Constructional Features of a Power BJT
Constructional Features of a Power BJT:
Fig: Constructional Features of a Power Bipolar Junction Transistor - Schematic of Construction
Power Transistors are predominantly of the n-p-n type. Some of the characteristics of the Power transistors are given as:
- A power BJT has a vertically oriented alternating layer of n type and p type semiconductor materials. The vertical structure 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 lass is minimized.
- In order to maintain a large current gain “β” (and hence reduce base drive current) 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 function of this drift region is similar to that in a Power Diode.
- However, 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 cannot be made as small as that in a signal level transistor. This comparatively larger base width has adverse effect on the current gain (β) of a Power transistor which typically varies within 5-20.The collector drift region has significant effect on the output characteristics of a Power BJT.
- Practical Power transistors have their emitters and bases interleaved as narrow fingers. This is necessary to prevent “current crowding” and consequent “second break down”. In addition multiple emitter structure also reduces parasitic ohmic resistance in the base current path.