What is a transistor

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.
  The symbol and simplified structural diagram of a bipolar transistor (BJT) are shown in Figure-2 (a). In the simplified structural diagram, for example, an NPN type is shown with a thin P-layer base sandwiched between N-layer emitter and collector, but this is a schematic diagram to make it easier to understand the operating principle, and does not directly represent the physical structure of an actual BJT.

• 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.
  The symbol for a MOSFET and a simplified structural diagram are shown in Figure-2 (b). The figure shows an N-channel MOSFET and a P-channel MOSFET as cross-sectional views of a lateral structure, with the N-channel MOSFET using a P-type substrate and the P-channel MOSFET using an N-type substrate. The source (S) and drain (D) are formed in the substrate, and the gate (G) is placed on top of them via an insulating film, allowing the conduction state between the drain and source to be controlled by the gate voltage.

• 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.
  The symbol for a JFET and a simplified structural diagram are shown in Figure-2 (c). For example, an N-channel JFET is expressed as a structure in which an N-layer is used as the substrate and P-layers are placed above and below it. However, this structural diagram is a schematic diagram intended to make it easier to understand the operating principle of a JFET, and it differs in some respects from the actual device structure. A distinctive feature of a JFET is that the channel width is controlled by the gate (G).

• 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.
  The symbol for an IGBT and a simplified structural diagram are shown in Figure-2 (d). The figure shows an equivalent circuit for an N-ch IGBT as an example, along with a simplified structural diagram that overlays that equivalent circuit. An IGBT is a device that uses an N-ch MOSFET as its basic structure and combines elements of a bipolar transistor in the output stage, and is characterized by combining the high-speed switching characteristics of a MOSFET with the low on-voltage characteristics of a bipolar transistor.

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. Figure-5 shows the energy band diagram of an NPN transistor.
Figure-5 shows the state when there is no bias and when a forward bias is applied between the base and emitter. When there is no bias, the energy barrier formed at the pn junction between the emitter and base prevents free electrons in the emitter from moving to the base, and no current flows between the collector and emitter. On the other hand, when a forward bias is applied between the base and emitter, this energy barrier is lowered, and some of the free electrons in the emitter pass over the base and flow into the collector. This is the collector current.

If the impurity concentrations of the emitter, base, and collector of an NPN transistor are NE, NB, and NC, respectively, then NE >> NB > NC. 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 Figure 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 NE >> NB, 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)
  Figure-6 shows an example of the simplest bias circuit for an N-channel MOSFET. The diagram shows a configuration in which a bias voltage V_GS is applied between the gate and source, and a bias voltage V between the drain and source. A channel is formed by the gate voltage V_GS, but because the gate is insulated, no gate current flows. Therefore, a resistor to limit current, as with a BJT, is not required. However, a resistor may be inserted at the gate to adjust the rise time or prevent oscillation.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.

Figure-7 illustrates the bias conditions and inversion layer formation in an N-channel MOSFET when V_GS is applied. The cross-sectional diagram shows three conditions: no bias (V_GS = 0 V), a forward bias condition below the threshold voltage (V_GS < V_T), and a forward bias condition above the threshold voltage (V_GS ≥ V_T). For simplicity, the drain (D) and source (S) are shown externally shorted. For information on non-shorted conditions, see the FAQ below.

FAQ: Understanding MOSFET Operation Principles and Mechanisms | Cutoff, Linear, and Saturation Regions Explained

Without bias, there is no significant change in the P-type substrate directly below the gate. However, when V_GS < V_T, a depletion layer forms. Furthermore, when V_GS ≥ V_T, an inversion layer forms directly below the gate, forming a channel electrically connecting the drain and source.
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 the change in the depletion layer as a function of the bias state in an N-channel JFET. The figure shows the simplest operating circuit, as well as cross-sectional views with no bias and with a bias applied. As the gate-source voltage V_GS increases, the depletion layer formed at the gate junction expands, gradually narrowing the channel width.
  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)
  IGBT is a device with a p-layer added to the drain (n-layer) of an N-ch MOSFET to serve as a collector. The input section is a MOSFET structure with fast switching and low drive current, and the output section is a BJT structure that allows for high current and high voltage resistance. This allows for a device that is faster than a BJT and has a higher voltage resistance and lower on-resistance than a MOSFET.
  Figure-9 shows the current flow when an IGBT is on. (a) is a circuit diagram, (b) is a diagram of the IGBT equivalent circuit superimposed on a structural diagram, and (c) is a structural diagram showing the current path in the on state. When voltage is applied between the gate and emitter, the MOSFET in the input stage turns on, which activates the bipolar transistor in the output stage. The arrows indicate the current path when on.
  For details on operation, please refer to the FAQ below.
  
  FAQ: What is an IGBT?
  
  As shown in the circuit diagram, apply voltage V_GE between gate (G) and emitter (E), and voltage V_CE between 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 MOSFET turns on. This makes the base of the PNP transistor equipotential to the emitter, turning the PNP transistor on and causing a collector current I_C to flow.
  If V_GE falls below the threshold value 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 that reduces the on-resistance when the device is turned on.

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.