What is a transistor

Fig. 1 shows the classification of transistors. There is also an explanation in the FAQ below.
FAQ: What types of transistors are available?

A simplified structural diagram and symbols for various transistors are shown in Fig. 2.

• Bipolar Junction Transistor (BJT)
  An NPN BJT has a p-layer base sandwiched between the n-layer collector and emitter. A PNP BJT has an n-layer base sandwiched between the p-layer collector and emitter.
  To increase the DC current gain h_FE, the base is very narrow (thin) and the impurity concentration is collector < base << emitter.

• Metal Oxide Semiconductor Field Effect Transistor (MOSFET)
  In an N-ch MOSFET, the drain and source, which are n layers, are embedded in the p layer. (There is also a vertical structure, but here we will explain the basic horizontal structure.) For the gate, which serves as the control terminal, an insulating film (oxide film) is formed directly above the p-layer, covering one end of the drain and source n-layers, and the gate electrode is placed on top of it. When voltage is applied to the gate, a current path is formed in the p-layer in contact with the insulating film, electrically connecting the n-layer drain and source. Because a path (channel) equivalent to that of the n-layer is formed in the p-layer in this way, it is called an n-channel (N-ch) MOSFET. A P-ch MOSFET has a structure in which the drain and source (p layers) are embedded in the n layer. The gate electrode is placed directly above the n-layer through an insulating film, covering one end of the p-layer for the drain and source.

• Junction Field Effect Transistor (JFET)
  N-ch JFET has a p-layer, which is the gate, embedded in the n-layer, which is the drain and source. The n-layer and p-layer that form the junction are used in reverse bias, so no current flows from the p-layer to the n-layer. Since the current path is the n-layer, it is called an n-channel (N-ch). This n-layer acts as a resistor. When a voltage is applied to the gate, the depletion layer expands in proportion to the voltage, narrowing the current path between the gates and increasing the resistance value.

• Insulated-gate bipolar transistors (IGBT)
  An IGBT is a composite device consisting of a MOSFET and a BJT. The input section has a MOSFET structure with fast switching and low drive current, while the output section has a BJT structure capable of high current and high withstand voltage. This allows for faster switching speeds than a BJT, and a device with higher withstand voltage and lower on-resistance than a MOSFET.

Let's think about the operation of BJT from a physical perspective. A transistor is a current-controlled element that amplifies the base current to pass the collector current, but here we will explain it in terms of the voltage applied to the base rather than the base current. Fig. 5 shows the energy band diagram of an NPN transistor.
If the impurity concentrations of the emitter, base, and collector of an NPN transistor are N_E, N_B, and N_C, respectively, then N_E >> N_B > N_C. There are free electrons or holes in proportion to this impurity concentration. In addition, the energy of each free electron follows the Fermi distribution (Fermi-Dirac distribution).
In a no-bias state, a built-in potential (the energy difference between the lowest limit of the emitter's conduction band and the lowest limit of the base's conduction band) occurs at each pn junction due to the generation of a depletion layer, creating an energy barrier. The emitter electrons cannot cross this barrier, and no current flows between the collector and emitter.
In the circuit shown in Fig. 3, applying a forward bias (V_BE higher than the on-voltage) between the base and emitter lowers the energy barrier.
Some of the free electrons (free electrons in the emitter with energy higher than this energy barrier) cross the barrier and diffuse into the base. At the same time, holes are injected into the base as base current. However, since N_E >> N_B, the number of free electrons diffused from the emitter is much greater than the number of holes in the base. Some of the diffused free electrons recombine with holes, but because the number of holes is small compared to the number of free electrons (difference in impurity concentration between the base and collector) and the thickness (width) of the base is thin, most of them drift due to the electric field between the base and collector and flow into the collector. This is the collector current.

• Metal Oxide Semiconductor Field Effect Transistor (MOSFET)
  N-ch MOSFETs are used by applying a bias as shown in Fig. 6. When a voltage (V_GS) above the threshold is applied between the gate and source, conduction occurs between the drain and source with a small resistance R_DS(ON), and the MOSFET turns on. When V_GS falls below the threshold, conduction does not occur between the drain and source, and the MOSFET turns off.
  The gate terminal of a MOSFET is insulated, so when turning it off, the gate voltage may not drop easily, and it may take some time. In such cases, a resistor may be inserted between the gate and source.

As shown in Fig. 7, an N-ch MOSFET has an N-type semiconductor embedded in a P-type semiconductor, which serves as the source and drain.
N-type semiconductors usually have an excess of electrons, and P-type semiconductors usually have an excess of holes. When a voltage (V_GS) is applied between the gate and source, the gate electrode and the P-type semiconductor directly below it act like a capacitor. When a voltage is applied between the gate and source, the electrode plate side becomes positively charged, and the P-type semiconductor side becomes negatively charged (electrons gather and there is an excess of electrons). In other words, negative charges gather on the surface of the P-type semiconductor directly below the gate, resulting in a state in which there are many free electrons. This state is called an inversion layer because it is a P-type semiconductor but has the same properties as an N-type semiconductor.
Fig. 7 shows the process by which an inversion layer is formed. A voltage V_GS is applied to the gate from an unbiased state. First, before the inversion layer is formed, the holes in the P-type semiconductor that faces the gate electrode disappear, creating a depletion layer. When the voltage called the threshold voltage V_T is reached, an inversion layer is formed. The source and drain are connected by this inversion layer. This inversion layer is called the channel, and in this example, since it is an N-type channel, it is an N-ch MOSFET.

