What is Transistor?
Transistor is a three-terminal electronic component. It has a control (“gate”) terminal, a collector terminal and an emitter terminal. Current flows through the emitter to the collector in one direction, and it can be interrupted by using the control terminal. An NPN transistor turns on or off in response to a positive voltage applied to its control terminal. A PNP transistor turns on or off in response to a negative voltage applied to its control terminal.
The transistor is a component in a semiconductor circuit. When the transistor is in the ON state, the circuit is ON, and when the transistor is in the OFF state, the circuit is OFF. shows a simple transistor circuit that controls the power supply to the circuit (ON when the base-emitter junction is forward biased, OFF when the base-emitter junction is reverse biased). The circuit has four transistors. The ON/OFF state of the circuit can be controlled by using the collector terminal as the output. For example, the ON state controls the circuit output to be in the high level (ON) state, and the OFF state controls the circuit output to be in the low level (OFF) state.
Transistor technology has been used in electronics and computing since the 1920s. At least one transistor was invented in 1933 by two scientists working independently, William Shockley and John Bardeen. The invention was the first practical device for amplifying or switching a flow of electricity in an electronic circuit. The first working devices were silicon transistors, invented by Julius Lilienfeld and Walter H. Weber at Bell Labs, New Jersey. The first working solid-state transistor was the germanium junction transistor invented by Philo Terman and Jack Kilby at Texas Instruments. This was followed shortly by several other working devices. In the 1950s, transistor technology reached the semiconductor industry where transistors began to be used in computers and other devices.
Transistors in a device
A transistor is a semiconductor device with two major parts. The first is the collector, which collects the current. The second part is the emitter, which supplies the current. These two parts are separated by a layer of dielectric material called a junction or barrier. The most common junction or barrier is the p-n junction between the region of the semiconductor that is p-type and the region of the semiconductor that is n-type.
The transistor can only pass current when a voltage is applied across the collector and emitter. Current cannot flow from the emitter to the collector in the absence of a voltage applied across the emitter and collector. The semiconductor used in the transistor must have the ability to pass current only when a voltage is applied across the semiconductor. In the absence of a voltage applied across the semiconductor, the current is stopped.
Transistors pass or amplify a current because they have a very small resistance when they are switched on. The resistance is usually very small and is given by the ratio of the current when the transistor is switched on to the current when the transistor is switched off. The term resistance is usually given to the resistance of the dielectric layers between the collector and emitter. As the size of the transistor is increased, the resistance of the dielectric layers tends to be reduced.
If the resistance of the dielectric layers between the collector and emitter is reduced below the resistance of the transistor when the transistor is switched on, the collector current is amplified. The resistance of the transistor when the transistor is switched on is given by this ratio. If the transistor has been properly designed, the resistance when the transistor is switched off will be very small. If it is very small, then the resistance when the transistor is switched on will be much larger. The resistance when the transistor is switched off is given by the voltage across the collector and emitter when the transistor is switched off. The voltage is divided by the emitter current and the ratio of the voltage divided by the emitter current is the resistance of the transistor when the transistor is switched off.
The base currency is the current that flows from the base to the emitter. The emitter current is given by the ratio of the current when the transistor is switched on to the current when the transistor is switched off. This ratio is the transistor's amplification or gain. The collector current is given by this ratio of the current when the transistor is switched on to the current when the transistor is switched off. The resistance of the transistor when the transistor is switched on is given by the voltage across the collector and emitter divided by the emitter current. The voltage is multiplied by the ratio of the emitter current to the collector current.
Summary
Voltage is the difference in electric potential between two points on a conductor. Voltage is defined by the amount of work done to move electric charge from one point on the conductor to another. Voltage is measured in volts or volts per amp. Current is the flow of electric charge past a conductor. Current is measured in amps or amps per centimeter. Resistance is the ratio of the voltage divided by the current.
Voltage is measured across the terminals of a component. Amperage is measured across the terminals of a component. Current is measured across the terminals of a component. Resistance is measured across the terminals of a component.
