of technology regarding construction of a solar cell is beyond our present discussion.