Offset generally includes voltage and current in the forward and reverse (negative). For example, an amplifier composed of transistors needs to amplify the signal voltage without distortion, and it is necessary to ensure the forward bias of the emitter junction of the transistor and the reverse bias of the collector junction. That is, it should set its working point. The so-called operating point is that the base, emitter and collector of the transistor are set to the required potential (according to the calculation) through the setting of the external circuit. These external circuits are called bias circuits (understandably, the circuit that sets the positive and negative bias of the PN junction). The current provided by the bias circuit to the transistor is called the bias current.

Forward bias

When the PN junction is positive, the direction of the external electric field is from the P area to the N area. Obviously the direction is opposite to the internal electric field. At this time, the external electric field drives the holes in the P area into the space charge area to cancel a part of the negative space charge. At the same time, the N area The free electrons enter the space charge region to cancel out a portion of the positive space charge. As a result, the space charge region is narrowed and the internal electric field is weakened. The weakening of the internal electric field causes the diffusion motion of the majority carrier to generally form a large diffusion current (the diffusion current is formed by the directional movement of many sons and is enhanced, and the PN junction pair is simply referred to as the current). In a certain range, the stronger the external electric field is, the larger the forward current is, and the current is in a low-resistance state. This situation is known as the forward conduction of the PN junction in the electronic technology.

In the absence of an applied voltage, the diffusion motion and drift motion of the semiconductor are in a dynamic equilibrium, and the current through the PN junction is zero in the dynamic equilibrium state. At this time, if voltage is applied across the PN junction, the balance of diffusion and drift motion will be destroyed, and the PN junction will show its unidirectional conductivity performance.

Within a certain range, the stronger the external electric field is, the larger the forward current is, and the PN junction is in a low-resistance state with respect to the forward current. This condition is referred to as the forward conduction of the PN junction in the electronic technology.

The forward conductivity of the PN junction is the main working mechanism of the semiconductor device.

Reverse bias

When the PN junction is reversed biased, the applied electric field is in the same direction as the internal electric field in the space charge region, which also leads to the destruction of the equilibrium state of diffusion and drift motion. The external electric field drives the holes and free electrons on both sides of the space charge zone to move away, widen the space charge zone, enhance the internal electric field, make the majority carrier diffusion motion difficult to carry out, and strengthen the drift movement of minority carriers. The reverse current flowing from the N region to the P region. However, because the minority carrier is constant at a normal temperature and the number is small, the reverse current is extremely small. The small current indicates that the reverse resistance of the PN junction is very high, and it is generally considered that the reverse biased PN junction is not conductive and is basically in an off state, which is called reverse blocking of the PN junction in the electronic technology.

When the applied reverse voltage changes within a certain range, the reverse current hardly changes with the applied voltage. This is because the reverse current is formed by the minority drift. Under thermal excitation, the number of minority sons increases and the reverse current of the PN junction increases. In other words, as long as the temperature does not change, the minority carrier concentration does not change. Even if the reverse voltage increases more than the allowable range, the number of minority sons cannot be increased, and the reverse current tends to be constant. The current is also called reverse saturation current. It is worth noting that reverse current is one of the major causes of circuit noise. Therefore, when designing a circuit, temperature compensation must be considered.