Main parameters of transistor

1. DC parameters

(1) The reverse saturation current Icbo between the collector and base, when the emitter is open (Ie=0) and a specified reverse voltage Vcb is applied between the base and collector, is only related to temperature and remains constant at a certain temperature. Therefore, it is called the reverse saturation current between the collector and base. A good transistor has a very small Icbo. The Icbo of a low-power germanium transistor is about 1-10 microamperes, while the Icbo of a high-power germanium transistor can reach several milliamps. The Icbo of a silicon transistor is very small, at the nanoampere level.

(2) When the collector emitter reverse current Iceo (penetration current) is open at the base (Ib=0), the collector current is determined by applying a specified reverse voltage Vce between the collector and emitter. Iceo is approximately β times that of Icbo, i.e. Iceo=(1+β) Icbo o. Icbo and Iceo are greatly affected by temperature and are important parameters for measuring the thermal stability of diodes. The smaller the value, the more stable the performance. The Iceo of low-power germanium diodes is larger than that of silicon diodes.

(3) When the collector is open and a specified reverse voltage is applied between the emitter and base, the current at the emitter is actually the reverse saturation current at the emitter junction.

(4) The DC current amplification factor β 1 (or hEF) refers to the ratio of the DC current output by the collector to the DC current input by the base when there is no AC signal input in the common emitter connection, that is:

β 1=Ic/Ib

2. Communication parameters

(1) The AC current amplification factor β (or hfe) refers to the ratio of the change in collector output current △ Ic to the change in base input current △ Ib in the common emitter connection, that is:

β=△ Ic/△ Ib

The beta of a typical transistor is approximately between 10-200. If the beta is too small, the current amplification effect is poor. If the beta is too large, although the current amplification effect is large, the performance is often unstable.

(2) The common base AC amplification factor α (or hfb) refers to the ratio of the change in collector output current △ Ic to the change in emitter current △ Ie when connected in a common base configuration, that is:

α=△ Ic/△ Ie

Because Δ Ic<Δ Ie, therefore α<1. If the alpha value of the high-frequency transistor is greater than 0.90, it can be used

The relationship between alpha and beta:

α=β/(1+β)

β=α/(1- α) ≈ 1/(1- α)

(3) The cut-off frequencies f β and f α are 0.707 times the frequency when β drops to low frequencies, which is the cut-off frequency f β of the common emitter; When α decreases to 0.707 times the frequency of low frequency, it is the cut-off frequency f α o f β of the common base. f α is an important parameter indicating the frequency characteristics of the diode, and their relationship is:

F β ≈ (1- α) f α

(4) The characteristic frequency fT decreases as the frequency f increases, and when β decreases to 1, the corresponding fT is an important parameter that comprehensively reflects the high-frequency amplification performance of the transistor.

3. Limit parameter

(1) When the collector current Ic increases to a certain value, causing the β value to decrease to 2/3 or 1/2 of the rated value, the maximum allowable current ICM of the collector is called ICM. So when Ic exceeds ICM, although it does not cause damage to the diode, the β value significantly decreases, affecting the amplification quality.

(2) When the emitter is open, the reverse breakdown voltage of the collector junction is called BVEBO.

(3) When the collector is open, the reverse breakdown voltage of the emitter junction is called BVEBO.

(4) Collector emitter breakdown voltage BVCEO is the maximum allowable voltage applied between the collector and emitter when the base is open. If Vce>BVCEO during use, the transistor will be broken down.

(5) The maximum allowable dissipated power of the collector PCM when the collector current exceeds Ic and the temperature increases, and the parameter changes of the diode due to heating do not exceed the allowable value, is called PCM. The actual dissipated power of the diode is the product of the collector DC voltage and current, i.e. Pc=Uce × Ic When using, make Pc<PCM.

PCM is related to heat dissipation conditions, and adding heat sinks can improve PCM.