The diode is an electronic instrument of great importance, one of them being using the diode as a switch. It is interesting to note that under certain conditions we can use a diode as a switch. In this article, we will study what is an electrical switch and how it differs from a mechanical switch. We will look at the working of the diode as a switch and we will even understand the diode switching times.
In the end, we will see the advantages and disadvantages of a diode as a switch. We will understand the advantages through the applications of the diode as a switch. The article will conclude with some Frequently Asked Questions that readers can refer to.
What is an Electrical Switch?
We can use devices like diodes to manufacture electrical switches. The PN junction in a diode can be used in different modes to operate as a switch. In the forward mode of the diode, the switch is considered to be 'ON', and in reverse mode, it is considered to be 'OFF'.
Electrical switches are preferred over conventional mechanical switches due to multiple reasons electrical switches are less prone to oxidation as compared to mechanical switches due to the material used for making them. This makes electrical switches more reliable and useful.
Electrical Switches Over Mechanical Switches
There are certain benefits that electrical switches offer over mechanical switches that make electrical switches a better choice. Some of these are:
- Electrical switches are less prone to oxidation as compared to mechanical switches mainly due to different materials used for designing them.
- The contacts in electrical switches are fixed and can be moved only physically whereas mechanical switches have movable contacts.
- The designing of mechanical switches makes them less capable of handling stress and strain whereas electrical switches can withstand more stress and strain.
- Any impact on the mechanical switches will directly affect their performance therefore one has to be very cautious with them.
- Electrical switches offer more features than mechanical switches like automating the task of turning ON and OFF.
Working of Diode as a Switch
A diode has a PN junction which is doped up with impurities. The P-region in a diode is lightly doped with impurities whereas the N-region is heavily doped with impurities. Different amounts of dopants are used for this purpose.
The diodes like silicon have a certain reference voltage. Any voltage applied to the diode below this reference voltage decreases the resistance of the diode which makes the diode forward-biased. In forward-biased conditions, the diode acts as a closed switch. When the applied voltage increases the threshold, it increases the resistance of the diode. Under this condition, the diode is reverse-biased. This makes the circuit open and the diode acts as an open-switch. Let us see the working through the circuit diagram.
(NOTE: Here, the reference voltage is used as a standard for comparison and is assumed to be a negative voltage. Changing this voltage alters the polarity of the terminal connected to the diode, which in turn affects the biasing of the diode as described above.)
Diode As a SwitchAs explained above, when the voltage applied to the diode is greater than the reference voltage, the resistance of the diode increases due to increase in the width of the depletion region. Under this reverse biased condition, a reverse current flows through the diode for a small period of time. After this small period of time, the switch turns OFF , the reverse current is directly proportional to the leakage current.
Note that this can lead to a phenomena called Ringing. Ringing refers to the oscillations that occur in the diode due to sudden flow of reverse current in a diode under reverse-biased condition. This ringing is a lossy phenomena and therefore should be minimized.
Diode Switching Times
Whenever the diode stays in a state for a longer period of time ,it becomes transient and then moving from that state to any other state can require certain amount of time. This time taken by circuit is often described as transient response of system and is an important parameter to analyze the electric circuit.
Here are certain responses of the system :
- Recovery Time
- Forward Recovery Time
- Reverse Recovery Time
Let's discuss them briefly :
- Recovery Time: It is the time taken by diode before switching from forward-bias to reverse-bias or vice versa is called the recovery time of diode.
- Forward Recovery Time: It is the time taken by diode before switching from reverse-bias to forward-bias is called the forward recovery time of diode.
- Reverse Recovery Time: It is the time taken by diode before switching from forward-bias to reverse-bias is called the reverse recovery time of diode.
What is Carrier Concentration?
Let us study the concentration of charges at the junction of diode. We can say that the majority "minority carriers" lie at the junction and decrease exponentially from there which means that majority carriers are present at the ends. When a voltage is applied above the cut off voltage, the diode is in forward biased condition. The electric field develops at the junction in such a way that electrons from n side move towards p side and holes from p side move towards n side. This means the concentration will increase at the junction whereas the sides will have minority carriers.
Let us study the different cases.
- Forward Bias Condition
- Reverse Bias Condition
Case I: Forward Bias Condition
Here is a pictorial view of charges during the forward biased condition. Note how the concentration in excess at decrease from there onwards. The dotted line represents the concentration at equilibrium.
Forward biased carrier distributionFrom the above graph, we can see that in forward biased condition the minority charges in P-type region is pn-pno .Steady state occurs when concentration becomes pno. The minority charges in N-type region is np-npo. Steady state occurs when concentration becomes npo.
Case 2: Reverse Bias Condition
Here is a pictorial view of charges during the reverse biased condition. Note that majority carriers don't don't generate current in reverse biased condition due to excess resistance of diode.
