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What is a Transistor?

A transistor is a semiconductor device that serves as a fundamental building block in modern electronics. It is primarily used to amplify or switch electrical signals and power. Transistors are composed of semiconductor materials of either P-type or N-type, where these materials are sandwiched together to form P-N junctions. These junctions are tapped into via terminals, and transistors typically have at least three terminals for connecting to an electronic circuit: the emitter, base, and collector in bipolar junction transistors (BJTs), or the source, gate, and drain in field-effect transistors (FETs). More on P-N junctions, BJTs and FETs later.

Applications of Transistors

Transistors are used in a wide range of applications, including:

• Amplifiers: They can increase the strength of weak signals, which is crucial in audio and communication devices.
• Switches: Transistors can turn electrical signals on and off, making them vital in digital circuits and computing.

Transistors are versatile and essential components in electronics, enabling the control and manipulation of electrical signals in countless devices and systems.

Why Use a Transistor for Amplification?

Transistors are preferred for amplification in many electronic applications due to several key advantages over other components. Here are some reasons why transistors are often the popular choice:

1. High Gain: Transistors can provide significant amplification of weak signals. For instance, a small input current at the base of a bipolar junction transistor (BJT) can control a much larger current flowing from the emitter to the collector (for PNP transistors, more on this later), effectively amplifying the signal.
2. Low Power Consumption: Transistors can amplify signals with minimal power loss. They can control large power flows (like 1000W) using a very small input signal (as little as 1W), making them highly efficient for amplification purposes.
3. Wide Frequency Range: Transistors can operate effectively across a broad range of frequencies, making them suitable for various applications, from audio amplification to radio frequency transmission. This versatility is crucial in designing amplifiers for different types of signals.
4. Compact Size: Transistors are small and can be integrated into compact circuits, allowing for the design of miniaturized electronic devices. This is particularly important in modern electronics, where space is often limited.
5. Robustness and Reliability: Transistors are generally more robust and reliable compared to other amplification components, such as vacuum tubes. They are less susceptible to damage from physical shocks and have a longer operational life.
6. Temperature Stability: Transistors can maintain their performance over a range of temperatures, which is essential for applications that may experience varying environmental conditions.

Transistors are favored for amplification due to their high gain, low power consumption, versatility across frequencies, compact size, robustness, and temperature stability. These characteristics make them indispensable in a wide array of electronic devices and systems.

Why Use a Transistor for Switching?

Transistors are commonly used for switching applications due to several advantages over other electronic components. Here are some key reasons why transistors are preferred for switching purposes:

1. High Efficiency: Transistors can switch on and off very quickly, allowing for efficient control of power. This rapid switching capability is essential in applications like pulse-width modulation (PWM) for motor control, where precise timing is crucial.
2. Low Control Power: A small input current at the base (for BJTs) or a small voltage at the gate (for FETs) can control a much larger current flowing through the collector and emitter (or drain and source). This means that transistors can effectively control high-power devices without requiring significant power themselves.
3. Compact Size: Transistors are small and can be easily integrated into compact circuits. This is particularly beneficial in modern electronics, where space is often limited, allowing for the design of smaller and more efficient devices.
4. Versatility: Transistors can be used in various configurations (like common emitter, common collector, or common source) to achieve different switching behaviors. This versatility makes them suitable for a wide range of applications, from simple on/off switches to complex control systems.
5. Robustness: Transistors are generally more robust than mechanical switches. They are less prone to wear and tear, which enhances their reliability and lifespan in electronic circuits.
6. Isolation: Transistors can provide electrical isolation between the control circuit and the load. This is particularly useful in protecting sensitive components from high voltages or currents that may be present in the load circuit.
7. Integration with Digital Logic: Transistors can easily interface with digital logic circuits, making them ideal for use in microcontrollers and other digital systems. This allows for seamless integration in applications like automated control systems and robotics.

Transistors are favored for switching applications due to their high efficiency, low control power requirements, compact size, versatility, robustness, ability to provide isolation, and compatibility with digital logic. These characteristics make them essential components in a wide variety of electronic devices and systems.

