Learn the basics of embedded system circuits – diode/transistor/FET

Substances that conduct electricity between conductors and insulators – semiconductors

Silicon and germanium are located between conductors such as silver and aluminum and insulators such as quartz and ceramics. They are raw materials used to manufacture semiconductor devices and have a certain resistivity. The different resistivities produced by different substances are caused by different amounts of movable electrons. Such movable electrons are called “free electrons”. Generally, substances that can change the number of free electrons and control the flow of electricity by doping them with impurities are called semiconductors.

Learn the basics of embedded system circuits – diode/transistor/FET

  根据电流流动的构造,可将半导体分为N型和P型两类。

  半导体的电流流通原理

  (1) N型半导体

  图1是在硅晶体中掺入杂质磷(P)元素的概要图。磷原子持有的5个价电子中4个和硅(Si)原子一样,通过共价键,与邻接原子紧密结合。剩下1个价电子不发生共价键,而是根据室温高低成为自由电子。这个自由电子将旁边的价电子赶出,取代它的位置,而原有价电子变为自由电子,再将旁边的其他价电子赶出。通过这样的重复过程,使自由电子不断移动从而形成电流。由电子作为载流子(输送电流)的半导体称为“N型半导体”。施主原子的电子不足时,带正电荷。

  

Figure 1 N-type semiconductor structure

(2) P-type semiconductor

FIG. 2 is a schematic diagram of doping an impurity boron element in a silicon crystal. The boron element has three valence electrons, which is one valence electron less than that of silicon. The valence electrons in the adjacent silicon atoms are converted into free electrons by a small amount of thermal energy, which are absorbed by the acceptor atoms. The original positions of the absorbed valence electrons are called holes, which further absorb valence electrons in adjacent silicon atoms. Through this repeated process, the holes move, creating an electric current. Semiconductors with holes as carriers are called “P-type semiconductors”. The acceptor atom has too many electrons and is therefore negatively charged.

  

Figure 2 P-type semiconductor structure

Diodes are Electronic devices that conduct unidirectionally

The diode is formed of a P-type semiconductor and an N-type semiconductor and has a simple structure. Around the interface of the P-type and N-type junctions, the respective carriers diffuse and combine, so that a carrier-free region appears. In this region, charged impurities form a barrier electric field that hinders bonding by preventing carrier diffusion. We call this barrier electric field without carriers the depletion layer.

Figure 3 Structure of a PN junction diode

At both ends of the diode, a positive voltage is applied to the P-type region, a negative voltage is applied to the N-type region, and energy is added to the direction of the narrowing of the depletion layer, the carriers are very easy to drift to both sides, recombination occurs again, and disappears due to recombination The carriers of the ions are replenished by the current of the applied voltage, forming a directional current. On the contrary, when a negative voltage is applied to the P-type region and a positive voltage is applied to the N-type region, and energy is added to the direction in which the carriers are attracted by the electrode, the depletion layer becomes wider and the current hardly flows. The unidirectional flow of the above current is the basic principle of the diode – rectification. The direction of easy current flow is called forward direction, and the direction of less current flow is called reverse direction.

Voltage and Current Characteristics of Diodes

The voltage and current characteristics of the diode are shown in Figure 4. It should be noted that even in the forward direction, if a certain level of voltage is not applied, the current will not flow. The applied voltage required for silicon diodes is 0.7-0.8V, Schottky diodes are about 0.2V, and light-emitting diodes (LEDs) are more than 2-5V, allowing current to flow forward. When a certain voltage is applied in the reverse direction, a current can also be suddenly generated, and this phenomenon is called breakdown. The breakdown voltage is hardly affected by the current, so it is often used as a constant voltage source.

  

Figure 4 Voltage and current characteristics of diodes

Basic components of electronic circuits (the first solid-state active components put into use)

A transistor (to avoid confusion with the FET below, it may also be referred to as a bipolar transistor) is an element in which a P-type semiconductor and an N-type semiconductor are superimposed on each other in a sandwich structure. According to the different stacking order, it can be divided into two types: NPN type and PNP type.