Reverse biased carrier distributionIn reverse biased condition, the diode behaves like a short-circuit due to the increased width of depletion region. The current in reverse biased condition, the minority carriers will cross the junction resulting in a reverse saturation current. This current is enough to turn the circuit ON and this current flows till the excess carriers are present in the diode.
Characteristics of Switching Diode
Switching times are used in diodes to study the characteristics of switching diode. Let us study them
- Turn-on time: Turn-on time is the time required to turn on the switch from switching diode from the cut-off . We require this time to be short so that appliances start working as soon as they are turned on. Therefore, we use appliances with minimum turn-on time.
- Reverse Recovery Time:Once a diode has been turned on then it may require some time to go to the 'OFF' state. The time required by a diode from going to 'OFF' state from 'ON' state is known as reverse recovery time of the diode. The reverse recovery time of diode is usually larger than the turn on time.
- Switching Time:Switching Time is defined as the sum of turn-on time & reverse recovery time. The speed of switching from one state to the other state is quite fast in the diodes. If we compare two materials then we can say that Ge has a reverse recovery time of .few hundred nanoseconds whereas silicon diodes have a reverse-recovery time of few nanoseconds.
The resistance of diode directly impacts the switching speed of diodes. Minimum resistance of diode occurs when high forward voltage is applied to the diode which reduces the switching time of diode.
Factors Affecting the Switching Time
Let us now see the factors that affect the switching time of diode:
- Capacitance Developed: The P side and N side of a diode behave like two metal plates of a capacitor with dielectric material in between. This PN junction capacitance changes depending upon the bias conditions mainly because of variations in doping concentrations of dielectric i.e. junction.
- Resistance Developed: The deplition layer developed in the diode works as a resistance .Depending on the width of the region ,the resistance offered by the diode changes controlling whether the amount of current generated is in milli-amps or micro-amps..
- Doping Concentration: The level of doping of the diode, affects the diode switching times. The diffusion rate is indirectly dependant on this doping concentration of majority and minority carriers therefore it affects the switching time.
- Depletion Width Developed: The depletion width formed due to carriers also affects the switching time of diode. The narrower the width of the depletion layer, the faster the switching will be since less time will be required for carriers to cross the depletion region.
Advantages of Diode as a Switch
- Diode switches are made up of oxidation proof material which prevents them from getting oxidized which makes them a reliable and long-term switch.
- The simple design of diode as switches makes it easy to design them. The overall circuit design is simple and straightforward .This can make it easy to integrate the switch on the whole circuit board.
- Diode switches are energy-efficient devices since they consume low power. Devices that are battery powered can used diode switches for energy efficiency.
- The diode switches have fast switching ability. They are suitable for high-frequency appliances that use high frequency for performing tasks. This is mainly useful in digital circuits that require fast switching actions.
- Switch diodes are very cost-efficient devices because they are made up of inexpensive components . This helps to produce these switches on a large scale without being concerned about spending large amounts of money.
Disadvantages of Diode as a Switch
- Diode switches are made of very few components which limits the tasks they can perform . The limited functionality of diode switches is a major disadvantage and limits their applications in other devices like bidirectional circuits.
- Diode switches are temperature sensitive devices. This means the voltage at which forward bias condition occurs is dependent on temperature. The temperature sensitivity affects the overall performance of diodes.
- Diode switches are not suitable for high-power appliances due to their ability to carry a limited amount of current. High power appliances require the use of devices like transistors that don't have a small limit for carrying current.
- Use of diode switches introduces reverse-recovery time in the circuit. This refers to the time taken by diode to switch from the conducting state to the non-conducting state. Reverse -recovery time can therefore introduce delays in circuits affecting diode performance.
- Diodes have a certain threshold voltage such as 0.7V for silicon. Low voltage appliances can face disadvantage by using diode as a switch since every bit of voltage matters.
Applications of Diode as a Switch
- Diodes are used in High speed rectifying circuits and High speed switching circuits. Due to the high speed operating ability of diode, it can be used for ideal high-speed switching circuits.
- A PN junction diode has certain rectification properties which can be used as an RF switching diode. These RF switches have very low p-n junction capacitance.
- Switching Diodes are an ideal choice for telecommunication appliances .They are used in small signal analysis, in rectifiers for AC line rectification. They can be used for voltage ranges less than tens of volts.
- Diodes are used in Automotive applications for different purposes. The diode protects essential elements of the car from current spikes by controlling the voltages levels in case of a sudden increase.
- Switching Diodes have some general purpose applications like consumer applications. They are even used in switches, transient voltage suppressors, diode logic, voltage clamping circuits, and voltage detection circuits.
Conclusion
We have seen how diodes have many functionalities. One of the major functionality of diodes is their use as a switch in different appliances. The working of diode as a switch has been explained clearly to ensure proper understanding of the concept. We have seen how using a diode as a switch has many advantages over the conventional mechanical switches. Despite the various features it offers, there are some limitations of using diodes as a switch. The major disadvantage is the reverse recovery time of diodes. Developments are being made in this field to enhance the functionalities of diode and use it in more number of appliances with time.
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