Common Gadgets Using Transistors

Transistors are integral components in a wide variety of everyday gadgets. Here are some examples of common devices where transistors are frequently used:

1. Computers: Transistors are the building blocks of microprocessors and memory chips. Millions, or even billions, of transistors are embedded in a single integrated circuit, enabling the complex processing and storage functions of modern computers.
2. Smartphones: Similar to computers, smartphones rely heavily on transistors for their processors and memory. They help manage everything from running applications to processing audio and video.
3. Televisions: Transistors are used in the circuitry of televisions to amplify signals and switch channels. They play a crucial role in both analog and digital TVs.
4. Radios: In radios, transistors are used to amplify audio signals and switch between different frequencies, making them essential for receiving and processing radio waves.
5. Digital Cameras: Transistors are found in the image sensors and processing units of digital cameras, where they help convert light into electronic signals and manage image processing.

These examples illustrate the versatility and importance of transistors in modern electronic devices, enabling them to function efficiently and effectively.

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How Does a Transistor Really Work?

Simply put, transistors operate by controlling the flow of electrical current. In the case of a BJT, a small current at the base terminal can control a larger current flowing from the emitter to the collector for PNP transistors, or from the collector to the emitter for NPN transistors. This property allows transistors to amplify signals, making them essential in applications like audio amplifiers and radio transmitters. In FETs, the voltage applied to the gate terminal creates an electric field that influences the conductivity of a channel between the source and drain. This allows for efficient switching and amplification of signals with very low power consumption.

Before we get too far into our discussion, how about we take a moment to explain what bipolar junction transistors (BJTs) and field-effect transistors (FETs) are. If we can get a general sense of what they are, what they do, and how they work, we can better understand what transistors as a whole are and how they work. Transistors are semiconductor devices consisting of an N-type and P-type material. Let’s first look into the basics of their structure and what N-type and P-type materials are.

What are P-N Junctions?

A P-N junction is formed when two types of semiconductor materials are joined together: P-type and N-type.

• P-type semiconductors have an abundance of “holes” (places where an electron could exist but doesn’t), which can carry positive charge.
• N-type semiconductors have extra or free electrons, which can carry negative charge.

When these two materials are put together, they create a junction where the electrons from the N-type region can fill the holes in the P-type region. This interaction creates a depletion region at the junction, where no charge carriers are present, leading to an electric field that affects how the junction behaves.

Role of P-N Junctions in Transistors

In transistors, particularly bipolar junction transistors (BJTs), there are two p-n junctions. For example, in an NPN transistor, the structure consists of:

1. N-type material (the emitter)
2. P-type material (the base)
3. N-type material (the collector)

The two p-n junctions in an NPN transistor are formed between the emitter and the base, and between the base and the collector. This configuration allows the transistor to control current flow. When a small current flows into the base (the P-type region), it allows a much larger current to flow from the collector to the emitter, effectively amplifying the signal.

More on Bipolar Junction Transistors (BJTs)

Just as was previously stated, a bipolar junction transistor, or BJT, is a semiconductor device that consists of semiconductor material, forming two p-n junctions. These p-n junctions can be either N-type (mostly consisting of electrons) or P-type (mostly consisting of holes). The two main configurations are:

• NPN Transistor: Composed of two N-type layers (the emitter and collector) and one P-type layer (the base) sandwiched in between.
• PNP Transistor: Composed of two P-type layers and one N-type layer.

We know now that BJT can be designed and made in a way that it consists of varying N-type and P-type materials that alters its conductive properties. Once its semiconductive properties have been established, we need a way to use it, as in connecting it to a circuit. The BJT has three terminals, each terminal connected to each of the semiconductor materials that make up its structure. Those terminals are called the emitter, base, and collector:

• Emitter (E): The terminal that emits charge carriers (electrons for NPN, holes for PNP).
• Base (B): The thin middle layer that controls the flow of charge carriers. It is lightly doped compared to the emitter and collector.
• Collector (C): The terminal that collects charge carriers from the emitter.

Operation of a BJT

• Current Control: In a BJT, a small current flowing into the base terminal controls a larger current flowing from the emitter to the collector. When a sufficient base current is applied, it allows electrons (in NPN transistors) or holes (in PNP transistors) to flow from the emitter to the collector, thus amplifying the input signal.
• Active Region: When the transistor is in the active region, it can amplify signals. The relationship between the base current and the collector current is defined by the transistor’s current gain (β), which indicates how much the input current is amplified.
• Switching: When the base current is removed, the transistor turns off, stopping the flow of current from the emitter to the collector. This on/off behavior allows the BJT to function as a switch.

