Difference between unipolar and bipolar families in electronics
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Answer:
Explanation:
- What is the difference between bipolar and unipolar devices ?Bipolar transistors can have both minority and majority carriers flowing, whereas FET's only have majority carriers flowing.The fact that bipolar transistors have two types of carriers flowing simultaneously results in the name 'bi-polar'. The monirity carrier flow is responsible for collector conduction modulation and transistor storage time.-Why is a bipolar transistor current driven and a FET voltage driven ?A FET has a gate that is either insultaed physically (MOSFET) or by a reverse biased junction (JFET). Hence no current is flowing through the gate. A bipolar transistor has the base electrically connected and needs base current to turn on. How does a bipolar transistor (BJT) work ?A BJT (NPN) will turn on by base current. For a positive base current, electrons will be drawn from the emitter into the base area.. Most of the electrons that are drawn from emitter into base area will be cought by the collector. For every electron that is taken from the base terminal, several electrons will reach the collector terminal. This is the multiplication effect, or current gain. The BJT is a current amplifier. The BJT has 4 modes of operation :1) Cutoff mode : the base voltage is below threshold (0.4-0.7V for Si) so no collector current is flowing. The base-collector juction is reverse biased, and the collector voltage is blocked with no collector current flowing.2) Linear mode : the collector-base junction is reverse biased, and the base-emitter junction is forward biased. Electrons are injected into the reverse-biased base-collector junction, constituting a collector current which is a multiple of the base current. The collector current is more or less independent from the collector voltage.3) Quasi Saturation : the base-collector junction becomes forward biased. This can appear even if the collector voltage is still higher than the base voltage because of conduction losses in the collector area. In this mode minority carriers will be injected into the collector area though the forward biased base-collector junction. These minority carriers will generate an electron-hole plasma effectively lowering the collector resistance. It is this lowered collector resistance that will offer BJT's a benefit over unipolar devices in switching applications.4) Saturation : the base-collector junction is forward biased and increasing the base current will no longer lower the collector resistance.
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