Diode structure
The basic structure of the diode is as follows: it is composed of a PN junction and the semiconductor where it is located, plus electrode leads and packaging; This is shown in the figure below.
1. Cathode/anode;
2. Housing;
3. Leads.
According to the specific structure of the diode, it can be divided into three types: point contact type, surface contact type, and silicon planar type.
As shown in the figure below, silicon planar diodes are generally used most in digital circuits, while diodes with other two structures also have different application scenarios.
1. Characteristics of point contact structure: the junction area is small, so the junction capacitance and allowable current are small.
Point contact diodes are suitable for high-frequency circuits due to their small parasitic capacitance (junction capacitance), so they are generally used in detection, rectification, modulation, mixing and limiting of high-frequency small signals.
2. Characteristics of surface contact structure: the junction area is large, so the junction capacitance and the allowable current are large.
Surface-contact diodes can not only pass large currents, but also have stable and reliable performance, and are mostly used in low-frequency rectification/high-current switching, pulse and low-frequency circuits.
3. Characteristics of silicon planar structure: large junction area is suitable for high-power rectification.
Planar junction diodes have a small junction area, small junction capacitance, and can pass large currents at the same time, and the performance is stable and reliable, and is mostly used in switching, pulse and high-frequency circuits.
Diode basic parameters
We know that the most important parameter of a diode is its unidirectional conductivity, that is, the normal operating state of the diode: it can only flow in from the positive (anode) of the diode and out from the negative (cathode).
Therefore, one-way conductivity can be divided into: forward and reverse characteristics.
1. Forward characteristics: The positive electrode is connected to the high potential end, and the negative electrode is connected to the low potential end is called forward bias.
1. When the forward voltage is less than Vth: the forward voltage of the diode is less than the built-in electric field of the PN junction, and the electric field of the PN junction is gradually overcome, and the current increases linearly with the voltage (small injection).
In this case, although the forward voltage of the diode is < Vth, the diode does not have no current flowing through, but generates some current.
2. When the forward voltage is greater than Vth: at this time, the diode is in the normal conduction state, the PN junction electric field is completely weakened, the diode conduction voltage drop remains unchanged, and the current increases with the voltage index (conductance modulation).
At this time, the forward conduction of the diode enters a state of "large injection" due to the conductance modulation effect.
2. Reverse characteristic: The positive electrode is connected to the low-potential end, and the negative electrode is connected to the high-potential end is called reverse bias.
1. The reverse voltage is in the range of 0V-Ubr: at this time, the diode is in the reverse cut-off state, because the reverse voltage is small, the reverse current is also very small, and the reverse leakage current is greatly affected by temperature.
For every 10°C increase in temperature, the current increases by a factor of 4; The reverse leakage current depends on the concentration of few sons in the semiconductor, whereas the concentration of the few sons depends mainly on the temperature.
2. The reverse voltage is greater than Ubr: at this time, the diode is in the reverse breakdown state, because the voltage is greater than the reverse breakdown voltage, the diode loses its unidirectional orientation, and the reverse current suddenly increases (exponentially increases);
3. If the reverse voltage continues to increase or the overvoltage time is longer, it will cause the diode breakdown to be damaged, and the damage is mainly caused by overheating of the device.
My question now is, how does the current in the "reverse cut-off zone" be generated in these states?
1. PN junction state of the "reverse cut-off zone": As shown in the figure below, the PN junction is externally reversely voltaged (negative in P region, positive in N region), and the external electric field is superimposed with the electric field in the space charge region, and the range of the space charge region becomes larger.
2. At this time, due to the enlargement of the space charge region, more electron-hole pairs will inevitably be generated: because the inverse space charge region has a strong electric field (electron concentration and hole concentration are 0), these intrinsically excited electron-hole pairs are swept out of the space charge region by the electric field once they are generated: the holes go to the P region, and the free electrons go to the N region.
The direction of the flow of this part of the charge is in the direction of the reverse bias current, forming the reverse current.