Input and Output characteristics of NPN transistor
Input and output characteristics of an N-P-N Transistor:
Fig: Input and output characteristics of an n – p – n transistor. (a) Input characteristics; (b) Output characteristics; (c) Current gain[β] characteristics
- When a biasing voltage VBB of appropriate polarity is applied across the junction JBE the potential barrier at this junction reduces and at one point the junction becomes forward biased. The current crossing this junction is governed by the forward biased p-n junction equation for a given collector emitter voltage.
- The base current iB is related to the recombination of minority carriers injected into the base from the emitter. The rate of recombination is directly proportional to the amount of excess minority carrier stored in the base.
- In a normal transistor the emitter is much more heavily doped compared to the base the current crossing JBE is almost entirely determined by the excess minority carrier distribution in the base. It can be concluded that the relationship between iB and VBE will be similar to the i-v characteristics of a p-n junction diode.
- As VCE increases reverse bias of JCB increases and the depletion region at JCB moves deeper into the base. The effective base width thus reduces, reducing the rate of recombination in the base region and hence the base current. Therefore iB for a given VBE reduces with increasing VCE
- Only a fraction (denoted by the letter “∝”) of the total minority carriers injected into the base reaches junction JCB where they are swept in to the collector region by the large electric field at JCB.Minority carriers constitute the major component of the total collector current. The other component of the collector current consists of the small reverse saturation current of the reverse biased junction JCB.
- Therefore, IC = ∝IE Ics
Where Ics is the reverse saturation current of junction JCB , But IE = IB IC
- β is called the large signal common emitter current gain of the transistor and remains fairly constant for a large range of IC
- With VBB = 0 or negative there is little injected minority carrier into the base from the emitter side. Therefore, iB = 0 and iC is negligibly small. The transistor is said to be in the “cut off” region under this condition.
- As VBB is increased from zero, base current starts flowing. The component “∝IE” of collector current will increase. ICS is ,for all practical purpose, independent of VCE. This is the active or “amplifier mode” of operation of a transistor.
- In the active region as iB increases iC also increases. For a given value of VCC, VCE reduces with increasing iC due to increased drop in an external load.
- At one point the junction JCB becomes forward biased. VCE, now is just the difference between the voltages across two forward biased junction JBE and JCB (a few handed milli volts). This is when the transistor enters the saturation mode of operation.
- The ratio iC/iB at the onset of saturation is called βMin and is an important parameter for a power transistor. In saturation iC is almost entirely determined by the external load and further increase in iB changes iC or VCE very little.