1. Forward conduction and reverse end. When the forward voltage reaches a certain value (0.7V for silicon tube and 0.3V for germanium tube), the current changes exponentially with the voltage. Compared with resistance, it has non-linear characteristics, so its characteristic curve is generally non-linear.
2. There are two kinds of carriers, namely electrons and holes.
3. The temperature influence is relatively large, because the temperature change affects the movement speed of the carrier and the degree of intrinsic excitation, so the temperature problem is often considered in the design or application.
Breakdown characteristics of PN junction
As shown in the figure, when the reverse voltage applied to the PN junction increases to a certain value, the reverse current suddenly increases sharply, and the PN junction produces electrical breakdown—this is the breakdown characteristic of the PN junction. The reverse bias voltage when the breakdown occurs is called the reverse breakdown voltage VBR of the PN junction.
The electrical breakdown of the PN junction is a reversible breakdown. If the bias voltage is lowered in time, the PN junction will restore its original characteristics. The characteristics of electrical breakdown can be used (such as a Zener tube). Thermal breakdown means burnout, which is irreversible breakdown. Try to avoid it when using it.
After the PN junction is broken down, the voltage drop across the PN junction is high, the current is large, and the power is large. When the power dissipation on the PN junction causes the PN junction to heat up and exceeds its dissipation power, the PN junction will undergo thermal breakdown. At this time, a vicious circle occurs between the current and temperature of the PN junction, which will eventually cause the PN junction to burn.
Unidirectional conductivity of PN junctionWhen forward biased, the space charge area shrinks and weakens the internal electric field. After the external electric field increases to a certain value, the diffusion current increases significantly, forming an obvious forward current, and the PN junction is turned on.
When reverse biasing, the space charge region expands, the internal electric field is strengthened, the diffusion motion is greatly weakened, and the drift motion of minority carriers is strengthened and dominated. However, the minority carrier concentration of doped semiconductors at room temperature is very low, and the reverse current is much smaller than the forward current.
When the temperature is constant, the minority carrier concentration is constant, and the PN junction reverse current is almost independent of the applied reverse voltage, so it is also called reverse saturation current.
In addition to nonlinear resistance characteristics, PN junctions also have nonlinear capacitance characteristics, mainly including barrier capacitance and diffusion capacitance.
1. Barrier capacitance: The barrier area is similar to a plate capacitor, and the two sides of the boundary store ionic charges of equal value and opposite polarity. The amount of charge changes with the applied voltage, which is called the barrier capacitance, which is represented by CT.
CT=dQ/dV
PN junction has sudden change junction and slow change junction. Now consider the situation of sudden change junction (see "Transistor Principle" for slow change junction). PN junction is equivalent to a plate capacitor. Although the applied electric field will widen or narrow the barrier area, this change is relatively small. Can be ignored,
Then CT=εS/L, and the width L0 of the barrier layer under dynamic balance is known, and substituting the above formula can be obtained:
CT is not a constant value, but changes with V. Using this characteristic, a varactor diode can be made.
2. Diffusion capacitance: multiple sons cross the PN junction and become the minority son of the other party during the diffusion process. When the PN junction is in equilibrium (no applied voltage), the minority sons are called balanced minority sons, which can be considered as balanced minority sons in the area outside the barrier layer. When the PN junction is forward biased, the multi-carrier free electrons in the N region diffuse to the P region and become the unbalanced minority carriers in the P region. Due to the difference in concentration, they will also diffuse to the depth of the P region, and the distance from the interface is Far, the lower the non-equilibrium minority carrier concentration, the distribution curve is shown in [Voltage Ampere Characteristics of PN Junction]. When the applied forward voltage increases, the concentration distribution curve shifts upward, and the non-equilibrium minority carrier concentration on both sides increases, that is, the amount of charge increases. In order to maintain electrical neutrality, the non-equilibrium multi-carrier concentration in the neutral zone also increases accordingly. That is to say, When the applied voltage increases, the amount of holes and free electrons stored in the P and N regions also increase. This effect is equivalent to connecting a capacitor in parallel with the PN junction. Because it is caused by carrier diffusion, it is called For the diffusion capacitance CD, CD=(I+Is)Ï„p/VT is derived from semiconductor physics, please refer to "Transistor Principle".
When the reverse voltage is applied, I=Is, and CD tends to zero.
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