Current Components in a Transistor
The conduction of current in NPN transistor is owing to electrons and in PNP transistor, it is owing to holes. The direction of current flow will be in opposite direction. Here, we can discuss the current components in a PNP transistor with common base configuration. The emitter-base junction (JE) is forward biased and the collector-base junction (JC) is reversed biased as shown in figure. All the current components related to this transistor are shown here.
We know that, the current arrives the transistor through the emitter and this current is called emitter current (IE). This current consists of two constituents – Hole current (IhE) and Electron current (IeE). IeE is due to passage of electrons from base to emitter and IhE is due to passage of holes from emitter to base. Normally, the emitter is heavily doped compared to base in industrial transistor. So, the Electron current is negligible compared to Hole current. Thus we can conclude that, the whole emitter current in this transistor is due to the passage of holes from the emitter to the base.
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Some of the holes which are crossing the junction JE (emitter junction) combines with the electrons present in the base (N-type). Thus, every holes crossing JE will not arrive at JC. The remaining holes will reach the collector junction which produces the hole current component, IhC. There will be bulk recombination in the base and the current leaving the base will be The electrons in the base which are lost by the recombination with holes (injected into the base across JE) are refilled by the electrons that enter into the base region. The holes which are arriving at the collector junction (JC) will cross the junction and it will go into the collector region.
When the emitter circuit is open circuited, then IE = 0 and IhC = 0. In this condition, the base and collector will perform as reverse biased diode. Here, the collector current, IC will be same as reverse saturation current (ICO or ICBO). ICO is in fact a small reverse current which passes through the PN junction diode. This is due to thermally generated minority carriers which are pushed by barrier potential. This reverse current increase; if the junction is reverse biased and it will have the same direction as the collector current. This current attains a saturation value (I0) at moderate reverse biased voltage.
When the emitter junction is at forward biased (in active operation region), then the collector current will become The α is the large signal current gain which is a fraction of the emitter current which comprises of IhC. When the emitter is at closed condition, then IE ≠ 0 and collector current will be In a PNP transistor, the reverse saturation current (ICBO) will comprises of the current due to the holes passing through the collector junction from the base to collector region (IhCO) and the current due to the electrons which are passing through the collector junction in the opposite direction (IeCO). The total current entering into the transistor will be equal to the total current leaving the transistor (according to Kirchhoff’s current law).