Transistor Advantages and Disadvantages

This article discusses the pros and cons of transistors, focusing on Bipolar Junction Transistors (BJTs) for explanations on their operation and applications.

What is a Transistor?

• A transistor is a solid-state device, serving as a modern equivalent to the triode vacuum tubes used in early radios and computers.
• It’s constructed from three layers of semiconductor materials, commonly silicon or germanium.
• Transistors can act as both insulators and conductors.
• This three-layered type is also known as a Bipolar Junction Transistor (BJT). The term “bipolar” arises from the use of both electrons and holes as charge carriers within the transistor’s structure. BJTs are categorized into NPN and PNP types.
• A transistor has three regions: the emitter, base, and collector. The emitter is heavily doped, the base is lightly doped, and the collector is moderately doped.
• The basic planar structure of a BJT transistor is illustrated below:

• The symbols for NPN and PNP transistors are shown in figure 2.
• There are various types of transistors, including BJTs, FETs, and MOSFETs.
• A BJT is a current-controlled device that amplifies the input signal. A FET, on the other hand, is a voltage-controlled device with source, gate, and drain terminals.

• For a transistor to operate, the Base-Emitter (BE) junction is forward biased, and the Base-Collector (BC) junction is reverse biased. This connection results in current flow, where I_E = I_B + I_C, as depicted in the figure below.
• A BJT transistor has three operating regions: cutoff, linear, and saturation. These can be likened to the different states of a water tap: OFF, ON, and fully ON, respectively.

• Transistors can function as both switches and amplifiers. As a switch, it operates in either the cutoff or saturation region. As an amplifier, small changes in the base current cause the collector current to mimic the input base signal with a higher amplitude.

Advantages of Transistors

Here are the benefits of using transistors:

• High Input Impedance: Common collector BJT amplifiers have the highest input impedance and the lowest output impedance. Darlington pairs, which offer very high current gain, are used where very high impedance is required.
• Current Control: Transistors can be used as current-controlled current sources.
• Fast Switching: They are suitable for fast switching applications.
• Low Cost: Transistors are readily available at a low cost.
• Small Size: They are significantly smaller in size compared to vacuum tubes.
• Long Lifespan: Transistors have a longer operational life.
• Low Voltage Operation: They operate at low voltages, improving safety.
• No Cathode Heater Power Consumption: Unlike vacuum tubes, transistors do not require power for a cathode heater.

Disadvantages of Transistors

Here are the drawbacks of using transistors:

• Difficult Troubleshooting: Due to their small size, it can be difficult to identify faulty transistors. Soldering and replacing them can also be challenging.
• Complex Manufacturing: The manufacturing process is complex and requires a clean room environment.
• Non-Zero ON Resistance: Transistors have a non-zero ON resistance. This means that when a transistor is in the ON state, the voltage across it is never truly zero.
• Leakage Current: Even in the OFF state, a small leakage current flows. As a result, transistors don’t function as efficiently as mechanical switches, electrical switches, or relays.