Example NPN Transistor Circuit Calculations

Let’s say that we have a circuit containing an NPN transistor (Q1), a 9V battery (BT1), a switch (S1) and a resistor (R1) with a value of 1MΩ at the base (B) of the NPN transistor, a resistor (R2) with a value of 1kΩ and an orange LED (D1), both in series to the collector (C) of the NPN transistor, all forming the circuit shown in the schematic below (note that switch S1, is closed):

Let’s say that we’re given that the gain (β) of the NPN transistor is 200. With this information, let’s say that we want to calculate the following values:

1. The base current (I_B).
2. The collector current (I_C).
3. The voltage (V_CE) that the multimeter (M1) will read in the schematic shown above.

Recall Kirchhoff’s Voltage Law (KVL), where it states that the sum of the voltages in a closed loop path must sum to zero. Remember, KVL deals with the conservation of energy, so energy must be conserved. What this means is that if we start at the point of higher potential of a closed loop path — in this case point A, which has a potential of +9V, as shown on our NPN transistor circuit below — then make our way around the closed loop path, shown by the green loop, voltage drops occur as current goes through elements within the circuit loop — in this case, the resistor R1, and the transistor Q1 — to ground (or to the negative terminal of battery, BT1), which has a voltage potential of 0V. The sum of the voltages starting from point A, through all the voltage drops across each element within the closed loop path, to ground, should equal zero.

Following the Closed Loop Path

First, let’s look at this process, in depth, to understand what we’re trying to do. For our first task, we’re asked to find the base current (I_B), which happens to be the current that leaves the positive terminal of our 9V battery. Let’s follow the current of the closed loop path (the green arrows), starting from the battery’s positive terminal:

• Current path starts from positive terminal of 9V battery, BT1.
• Current continues, going to point A, which has an electric potential of +9V from the battery.
• Current goes through the closed switch, S1. No voltage drop here, because the switch acts as a conductor and allows current through with no resistance, ideally.
• Current continues through resistor R1, which has a value of 1MΩ. There will be a voltage drop here! Recall Ohm’s Law:

\begin{equation}
V = IR
\end{equation}

• Current continues toward the base (B) of the NPN transistor (Q1) and through to the emitter (E) of the transistor. There’s a voltage drop here too! The voltage across the base and the emitter of the NPN transistor (V_BE) typically varies between 0.6V and 0.7V (check your transistor’s datasheet). We’ll say that V_BE = 0.6V, for our case, therefore saying that the electric potential of the base (B) of the transistor is 0.6V higher than the emitter (E) of the transistor.

\begin{equation}
V_{BE} = 0.6V
\end{equation}

• Current continues from the emitter of the transistor to ground, or back around to the negative terminal of the 9V battery (BT1), which has an electric potential of 0V, therefore finishing the closed loop path.

Our first voltage drop comes from the part of the closed loop path where current goes through the resistor, R1. We now know the electric potentials across R1, those being +9V at point A, before current goes through the resistor — remember, there’s no voltage drop across the switch, so the potential after the switch, before R1, is still +9V — and 0.6V at the base of the NPN transistor, because the potential at the base is 0.6V higher than at the emitter, which is connected to ground, or 0V. So, the electric potential at the emitter is 0V. From this information, we can calculate the current flowing through the resistor (R1), which is the base current (I_B).

1.) The base current (I_B)

Recall Ohm’s Law (V = IR). Rearranging Ohm’s Law to find the current, we get:

\begin{equation}
I = \frac{V}{R}
\end{equation}

So, the current we’ll find is for the base current (I_B), the resistance (R) is from the resistor value of R1, and the voltage (V) is the voltage across R1, which is the electric potential difference of the voltage values on each side of the resistor, R1:

\begin{equation}
I_B = \frac{(9V-0.6V)}{1MΩ}
\end{equation}

\begin{equation}
I_B = \frac{(9V-0.6V)}{1,000,000Ω}
\end{equation}

\begin{equation}
I_B = \frac{8.4V}{1,000,000Ω}
\end{equation}

\begin{equation}
I_B = 0.0000084A
\end{equation}

\begin{equation}
\boxed{I_B = 0.0084mA}
\end{equation}

So, the base current of the NPN transistor is 0.0084 milliamps.

