Why transistors are not used as switching devices

The name bipolar indicates that two types of charge carriers i. The transistor has three regions, namely base, emitter and collector. The emitter is a heavily doped terminal and emits electrons into the base. Base terminal is lightly doped and passes the emitter-injected electrons on to the collector. The collector terminal is moderately doped and collects electrons from base. This collector is large when compared to the other two regions so it can dissipate more heat.

Both these function in the same way but they differ in terms of biasing and power supply polarity. These two transistors can be configured into different types like common emitter, common collector and common base configurations. Depending on the biasing conditions like forward or reverse, transistors have three major modes of operation namely cutoff, active and saturation regions. In this mode, the transistor is generally used as a current amplifier.

In active mode, two junctions are differently biased that means emitter-base junction is forward biased whereas collector-base junction is reverse biased. In this mode, current flows between emitter and collector and the amount of current flow is proportional to the base current. In this mode, both collector base junction and emitter base junction are reverse biased. As both the PN Junctions are reverse biased, there is almost no current flow except small leakage currents usually in the order of few nano amps or pico amps.

In this mode of operation, both the emitter-base and collector-base junctions are forward biased. Current flows freely from collector to emitter with almost zero resistance. In this mode, the transistor is fully switched ON and is essentially a close circuit. The below figure shows the output characteristics of a BJT.

In the below figure, the cutoff region has the operating conditions when the output collector current is zero, zero base input current and maximum collector voltage. Therefore, the transistor is completely in OFF condition. Similarly, in the saturation region, a transistor is biased in such a way that maximum base current is applied that results in maximum collector current and minimum collector-emitter voltage.

This causes the depletion layer to become small and to allow maximum current flow through the transistor. Therefore, the transistor is fully in ON condition. This type of switching application is used for controlling LEDs, motors, lamps, solenoids, etc.

A transistor can be used for switching operation for opening or closing of a circuit. This type solid state switching offers significant reliability and lower cost when compared to conventional relays. Some of the applications use a power transistor as switching device, at that time it may necessary to use another signal level transistor to drive the high-power transistor. Based on the voltage applied at the base terminal of a transistor switching operation is performed.

Therefore, the transistor acts as a short circuit. Similarly, when no voltage or zero voltage is applied at the input, transistor operates in cutoff region and acts as an open circuit.

In this type of switching connection, load here an LED is used as a load is connected to the switching output with a reference point. Thus, when the transistor is switched ON, current will flow from source to ground through the load. At the base, an input signal varying between 0V and 5V is given. We are going to see the output at the collector by varying the V I at two states that is 0 and 5V as shown in figure.

And the corresponding base current for this collector current is So, it is clear that when the base current is increased beyond the Consider the case when zero volt is applied at the input. This causes the base current to be zero and as the emitter is grounded, emitter base junction is not forward biased. Therefore, the transistor is in OFF condition and the collector output voltage is equal to 5V.

Thus, the output at the collector becomes approximately zero. This type of switching is used for negative ground configurations. Since transistors can be used to switch power flowing to ICs, they are essentially driving capacitive loads and there is a slightly delayed response seen at the IC pin due to its input capacitance.

This back EMF is normally damped with a flyback diode to prevent damage to other components in the system. The activating circuit in a relay is galvanically isolated from the energized side of the relay, which provides a major level of safety when relays are used for switching high voltages. In contrast, a transistor does not have any isolation, and an ESD event at one terminal can propagate to the two other terminals.

Transistors used in high power systems that need some ESD protection will require some extra components for user protection and to ensure circuits are not damaged. A relay can be used with AC or DC power over a very broad range of power levels. A transistor is generally meant to be used with DC power or digital signals, but they can be used with AC signals as well.

However, a transistor must be carefully designed to operate in its linear range to prevent the transferred AC signal from clipping and producing harmonic distortion. For this reason, transistors are less desirable for use in high power AC systems, but they are still useful as analog components generally, as long as they are operating in the linear range. Relays are not meant to be switched repeatedly, as their electrical contacts will wear out over time. In contrast, a transistor has no moving parts, so it will have an exceedingly long lifetime and can be switched repeatedly without experiencing wear as long as it is not driven beyond its absolute maximum ratings.

This is why transistors are used as switching elements in switched-mode power supplies and power converters. Over time, the contacts in the top-left portion of this image will wear out due to friction and arcing.

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Register today to start searching the right components for your next design. Sign Up. Relays vs. Transistors: Which Is the Correct Choice? February 9, Share Tweet Pin. Transistors: Their Unique Characteristics Relays and transistors are multi-terminal devices that provide switching functions. Relay switching: A relay is a mechanical switch triggered by using an electric current to generate a magnetic field near an armature.

A magnetic field is generated in a coil, which pulls the mechanical switch armature closed or pushes it open. Transistor switching: Transistors are semiconductor devices , and the electrical conductivity of the conduction channel is modulated by applying a voltage for FETs or current for bipolar transistors to the third terminal. Application area and specification Relay Transistor Power level Can be used with very high voltages and currents that would destroy transistors.