Before understanding the advantages over each other, first we need to understand what they are and then we will have a better idea that how and why CMOS has better advantages than BJT. So, let’s start with CMOS switches so that we can understand why it became most used technology in VLSI Chips. CMOS stands for complementary metal oxide semiconductors; it is basically a technology for constructing integrated circuits. CMOS technology is used in digital logic circuits but this technology is not limited to digital logic, we have seen CMOS sensor is used as image sensor which is a perfect example of using CMOS for analog circuits. CMOS is also known as complementary-symmetry which is basically because it uses two types of semiconductors which are: p-type and n-type and CMOS uses these two semi-conductors in complementary and symmetry with each other. The best feature of CMOS switches are that they prevent high noise and consume very less power. Also, the best part of these switches is they do not produce much heat waste and allow high density local function at chip level.
Now, let’s talk about Bipolar Junction Transistor switches. The basic function of BJT is to amplify current, for that these transistors uses two types of charge carrier that are hole and electron. For operations they use two junctions between two semi conductors i.e. n-type and p-type and hence they are available in two individual component type which are NPN and PNP unlike uni-polar switches like CMOS which uses p-type and n-type semi conductors integrated.
Since we pretty much know on the advantages, functions and how CMOS switches works let’s move to the reasons why it is better than the Bipolar junction transistors as a switch. Here we go:
- Voltage Drop comparison:
First and the most important advantage of CMOS is that it consumes less power and the does not drop voltage between the base and emitters but on the other hand BJT switches drops almost 0.7V between base-emitters node. It is basically because BJT’s are current operated device in compare to the CMOS which is a voltage operated device.
- Temperature resistance:
Temperature co-efficient for resistance is one of the major components that work as an advantage when it comes to switches. Since CMOS has positive temperature co-efficient resistance parallel operations are possible without current sharing resistor but on the other hand BJT has negative temperature co-efficient for resistance which makes it mandatory to use a current sharing resistor for parallel operations.
- Secondary break down issue:
Secondary breakdown issue is basically a failure mode in transistor with a large junction area. It happens because current gets concentrated in a small spot of the base emitter junction that leads to local heating and causes a short between collector and emitter. This is one of the biggest issues that BJT switches have and on the hand CMOS does not has this issues.
After analyzing the different function and feature, we can pretty much draw the conclusion that CMOS offers better advantages than Bipolar switches.
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