2.) The collector current (I_C)

Now, that we know the base current (I_B), we can find the collector current using the formula for the gain of an NPN transistor, which was:

\begin{equation}
\beta = \frac{I_C}{I_B}
\end{equation}

If we rearrange the formula for gain of the NPN transistor, we can find the collector current as:

\begin{equation}
I_C = \beta{I_B}
\end{equation}

\begin{equation}
I_C = (200)(0.0084mA)
\end{equation}

\begin{equation}
I_C = (200)(0.0000084A)
\end{equation}

\begin{equation}
I_C = 0.00168A
\end{equation}

\begin{equation}
\boxed{I_C = 1.68mA}
\end{equation}

So, the collector current of the NPN transistor is 1.68 milliamps. The collector current (I_C), is the current that flows from the node, point A, through resistor R2, and the orange LED (D1), down through to the collector (C) of the NPN transistor, as shown by the blue path in our example circuit schematic below:

Now that we know the collector current (I_C), we can obtain the voltage potentials across resistor R2 and the orange LED (D1) — two of the three elements along the closed loop path of the blue arrow loop. The collector current (I_C) that goes through the collector of the NPN transistor, down through its emitter, is only activate when the transistor is activated by the small base current (I_B) coming through at the base when the switch (S1) is closed.

Why do we want to know the voltages around R2 and D1? Well, because we still need to figure out what the voltage output of the multimeter is going to be. To do that, we’re going to use KVL again —we’re going to look at the voltage drops along the blue arrow closed loop path, which includes the path through the collector (C) and emitter (E) of the transistor (Q1), right where the terminals of the multimeter (M1) are probing.

3.) The voltage (V_CE) across the multimeter (M1)

So, if we observe the blue arrow closed loop path, starting at the positive terminal of the battery (BT1), we come to node A, where we’ve stated previously that at this location, there it has the same electric potential as the battery, essentially, +9V. As we move along our blue arrow loop, we come in contact with resistor R2, still with a voltage potential of +9V. As we move along through R2, there will be some other electric potential value between R2 and the LED (D1), point D, meaning that there is some potential lower there than was on the node A side of R2, but a potential still higher than the cathode side of the LED (D1), or the side where the LED is connected to the collector of the transistor (Q1).

Since we know the current flowing through R2 is the collector current (I_C), we can calculate the potential on the other side of R2, at point D. Recalling Ohm’s Law, the voltage across the resistor R2, is the product of the collector current that flows through R2, and the value of the resistor, R2. The voltage across R2 is the potential difference from our starting point A, to point D. The potential difference (V_R2) across R2, is equal to the voltage at point A (V_A) minus the voltage at point D (V_D):

\begin{equation}
V_{R2} = V_A – V_D
\end{equation}

\begin{equation}
{I_C}{R_2} = V_A – V_D
\end{equation}

Rearranging to find the voltage at point D (V_D):

\begin{equation}
{I_C}{R_2} = V_A – V_D
\end{equation}

\begin{equation}
V_D = V_A – ({I_C}{R_2})
\end{equation}

\begin{equation}
V_D = 9V – (1.68mA)(1kΩ)
\end{equation}

\begin{equation}
V_D = 9V – (0.00168A)(1000Ω)
\end{equation}

\begin{equation}
V_D = 9V – 1.68V
\end{equation}

\begin{equation}
V_D = 7.32V
\end{equation}

Now, as we continue to move along the blue arrow loop, we come in contact with the orange LED (D1). This means that there will be a lower potential at the point where the collector (C) of the NPN transistor meets the cathode of the LED in the schematic, as compared to the potential we just calculated at the anode side of the LED at the point D. To calculate the voltage potential at point C (at the collector of the transistor), we need to know the voltage drop across the LED.

