Bipolar transistors are generally preferred over MOS transis- tors because of their lower spread. However, at deep-cryogenic temperatures, the performance of BJTs deteriorates due to a significant reduction in current gain and a substantial increase in the base resistance.

How does temperature affect a BJT?

As temperature increases , Ibo or reverse saturated current through emitter junction increases and hence overall current through Je reducess, and hence Vbe also get reduces proportionally.

Does temperature affect transistor?

An increase in temperature produces an increase in the minority carrier current, but a negative change in V_BE, so both effects lead to an increase in collector current with temperature.

Are transistors sensitive to temperature?

As described in Chapter 1, one of the characteristics of transistors and other semiconductor devices is that the electrical characteristics are very sensitive to temperature. Therefore, when creating a circuit design, it is necessary to consider changes in the operating point due to changes in temperature.

What are the limitations of BJT?

Disadvantages of BJT :

• BJT has a low thermal stability.
• BJT is most effective by radiation.
• BJT has more noise produced.
• BJT has a low switching frequency.
• BJT has a very complex control.
• The switching time is not very fast compared to a high alternating frequency of current and voltage.

44 BJT Breakdown and Temperature Effects

Why we prefer MOSFET over BJT?

They are portable, use low power, draw no current and are compatible with silicon processing technology. Their lack of gate current results in high input impedance. One additional major advantage of MOSFET over BJT is that it forms the basis of a circuit with switches of analog signals.

Why are MOSFET better than BJT?

BJTs are much more suitable than MOSFETs for driving low-power LEDs and similar devices from MCUs. MOSFETs are better for high-power applications because they can switch faster than BJTs, enabling them to use smaller inductors in switch-mode supplies, which increases efficiency.

Why is thermal stability required for a transistor?

Abstract: When the junction temperature of a transistor increases, the collector current increases, as a result of increases in 1) saturation currents and 2) dc conductances. Thermal instability occurs when junction temperature and collector current increase in regenerative and uncontrollable fashion.

How temperature affects the maximum power output in transistor?

The maximum power output available from a power transistor is closely linked to temperature, and above 25°C falls in a linear manner to zero power output as the maximum permissible temperature is reached.

Why does VBE decrease with temperature?

It's related to the energy required for the carriers to cross the PN junction. The thermal motion of the carriers reduces the amount of added energy needed, thus as the temperature increases, the forward voltage drop goes down.

What is thermal runaway in BJT and how does it occurs?

Thermal runaway

The problem with increasing temperature causing increasing collector current is that more current increase the power dissipated by the transistor which, in turn, increases its temperature. This self-reinforcing cycle is known as thermal run away, which may destroy the transistor.

How thermal runaway can be avoided in BJT?

I2 * R. So, as collector current increases, power dissipated increases which in turn increases the collector base junction temperature. So the process is cumulative leading eventually to the destruction of the transistor. Thermal runaway can be prevented by using aheat sink.

What is thermal runaway of BJT?

Thermal runaway: The thermal runaway takes place in a BJT. Thermal Runway in BJT is a process of self-damage of BJT because of overheating at the collector junction due to an increase in Ic with Ico. If T↑, then Ico (Reverse separation current) ↑, which results in an increase in the collector current, i.e. Ic ↑.

What is stability factor in BJT?

Stability factor (S):

It is the rate of change of collector current (I _C) with respect to the reverse saturation current of collector junction (I _CO), i.e. S = ∂ I C ∂ I C O ---(1)

What are the factors that affect the amplification of BJTs?

Mainly three parameters are used to define BJT transistor performance. Current Amplification factor, Base Transport Factor, Emitter Injection Efficiency parameters shows the performance of NPN transistor and PNP transistor.

What happens in base emitter resistance if temperature of a transistor goes up?

Right Answer is: A

Due to an increase in temperature, base-emitter voltage (V_BE) decreases and hence base-emitter resistance decreases.

Why are power transistors provided with heat sink?

Heat sinks are used for power transistors as the power dissipated at their collector junction is large. If heat dissipation is not done, this will cause large increases in junction temperature.

Why do we mount a transistor on a heat sink?

Its purpose is to fill in any gaps between the heatsink, washer and transistor to improve heat conduction. The securing plate goes on the other side of the heatsink. On the right you can see that the electrically conductive parts extend through holes in all these layers to reach the other side.

What is thermal runaway in BJT explain the condition of thermal stability?

Thermal runaway refers to the rise in temperature that can cause the destructive result. The energy emitted or released during the process is accelerated by the increasing temperature. It is not a topic of any particular subject. Thermal runaway can occur in almost every field and subject related to the temperature.

What is condition for thermal stability?

A material is thermally stable if it does not decompose under the influence of temperature.

Why is a BJT preferred over MOSFET while designing low current bias circuit?

The current output of the BJT can be controlled through the i/p base current. The current output of the MOSFET can be controlled through the i/p gate voltage. In BJT, Electrostatic Discharge is not a problem. In MOSFET, Electrostatic Discharge is an issue, so it can cause a problem.

Why BJT is faster than CMOS?

BJT is overall faster due to less junction capacitance in comparison with the large gate-source capacitance of MOS.

Why is MOSFET slower than BJT?

MOSFETs are slower than BJTs because of enormous capacitances at its junctions, formed by metal, substrate and the oxide layer.

What are the disadvantages of MOSFET over BJT?

The disadvantage of a typical MOSFET as compared to BJT is

• Increased power-handling levels.
• Reduced power-handling levels.
• Increased voltage-handling levels.
• Reduced voltage-handling levels.

What is the major difference between a BJT and a MOSFET?

BJT is a Bipolar Junction Transistor, while MOSFET is a Metal Oxide Semiconductor Field-Effect Transistor. 2. A BJT has an emitter, collector and base, while a MOSFET has a gate, source and drain.