EFFECTS OF THE TEMPERATURE ON THE CHARACTERISTICS OF THE TRANSISTOR

As saw it to you, if one polarizes only the junction collector-bases, a residual current ICBO circulates in the circuit of the collector.

This junction being polarized in opposite direction, current ICBO is due to the minority carriers consisted the couples electron-positron pair. The number of these couples increase when the temperature grows. Consequently, current ICBO increases too.

This increase in current ICBO can influence the normal operation of the transistor.

To determine this influence, it is first of all necessary to know the relation between ICBO and the ambient temperature. For that, it is necessary to carry out the assembly of figure 19.

Current ICBO is measured with a microammeter. One heats the transistor to follow the evolution of ICBO.

One realizes whereas ICBO doubles when the temperature increases 10° C (transistor with germanium). For a transistor with silicon, ICBO doubles for an increase in temperature of 6° C.

Let us give an example for the transistor to germanium.

For a temperature of 25° C, ICBO = 5 µA ; with 35° C, ICBO = 5 x 2 = 10 µA with 45° C, ICBO = 10 x 2 = 20 µA.

One can thus plot a curve representing the variation of ICBO according to the temperature for a given transistor. Indeed, for a given temperature, ICBO can be different according to the examined transistor.

One can then plot the graphics of figure 20 which represents the relative increase in current ICBO according to the temperature.

It is seen that ICBO is multiplied by 30 when the temperature passes from 25° C to approximately 75° C. 

Now let us see the influence of the temperature on the collector current during the normal operation of the transistor.

Let us consider the transistor assembled in base common, illustrated to the figure 21-a.

The current of transmitter IE is worth 2 mA, ICBO are worth 5 µA with 25° C and IC are worth 1,965 mA.

The following relation binds these various values.

Let us calculate current IC with 50° C. current ICBO is multiplied by approximately 6 (see figure 20). 

Therefore, it is worth 5 µA x 6 = 30 µA = 0,030 mA with 50° C.

From where IC = (0,98 x 2) + 0,030 = 1,990 mA. (Figure 21-b)

Current IC increased by 25 µA, which corresponds to the increase in residual current ICBO.

As the residual current is always weak compared to the collector current in the assembly bases common, one can say that the temperature has little influence on the collector current.

Now let us consider the transistor assembled out of common transmitter (figure 21-c).

IB = 35 µA, ICEO = 250 µA with 25° C and IC = 1,965 mA.

Recall:

• ICEO is current the residual collector-transmitter when the base is in the air.

These values are bound by the relation :

IC =  x IB + ICEO

It is known that :  =   / (1 - )

Current ICEO can be calculated using the following relation already seen:

This value relates identical to that to the assembly bases common (figure 21-a).

Let us calculate the value of IC for a temperature of 50° C.

For that, one must calculate ICEO

However, ICEO = ( + 1) x ICBO

Thus ICEO = (49 + 1) x 30 µA = 1500 µA = 1, 5 mA

And IC = 49 x 0,035 + 1,5 mA = 3,215 mA (Figure 21-d)

The absolute increase in current IC is worth :

3,215 - 1,965 = 1,250 mA is 1250 µA

Under the same conditions of temperature, it was not that of 25 µA for the assembly bases common.

In conclusion, the collector current of a transistor assembled out of common transmitter is notably influenced by the temperature.

It is in addition the greatest disadvantage of the common transmitting assembly.

Figure 22 makes it possible to see the effect of the temperature on the network of characteristics of exit.

You notice that when the temperature passes from 25° C to 55° C, the whole of the characteristics is shifted upwards. The temperature also has an effect on the position of the point of operation.

It is what we will see with the assembly of figure 23.

In figure 22, one plotted the two straight lines of load relating to this assembly.

Current IB is worth 20 µA.

Consequently, to 25° C, the point of operation A corresponds to :

VCE = 5,4 volts and with IC = 2,4 mA

With 50° C, the point of operation of the assembly moved (point A'). VCE decreased (4 volts) and IC increased (3,3 mA). If one wanted to preserve the same values for VCE and IC, it would be necessary that current IB is 10 µA (point A").

In certain cases, the increase in current IC involves an increase in the power dissipated by the transistor. That causes to increase the temperature of the transistor, from where increase in current IC and the dissipated power and so on. This phenomenon is the thermal runaway and can lead to the destruction of the transistor.

In order to avoid this phenomenon, it is necessary to resort to suitable assemblies.

Note that these problems involved in the temperature are especially sensitive with the transistors to germanium. In the case of the transistors with silicon, the residual currents are definitely lower and consequently, the effect of the temperature is more reduced.