Solid State Electronics ECE-1109 Md. Ebtidaul Karim

Electronics are Used as - To control electron flows which makes weak signal amplification possible. As switches which makes digital information processing possible.

History Invented in 1904 by John Ambrose Fleming vacuum tubes were a basic component for electronics throughout the first half of the twentieth century. The basic working principle of a vacuum tube is a phenomenon called thermionic emission. When we heat up a metal, thermal energy knocks some electron loose.

History But Vacuum tube technology had various limitation. Such as: Bulky Higher Operating voltage generally required. High Power consumption. ( Heat wastage). Glass tubes are fragile. As a result scientist started to look for alternative of vacuum tube.

Advantage of Transistor over vacuum tube It is smaller and lightweight. It has no heat requirement or heat loss. It is more efficient since less power is absorbed by the device itself. It is instantly available since for use, requiring no warm-up period. It has low operating voltage.

Transistor It is a three-layer semiconductor device consisting of either two n and one p type layers of material or two p and one n type layers of material. Three terminals are : Emitter Base Collector

Transistor Fig 1: PNP Transistor Fig 2: Symbols

Transistor Fig 3: NPN Transistor Fig 4: Symbol

Transistor Emitter-Base junction in both PNP and NPN transistor is forward biased. Base-Collector Junction is reversed biased. These transistors are called BJT. BJT means Bipolar Junction Transistor. Why it is named so ? Because here both electrons and holes contribute in current flow. In PNP transistor majority carrier is hole and in NPN transistor majority carrier is electron.

Transistor Emitter layer is heavily doped, the base is lightly doped and the collector is only lightly doped. Why emitter is heavily doped ? In NPN transistor once the electrons are injected by the emitter enter into the base, they become minority carriers. The base is made very thin and is very lightly doped. Because of this only few electrons traveling from the emitter to base region recombine with holes. The collector is so named because it collects electrons from base.

Transistor The transistor has two junctions. One between emitter and the base and other between the base and the collector. Because of this the transistor is similar to two diodes, one emitter diode and other collector base diode. When transistor is made, the diffusion of free electrons across the junction produces two depletion layers. For each of these depletion layers, the barrier potential is 0.7 V for Si transistor and 0.3 V for Ge transistor.

Transistor Operation Fig 5: P-N Diode in Unbiased Condition

Transistor Operation Fig 6: P-N Diode Forward Bias

Transistor Operation Fig 7: P-N Diode reversed bias

Transistor Operation Depletion region has been reduced in width due to the applied bias, resulting in heavy flow of majority carriers from p- to the n- type material.

Transistor Operation Flow of majority carrier is zero, resulting in only a minority- carrier flow, Fig 9: Reverse Bias Junction of a PNP Transistor

Transistor Operation Fig 10: Majority and Minority carrier flow of a PNP transistor

Transistor Operation Large number of majority carrier will diffuse across the forward biased p-n junction into the n- type material. Since n-type material is very thin, a very small number of carrier will take this path to the base terminal. Larger number of these majority carrier will diffuse across the reverse biased junction into the p-type material connected to the collector terminal. Why ??

Transistor Operation Because in reverse junction injected majority carrier will appear as minority carriers in n-type material. Applying Kirchhoff’s current law: Collector current comprises of majority and minority carriers. Minority current component is called leakage current ( Current with emitter terminal open). …….(1) Minority current is heat sensitive.

Common Base Configuration Driving Point or Input Parameter: For a fixed value of as the base to emitter voltage increases, the emitter current increases in a manner of the diode characteristics. Increasing level of has very small effect on the input characteristics. With the increase 𝑉 𝐶𝐵 of we see that curve shifts leftwards. Why??

Common Base Configuration Ignoring the effect of collector voltage and the slope of the curve we obtain a equivalent model for dc analysis. Here once a transistor is in the ‘on’ state the base to emitter voltage will be assumed to be,

Common Base Configuration Output Characteristics: It has three basic region of interest, such as: Active Region Cut-off Region Saturation Region.

Common Base Configuration In the active region the base-emitter junction is forward biased, whereas the collector-base junction is reversed biased. From the characteristics curve we can see, In the cut-off region the base-emitter and collector base junction are both reversed biased. In the saturation region both the base-emitter and collector base junctions are forward biased.

Common Base Configuration Now, Output characteristics will suggest =1.But practical value of  extends from 0.90 to 0.998.Here  is known as common base, short circuit amplification factor. From equation (1) we can write, When ,then

Amplification From the input and output characteristics curve we see that input resistance is quite small in compare to the output resistance. The difference in resistance is due to the forward biased junction at the input and reversed biased junction at output. See the example in book in fig. 3.12 In common base configuration voltage amplification varies from 50 to 300. Here basic amplification was produced by transferring current from low to a high resistance circuit.

Transfer + Resistor Transistor Why the name is transistor?? Transfer + Resistor Transistor

Transistor Transistor acts as both switch and amplifier. How????

Common Emitter Configuration Fig 13: PNP transistor

Common Emitter Configuration

Common Emitter Configuration Input Parameter: Here the input current 𝐼 𝐵 is small, as base is lightly doped in compare to input current produced in common base configuration.

Common Emitter Configuration Output Parameter: Fig 16: Output Characteristics

Common Emitter Configuration Output Parameter: In the active region of a common-emitter amplifier, base-emitter junction is forward biased and collector-base junction is reversed biased. Here although transistor configuration is changed, the same current relation 𝐼 𝐸 = 𝐼 𝐵 + 𝐼 𝐶 is maintained. But the curve obtained are not horizontal as compared to the curves in common base configuration.

Common Emitter Configuration When 𝐼 𝐵 =0, 𝐼 𝐶 >0 Why ?? Because here 𝐼 𝐶 = 𝐼 𝐶𝐵𝑂 1−∝ . If ∝=0.9996 then we obtain 𝐼 𝐶 =250 𝐼 𝐶𝐵𝑂 .So here we cannot consider 𝐼 𝐶 =0 , when 𝐼 𝐵 =0. Again as we increase 𝑉 𝐶𝐸 width of the base region decreases due to reverse biasing. As a result there is a lesser chance for recombination within the "smaller" base region. So collector current increases.

Common Collector Configuration

Common Collector Configuration Here the input terminal has high input impedance and output terminal has low output impedance. As a result it is used for impedance matching. It has low voltage gain and high current gain. It is mostly used as voltage buffer.

Common Collector Configuration Now, 𝑉 𝐵𝐸 = 𝑉 𝐶𝐸 − 𝑉 𝐶𝐵 Increasing the level of 𝑉 𝐶𝐵 with 𝑉 𝐶𝐸 held constant, reduces the level of 𝑉 𝐵𝐸 and thus reduces 𝐼 𝐵

Common Collector Configuration Output Characteristics: Fig 19: Output Characteristics of common collector

Comparison Parameters Common Base Common Emitter Common Collector Input/ Output Phase Relationship 0° 180° Voltage Gain High Medium Low Current Gain Input Resistance Output Resistance Note: Example 3.1, 3.2 in Boylsted Example 4.1, 4.2 in Bell Example 8.1-8.15 in V.K. Mehta