When a MOSFET transitions from OFF to ON, a charging current is required for this capacitor, but once it is turned on, no current like the base current of a BJT is required, making it possible to save power. (However, MOSFETs for power applications have a large parasitic capacitor, and a drive circuit is required to quickly charge this capacitance and turn it on.)

• Junction Field Effect Transistor (JFET)
  There are two operating modes: enhancement type (no current flows when the voltage between the gate and source is 0V) and depletion type (current flows when the voltage between the gate and source is 0V).
  The operation of depletion type is explained below.
  The N-ch JFET is an N-type semiconductor with a P-type semiconductor embedded as the gate. A pn ​​junction is formed between the gate and the drain and source. This junction is used in a reverse bias state.
  Fig. 8 shows an example of bias application to an N-ch JFET and the state of the depletion layer when V_GS is changed.
  When a reverse bias V_GS is applied between the gate and source so that V_G < V_S, the depletion layer expands and the channel narrows. This inhibits the flow of current (it becomes resistant). If the reverse bias is further increased, the channel is blocked by the depletion layer.

• Insulated-gate bipolar transistors (IGBT)
  An IGBT is a device in which a p-layer is added to the drain (n-layer) of an N-ch MOSFET to form a collector. The equivalent circuit is the circuit enclosed in a red frame in the bias circuit in Fig. 9. A diagram of the equivalent circuit overlaid on the structure diagram is also shown. In this way, the input section has a MOSFET structure with fast switching and low drive current, and the output section has a BJT structure that allows high current and high voltage resistance. In other words, while taking advantage of the advantages of MOSFETs, the weak point of on-resistance is improved by using the conductivity modulation effect of bipolar devices. This realizes a device that is faster than BJTs and has higher voltage resistance and lower on-resistance than MOSFETs.
  As shown in Fig. 9 bias circuit, a voltage V_GE is applied between the gate (G) and emitter (E), and a voltage V_CE is applied between the collector (C) and emitter (E). At this time, if V_GE is equal to or higher than the threshold value of the N-ch MOSFET, the built-in MOSFET turns on, a drain current ID flows, and the IGBT turns on.
  The IC current of the PNP transistor in Fig. 9 equivalent circuit is configured so that it hardly flows, and most of the current flows from the emitter of the PNP transistor to the gate. When the PNP transistor is on, the emitter of the PNP transistor (C in IGBT) supplies holes, which are majority carriers, to the N-layer (base of the PNP transistor). At the same time, electrons are also supplied from the N-layer (E in the IGBT) through the MOSFET to maintain electrical balance. As a result, the N-layer has a large number of holes and free electrons, which increases the conductivity and improves the on-resistance. This is called the conductivity modulation effect.
  When V_GE falls below the threshold of the N-ch MOSFET, it turns off. However, a current called a tail current flows for a certain period of time before the IGBT turns off. This is due to the conductivity modulation effect, which reduces the on-resistance when on.

For more information on conductivity modulation, please refer to the following FAQ.

• What is conductivity modulation?
• What is the tail current of an IGBT?

Transistors are used in all kinds of electronic devices and are mainly used in circuits that switch (on/off) or amplify electrical signals. Examples of switches include circuits that turn on/off at logic signal levels (up to 5V) and switching power supplies. Amplification circuits are also used not only for simple signal amplification but also for oscillation circuits. As you can see, transistors are used in a wide range of applications.

Both BJTs and MOSFETs can be used in these circuits. The choice between BJTs and MOSFETs depends on which characteristics (speed, loss, gain, etc.) are important in the circuit and what voltage, current, and frequency are used.
For reference, we have listed keywords related to each device.
-MOSFET: low power consumption, high-speed switching, high input impedance
-BJT: analog (linearity, high gain), low noise, turns on at low voltage (on voltage of about 0.7V)
Some circuit examples are shown below.

If you are interested in switching power supplies, please also refer to the following video.
[e-Learning] Full Bridge Converter - Basics of Switching Power Supplies (5)
[e-Learning] DC-DC Converter - Basics of Switching Power Supplies (6)
[e-Learning] Resonant Half Bridge Converter - Basics of Switching Power Supplies (7)
[e-Learning] Bridgeless PFC - Basics of Switching Power Supplies (8)

Toshiba's BJTs and MOSFETs are available in a wide range of packages, from small packages to packages for power applications. We also offer composite types such as 2-in-1.