The forward voltage drop of a typical orange LED is about 2V, as can be seen from the details of an LED using it datasheet here. So, we’ll use this information and say that the voltage drop across our orange LED is 2V. So, we start at the potential at point D, current goes through the LED giving a voltage drop of 2V, and we arrive at the point C (at the collector of the transistor):

\begin{equation}
V_D – 2V = V_C
\end{equation}

\begin{equation}
7.32V – 2V = V_C
\end{equation}

\begin{equation}
V_C = 7.32V – 2V
\end{equation}

\begin{equation}
V_C = 5.32V
\end{equation}

So, if you were to take a multimeter and measure the voltage between the collector and the emitter of the NPN transistor of our example circuit above, you’d read a voltage of about 5.32V. This would be called the collector-emitter voltage, or V_CE. Note again, that the potential at the emitter side of the NPN transistor is 0V.

\begin{equation}
\boxed{V_{CE} = 5.32V}
\end{equation}

We can see, by the example done above, that a transistor can work as a switch. We saw that a small current can be used to drive a larger current in the circuit.

More on Field-Effect Transistors (FETs)

A Field Effect Transistor (FET) is a type of transistor that controls the flow of electrical current using an electric field. It has three main parts, or terminals: source, gate, and drain.

• Source: This is where the current enters the FET.
• Gate: This terminal controls the flow of current. By applying a voltage to the gate, you can create an electric field that influences how easily current can flow from the source to the drain.
• Drain: This is where the current exits the FET.

You can think of the gate as a valve. When you apply a voltage to the gate, it opens or closes the valve, allowing more or less current to flow from the source to the drain. This ability to control current flow makes FETs very useful in electronic circuits, especially for amplifying weak signals, like those in wireless communications.

Types of FETs

FETs can be categorized into several types, with the two primary types being:

1. Junction Field-Effect Transistor (JFET):
   • JFETs are characterized by their use of a P-N junction to control the flow of current. They operate by allowing current to flow through a channel formed by either N-type or P-type semiconductor material, depending on the type of JFET. The current flow is controlled by the voltage applied to the gate terminal, which creates an electric field that influences the conductivity of the channel.
2. Metal-Oxide-Semiconductor Field-Effect Transistor (MOSFET):
   • MOSFETs are widely used in digital and analog circuits. They consist of a gate insulated from the channel by a thin layer of oxide, allowing for very high input impedance. MOSFETs can be further divided into enhancement-mode and depletion-mode types, depending on how they operate with respect to the gate voltage. In enhancement-mode MOSFETs, applying a voltage increases the channel conductivity, while in depletion-mode MOSFETs, applying a voltage decreases it.

Operation of a FET

1. Voltage Control: In a FET, the current flowing between the source and drain is controlled by the voltage applied to the gate. When a voltage is applied to the gate, it creates an electric field that either enhances or depletes the flow of charge carriers in the channel.
2. Enhancement Mode: In enhancement-mode FETs, applying a positive voltage to the gate (for N-channel) increases the conductivity of the channel, allowing current to flow from the source to the drain.
3. Depletion Mode: In depletion-mode FETs, applying a voltage can reduce the conductivity of the channel, effectively turning the current flow off.
4. Switching: Similar to BJTs, FETs can also act as switches. When the gate voltage is above a certain threshold, the FET is “on,” allowing current to flow. When the gate voltage is below this threshold, the FET is “off,” stopping the current.

Comparison of BJTs vs. FETs

	BJTs	FETs
Operation	Current-controlled. BJTs require a biasing current applied through the base terminal to operate.	Voltage-controlled. FETs require a voltage applied to the gate terminal to turn the FET on or off.
Input Impedence	Smaller input impedence. BJTs draw a higher current from the power circuit supplying them, which may lead to circuit loading.	Offer a greater input impedence. FETs essentially draw no current, thus not loading the power circuit that supplies them.
Gain	Offer a greater gain at the output than FETs.	Have a smaller gain than BJTs.
Size	Tend to be larger in size than FETs.	Tend to be smaller in size than BJTs.
Cost	Cheap to manufacture.	More costly to manufacture than BJTs.
Popularity	Less popular and not as widely used as FETs.	More popular and are widely used, more so than BJTs.

Conclusion

You’ve learned that a transistor is a semiconductor device that is primarily used to amplify or switch electrical signals and power. We’ve gone over how much these amazing components of technology are used in a variety of devices that each of us use on a daily basis, and have gained a better understanding of how they really work.

In summary, the structure of a transistor varies between BJTs and FETs, but both types are designed to control the flow of electrical current. BJTs use current at the base to control larger currents between the emitter and collector, while FETs use voltage at the gate to control current between the source and drain. This ability to amplify signals and switch currents on and off makes transistors essential components in a wide range of electronic applications.

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