MOS tube

A MOS transistor (also known as metal-oxide semiconductor field effect transistor) is electrically conductive by a majority of carriers’ and it is a voltage controlled electrical device. It is also called a unipolar transistor. It has three main terminals; Gate (G), Drain (D) and Source (S). The Gate voltage determines the conductivity of the device and with change of applied voltage; the MOS transistor can be used for amplifying or switching electronic signals. Its characteristics are; high input resistance (10^7~10^12Ω), low noise, low power consumption, large dynamic range, easy integration, no secondary breakdown, wide safe working area, source and drain can be interchanged, it is voltage controlled device and conduction takes place through majority carriers (n-channel: electrons and p-channel: holes).

All MOS integrated circuits (including P-channel MOS, N-channel MOS, complementary MOS-CMOS integrated circuits) have an insulated gate to prevent voltage breakdown. Generally, the thickness of insulating Gate oxide layer of MOS transistor is 5 – 200 nm (about 25 nm, 50 nm, and 80 nm). In addition to the high-impedance gate of the integrated circuit, there is a resistor-diode network for protection. However, MOS devices are sensitive to voltage spikes and static electricity discharges and this can cause difficulties when we have to replace MOS devices especially complementary-symmetry metal-oxide semiconductor (CMOS) devices. Therefore, the protection network inside the device is not enough to avoid electrostatic damage (ESD) to the device. To minimize chances of damaging MOS devices during handling, special procedures have been developed to protect them from static shock. ICs are generally shipped and stored in special conductive-plastic tubes or trays. MOS devices safety is ensured by inserting ICs leas into aluminium foil or antistatic (conductive) foam – not Styrofoam. PC boards containing static sensitive devices are normally shipped in special antistatic bags, which are good for storing ICs and other computer components that could be damaged by ESD.

Experiments indicate that MOS device will fail during high-voltage discharge. The device may also fail for accumulation of multiple lower voltage discharges. According to the severity of the damage, there are many forms of electrostatic damage. The most serious and most likely to occur is the complete destruction of the input or output so as to be short-circuited or open to the power supply terminal VDD, and MOS device completely loses its original function. A little bit of serious damage is intermittent failure or degradation of performance, which is even more difficult to detect. There is also some electrostatic damage that can cause the device performance to deteriorate due to increased leakage current.

MOS tube definition

MOS tube is a MOS transistor or a metal-insulator-semiconductor. The source (S) and drain (D) of MOS tube can be reversed. They are all N-type regions formed in the P-type backgate. And in most cases, the two zones are same even if two ends are reversed. And it will not affect performance of the device. Such devices are considered to be symmetrical. MOS tube is a voltage-driven high-current type device, which is widely used in circuits, especially power systems. MOS tubes have some characteristics that should be paid special attention in practical applications.

MOS devices have body diodes formed by pn junction between source (S) and drain (D), and also known as parasitic diodes or an internal diode, are found in a single MOS device between the drain and the source. They are not used in integrated circuit lithography (standard method of printed circuit board (PCB), and microprocessor fabrication). This diode can provide reverse protection and freewheeling during high current drive and inductive loads. The forward voltage drop is about 0.7~1V. Because of this diode, the MOS device can’t simply see the function of a switch in the circuit. For example, in the charging circuit, after the charging is completed, the battery will reverse when the supply power is removed; this is usually the result we do not want to see. The general solution is to add a diode to prevent reverse power supply. This can be done, but the characteristics of the diode must have a forward voltage of 0.6~1V. Down, in the case of high currents, the heat is severe, and at the same time, the energy is wasted, and the energy efficiency of the whole machine is low. Another method is to add a back-to-back MOS tube and use the low on-resistance of the MOS tube to achieve energy saving. Another common application of this characteristic is low-voltage synchronous rectification. In practice, the body diode is a result of manufacturing process, and it is in between the source and drain and on an n-channel device, if the drains fall below voltage on the source, current will flow from source to drain.

Precautions

After the MOS tube is turned on, it has no directionality and in this state of operation, it behaves like a wire. It has a resistance characteristic only and there is no conduction voltage drop in this case. Usually, the saturation level on resistance is several to several tens of milliohms (mΩ). MOS tube is also non-directional therefore allowing both DC and AC currents to pass through.

