Power Bipolar Junction Transistor (BJT) was the first semiconductor device to closely approximate an ideal fully controlled Power switch. Other transistors have characteristics that are qualitatively similar to those of the BJT. The characteristics of a Power BJT differs significantly from its signal level counterpart due to the requirement for a large blocking voltage in the “OFF” state and a high current carrying capacity in the “ON” state.
Operating Principle of a Bipolar Junction Transistor(BJT)
A transistor junction consists of a semiconductor crystal in which a p-type region is sandwiched between two n-type regions which is called an n-p-n transistor. Alternatively an n-type region may be placed in between two p-type regions to give a p-n-p transistor. As shown in fig the circuit symbols and schematic representations of an n-p-n and a p-n-p transistor. Transistors consist of terminals are called Emitter (E), Base (B), and Collector (C).
If no external biasing voltages are applied i.e. VBB and VCC are open-circuited all-transistor currents must be zero. The transistor will be in thermal equilibrium condition. It is clear from the diagram that p-type carriers in the base region of an n-p-n transistor are trapped in a potential well and cannot escape. Similarly, in a p-n-p transistor p-type carriers in the emitter and collector regions are separated by a potential hill.
When biasing voltages are applied the base-emitter junction ( JBE ) becomes forward biased whereas the base-collector junction is reverse biased. The potential barrier and depletion layer width at JBE reduces. As the potential barrier at JBE is reduced a large number of minority carriers are introduced into Base and the Emitter regions.
A portion of the minority carriers reaching the base recombines with majority carriers. The rest, defuse to the edge of the depletion region at JCB where they are swept away to the collector region by the large electric field. Under this condition the transistor is said to be in the Active region.
As VBE is increased injected minority charge into the base region increases and so does the base current and the collector current.
For a fixed collector bias voltage VCC, the voltage VCB reduces with an increase in collector current due to the increasing drop in the external resistance RC. Therefore, the potential barrier at JCB starts reducing. At one point JCB becomes forward biased. Due to the forward biasing of JCB there will be minority carrier injection into the base from this junction. The total voltage drop between collector and emitter will be the difference between the forward bias voltage drops at JBE and JCB. Under this condition, the transistor is said to be in the saturation region. A similar explanation applies to a p-n-p transistor.
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.
The relationship between iB and VBE will be similar to the i-v characteristics of a p-n junction diode. VCE, however, have some effect on this characteristic. 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.
Therefore IC= ∝ IE+ICS
where ∝ = fraction of the total minority carriers injected into the base reaches junction JCB
ICS = the reverse saturation current of junction JCB
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.
Input and output characteristics of BJT
In the active region as iB increases iC also increases. For a given value of VCC,VCE reduces with increasing iC due to an 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 millivolts).
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.
Constructional Features of a Power BJT
- BJT has a vertically oriented alternating layers of n-type and p-type semiconductor materials. Vertical is preferred for power transistors because it maximizes the cross-sectional area through which the on-state current flows. Thus, on-state resistance and power loss is minimized.
- In order to maintain a large current gain “β” the emitter doping density is made several orders of magnitude higher than the base region. The thickness of the base region is also made as small as possible.
- In order to block large voltage during “OFF” state a lightly doped collector drift region is introduced between the moderately doped base region and the heavily doped collector region.
- The doping density donation of the base region being moderate the depletion region does penetrate considerably into the base. Therefore, the width of the base region in a power transistor can not be made as small as that is a signal level transistor.,
- Larger base width has an adverse effect on the current gain (β) of a Power transistor which typically varies within 5-20.
- Practical Power transistors have their emitters and bases interleaved as narrow fingers. This is necessary to prevent current crowding and consequent second break down.
- Addition multiple emitter structure also reduces parasitic ohmic resistance in the base current path.
- A Bipolar Junction Transistor is a minority carrier, current controlled unidirectional device.
- A BJT can be of n-p-n or p-n-p type with three terminals called the collector, the base and the emitter
- BJT can operate in cut-off, active or saturation regions.
- In the cut-off region the base emitter junction is reverse biased and the collector current is almost zero.
- In the active region the ratio of collector current to base current is fairly constant. This ratio is called the dc current gain (β).
- A transistor can be driven into saturation by increasing the base current for a given collector current.
- For power application normally, n-p-n type transistor in the common emitter configuration with the base as the control terminal is used. They operate either in the cut-off, or saturation mode.
- For safe operation power transistors must observe maximum current, maximum voltage, maximum power dissipation and second break down limits.