2016.09.08 Si Power Device
We begin with a basic discussion of the types of diodes, which are here considered as silicon power devices, and of the essentials of diodes, which can be called the most basic silicon semiconductor device.
Types of Si Diodes
When considering the types of Si diodes, there are probably a number of ways to categorize them, depending on the main object. Here, we have categorized diodes mainly according to how they are expected to be used, such as for power conversion, and so on.
As our general framework, we have categorized diodes as rectifying diodes, Zener diodes, and radio frequency diodes. Among these, diodes used mainly for rectification are further divided into those for general-use rectification, devices for high-speed rectification assuming switching, fast-recovery type diodes for applications in ultra-high speed rectification, and finally Schottky barrier diodes which feature fast operation and low VF. Hereafter each of these is explained in turn.
We have adopted these categories, but in all cases these devices consist essentially of an anode and a cathode, and parameters indicating their essential functions and features are pretty much the same. If the discussion turns to "then how are they different?", we can probably summarize things by saying that "different electrical characteristics are optimized according to the application".
Electrical Characteristics of Diodes
To ensure certainty, we shall verify the static and dynamic characteristics of diodes. If you examine the diagram below, a detailed explanation should not be necessary, but the following are keywords that are needed to explain the characteristics of diodes.
Basic static characteristics of diodes are the forward voltage VF and forward current IF, and the reverse voltage and current VR and IR.
The area surrounded by the orange dashed line in the diagram on the right indicates the usable area of rectifying diodes. Specifically, this is the area within the range of allowable IF, and within the breakdown voltage range in the reverse direction.
It should be noted that the area enclosed by the green dashed line is the usable area of Zener diodes, although these are not discussed in this chapter. This area isn’t usable for other diodes, and if this area is entered without any limits on the IR, device failure may occur.
The main dynamic characteristics of diodes are the reverse recovery time trr, and the static capacitance Ct.
trr is the time when, from the state in which a voltage is applied in the forward direction and forward current IF is flowing, the voltage is changed to the reverse direction and the reverse current IR returns to the steady state (essentially zero). As indicated in the diagram on the right, when the device changes from the ON-state in which an IF is flowing to the OFF-state, ideally IF would immediately go to zero. But in actuality, zero is passed, and a reverse current IR flows momentarily, which recovers to zero in time trr. The shorter trr is, the better is the device characteristic.
The capacitance Ct is the capacitance of the diode itself, and has the same effect as a capacitor. As in the diagram on the right, when a diode is turned on and off, if Ct is large, the so-called rounding of the waveform becomes more pronounced, and in some cases there may be the problem that the device begins turn-off operation before an applied voltage reaches a full level due to time constants. In a high-speed switching circuit, diodes with a low Ct are desirable.
Finally, we summarize the main maximum ratings and electrical characteristics.
Maximum ratings indicate continuous and instantaneous applied voltages and currents, and the temperature conditions.
For the definition of "instantaneous", either the description on the data sheet should be consulted, or if there is no clear description, the manufacturer should be consulted.
As the start of a new chapter, much of the above has been a discussion of basics, but these are matters that must be understood when considering diode characteristics and applications. When selecting actual diodes, these parameters must be examined.
・Review of the most basic essentials.
・The types and categories of diodes used as Si power devices were described.