Precautions for using MOS tubes

• In order to safely use the MOS tube, the limit value of the dissipated power of the tube, the maximum drain-source voltage, the maximum gate-source voltage, and the maximum current set values cannot be exceeded in the manufacturing design.

• When using various types of MOS tubes, they must be connected to the circuit in strict accordance with the required bias, and the polarity of the MOS tube bias should be observed. For example, the junction between the source and drain of the junction MOS transistor is a PN junction, the gate of the N-channel transistor can be positively biased; the gate of the P-channel transistor can be negatively biased.

• Since the input impedance of the MOS tube is extremely high, the lead pin must be short-circuited during transportation and storage, and the metal shield package should be used to prevent the external induced potential from penetrating the gate. In particular, it is important to note that the MOS tube cannot be placed in a plastic box. It should be placed in a metal box e.g aluminium foil when it is stored, and the tube should be protected from moisture.

• In order to prevent the gate breakdown of the MOS tube, all test instruments, worktables, soldering irons, and the circuit itself must be well grounded; when the pins are soldered, the source is soldered first; before being connected to the circuit, All the lead ends of MOS tube are kept short-circuited with each other, and the short-circuit material is removed after soldering; when removing MOS tube from the component holder, the grounding of the human body should be adhered to. The advanced gas-fired electric soldering iron is convenient for soldering MOS tubes and ensures safety. When the power is not turned off, it is absolutely impossible to insert or remove the tubes from the circuit. The above safety measures must be taken care of when using MOS tubes.

• When installing the MOS tube, pay attention to the location of the installation to avoid heating elements; to prevent the vibration of MOS tube, it is necessary to fasten MOS tube; when the lead is bent, it should be larger than the root size of 5 mm. Therefore it is important to prevent bending of the pins and causing air leaks.

• When using a VMOS tube, a suitable heat sink must be added. Taking VNF306 as an example, the maximum power can reach 30W after it is equipped with a 140×140×4 (mm) heat sink.

• After the multiple MOS tubes are connected in parallel, the high-frequency characteristics of the amplifier are deteriorated due to the corresponding increase in the inter-electrode capacitance and the distributed capacitance and high-frequency parasitic oscillation of the amplifier is easily caused by the feedback. For this reason, the parallel composite MOS tubes generally do not exceed four, and the anti-parasitic oscillation resistors are connected in series to the base or the gate of each tube.

• The gate-source voltage of the junction MOS transistor cannot be reversed and can be saved in the open state. When the insulated gate MOS transistor is not used, the electrodes must be short-circuited since its input resistance is very high, so as to avoid an external electric field. The MOS tube is damaged by such action.

• When soldering, the soldering iron shell must be equipped with an external grounding wire to prevent damage to MOS tube due to electrification of the soldering iron. For a small amount of soldering, you can also solder the soldering iron after removing the plug or cutting off the power. Especially when soldering insulated gate MOS transistors, they should be soldered in the order of source-drain-gate, and the power should be cut off.

• When soldering with 25W soldering iron, it should be fast. If soldering with 45~75W soldering iron, use the tweezers to clamp the root of the pin to help dissipate heat. The junction MOS tube can qualitatively check the quality of the MOS tube by using the table resistance file (check the resistance between the forward and reverse resistance of each PN junction and the drain source), and the insulated gate field effect tube cannot be inspected with a multimeter, and the tester must be used. Moreover, the short-circuit line of each electrode can be removed after the tester is connected. When it is removed, it should be short-circuited and then removed. The key is to avoid the gate hanging.

When input impedance is a factor to consider during design process, it is necessary to take moisture-proof measures to avoid lowering the input resistance of the MOS tube due to temperature influence. If a four-lead MOS transistor is used, its substrate leads should be grounded. The ceramic packaged of the MOS tube has photosensitive properties and should be protected from light.

For power MOS tubes, there must be good heat dissipation conditions. Because the power MOS tube is used under high load conditions, it is necessary to design a sufficient heat sink to ensure that the temperature of MOS tube casing does not exceed the rated value, so that the MOS device can work stably and reliably for a long time.

In short, to ensure use of MOS tubes safely, there are many precautions to be adhered to, and the safety measures adopted are various. The vast number of professional and technical personnel required, especially the vast number of electronic enthusiasts, must proceed according to their actual conditions. Take practical measures to use MOS tubes safely and effectively.