FAQs on Transistor as a Device
1. What is a transistor and what are its three main terminals?
A transistor is a semiconductor device used to amplify or switch electronic signals and electrical power. It is a fundamental building block of modern electronics. It consists of three terminals:
- Emitter (E): This terminal supplies a large number of majority charge carriers.
- Base (B): This is the middle section which is very thin and lightly doped. It controls the flow of charge carriers from the emitter to the collector.
- Collector (C): This terminal collects the majority charge carriers supplied by the emitter.
2. What is the main difference between an NPN and a PNP transistor?
The primary difference lies in their construction and the type of charge carriers they use. An NPN transistor consists of a layer of P-type semiconductor sandwiched between two layers of N-type semiconductor. In an NPN transistor, the majority charge carriers are electrons. Conversely, a PNP transistor has a layer of N-type semiconductor between two layers of P-type semiconductor, and its majority charge carriers are holes. Their circuit symbols also differ to indicate the direction of conventional current flow.
3. How does a transistor function as an electronic switch?
A transistor acts as a switch by operating in two distinct regions: the cutoff region and the saturation region.
- OFF State (Cutoff Region): When no current is applied to the base, the transistor allows almost no current to flow from the collector to the emitter. This is equivalent to an open switch.
- ON State (Saturation Region): When a sufficient current is applied to the base, the transistor allows the maximum possible current to flow from the collector to the emitter. This is equivalent to a closed switch.
By controlling the small base current, we can turn the much larger collector current on or off.
4. Explain the principle of a transistor working as an amplifier.
A transistor works as an amplifier when operated in its active region. In this state, a small change in the input signal (applied as a small base current, I_B) results in a much larger, proportional change in the output signal (the collector current, I_C). The ratio of the change in collector current to the change in base current is called the current amplification factor (β). This amplification is possible because the small input signal controls the flow of a much larger current from an external power supply through the collector-emitter path.
5. Why is a Bipolar Junction Transistor (BJT) considered a current-controlled device?
A BJT is called a current-controlled device because its output current (collector current, I_C) is primarily controlled by its input current (base current, I_B). The relationship is defined by the equation I_C = β * I_B, where β is the current gain. A small change in the base current directly causes a large, proportional change in the collector current. This is fundamentally different from devices like FETs (Field-Effect Transistors), which are voltage-controlled.
6. In what operating regions does a transistor act as an amplifier versus a switch, and why?
A transistor's function is determined by its operating region, which is set by its biasing:
- Amplifier: A transistor acts as an amplifier only in the active region. This is because, in this region, the output collector current is a linear and magnified replica of the input base current, allowing for faithful signal amplification.
- Switch: A transistor acts as a switch by transitioning between the cutoff and saturation regions. The cutoff region represents the 'OFF' state (no current flow), and the saturation region represents the 'ON' state (maximum current flow). These two distinct states are ideal for representing binary logic (0 and 1).
7. How are transistors fundamental to the operation of modern digital logic gates?
Transistors are the core components of digital logic gates because they can be used as extremely fast electronic switches. For example, in a simple NOT gate (inverter), a transistor can be configured so that:
- A high voltage (binary '1') at the input turns the transistor ON, pulling the output voltage to a low state (binary '0').
- A low voltage (binary '0') at the input turns the transistor OFF, allowing the output voltage to rise to a high state (binary '1').
By combining transistors in various configurations, all other fundamental logic gates like AND, OR, and NAND can be constructed, forming the basis of all microprocessors and digital circuits.
8. Does a transistor create energy when it amplifies a signal?
No, a transistor does not create energy. This is a common misconception. A transistor acts more like a valve or a regulator for an external power source (like a battery). The weak input signal simply controls this valve. The amplified output signal is a larger-power version of the input, but the additional power comes from the external DC power supply connected to the collector circuit, not from the transistor itself. It adheres to the law of conservation of energy by converting DC power from the supply into the AC power of the output signal.