  

Figure 5 Schematic diagram of NPN transistor

Taking an NPN transistor (Figure 5) as an example, let’s see how it works.

The base region? The emitter region and the diode structure are the same. In addition, a forward voltage (about 0.7V) is applied to generate a base current (IB). A large number of free electrons flow into the base region from the emitter region, and the recombined carriers in the base region are less than those diffused out of the emitter region, so the free electrons remain. The remaining free electrons are attracted by the added E2 on the collector. The number of carriers diffused in the emitter region is 10 to several hundred times the number of recombination carriers, and this ratio is used to expand IB to generate collector current (IC). If IB is 0, there is no carrier diffusion in the emission region, and IC is also 0. That is to say, the forward current IB between the base region and the emitter region can control the current IC between the base region and the emitter region. This property applies to amplifiers and switches, which form the basic elements of electronic circuits. By combining such transistors, more complex electronic circuits can be formed.

The switching operation of the transistor

The transistor can get collector current several times greater than the base current. The ratio of collector current to base current is called the DC current amplification factor (HFE), and the ratio is about 100 to 700. In the circuit shown in Figure 6, when the applied voltage on IN is 0V, there is no current at the base and no current is generated at the collector, so there is no current through RL, and the output voltage on OUT is 12V. On the contrary, if a certain intensity voltage is applied between the base region and the emitter region (generally, a voltage above 0.7V is applied), the base electrode has a current passing through it, resulting in a collector current that is hFE times. However, the actual passing current is limited by the existence of the load resistance RL, (12V-Vce-sat (saturation voltage))/RL. Due to the large driving current of the switch circuit, it is often used in control occasions that cannot be directly driven by chips such as MCU and logic IC, such as the control of Power LEDs, relays and DC motors.

  

Figure 6 Switching operation of transistors

Contributors to achieve integration

FET (Field Effect Transistor: Field Effect Transistor) can be roughly divided into two categories: MOS (Metal Oxide Semiconductor: Metal Oxide Semiconductor) and junction type. Especially MOS type FET (MOSFET), compared with the above-mentioned bipolar transistor, its planar structure and the interference between adjacent components of the same type are extremely small, basically no need to be used separately, because it is easy to integrate, miniaturize and low power consumption, Therefore, it is an indispensable component in ICs and LSIs. Next, let’s take a look at the working principle of MOS type FET.

FIG. 7 is a schematic diagram of an N-type MOSFET. G is called the “gate”, the oxide film below G is an insulator, and the source S and drain D sandwich the gate. When the voltage between the gate and the source is 0V, a P-type semiconductor is sandwiched between the source and the drain composed of an N-type semiconductor to form a reverse bond and form an insulation. That is, no current flows between the source and drain.

When a voltage is applied to the gate, free electrons are attracted under the gate. The free electrons between the source and the drain increase, and current flows easily. That is, the current between the source and the drain can be controlled by applying a voltage to the gate.

It is mainly used in switching circuits and amplifier circuits. When the voltage applied to the gate is stable, the current between the source and drain is also stable, so it can be used as a constant voltage source.

When the current path under the gate is N-type, it is called an N-type MOSFET, and when the current path under the gate is P-type, it is a P-type MOSFET.

Figure 7 Overview of N-type MOSFET

Basic Elements of Digital Circuits CMOS

CMOS (ComplementaryMOS), as shown in Figure 8, is a complementary connection MOSFET. With this circuit configuration, no matter the IN voltage is 0V or VCC, only one of the MOSFETs is turned on. Therefore, basically no current flows from VCC to GND, which can be used to form an ideal circuit with extremely low power consumption. Today’s LSIs and ICs are basically composed of this CMOS.

Figure 8. inverter composed of CMOS

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