Lecturer, Dept. of EEE, KUET

Lecturer, Dept. of EEE, KUET
Semiconductor Devices and Circuits Presented by: Md. Shafiul Islam Lecturer, Dept. of EEE, KUET

Principle of electronics by V K Mehta
Semiconductor Devices and Circuits Reference Principle of electronics by V K Mehta

Outlines Semiconductor and it’s property Band diagram of semiconductor
Semiconductor doping Semiconductor devices Semiconductor circuits and applications

Semiconductor

Classification of materials
On the basis of electrical conductivity, materials can be classified into three types. Conductor Semiconductor (not good conductor but in between conductor and insulator) Insulator “Semiconductor can carry current under some condition”

Semiconductor “A semiconductor is a substance whose resistivity lies in between conductor and insulator”(according to electrical conductivity)

Properties of semiconductor
The resistivity of a semiconductor is less than an insulator but more than a conductor. Semiconductors have negative temperature co-efficient of resistance i.e. the resistance of a semiconductor decreases with the increase in temperature and vice-versa. For example, germanium is actually an insulator at low temperatures but it becomes a good conductor at high temperatures. When a suitable metallic impurity (e.g. arsenic, gallium etc.) is added to a semiconductor, its current conducting properties change appreciably.

Commonly used semiconductors
Germanium (32)

Commonly used semiconductors
Silicon (14)

Properties of semiconductor
A semiconductor is a substance which has almost filled valence band and nearly empty conduction band with a very small energy gap (1.1 eV) separating the two.

Energy band diagram

Classification of semiconductor
Intrinsic semiconductor Extrinsic semiconductor(this can be classified into two types) n-type semiconductor p- type semiconductor Intrinsic semiconductor: An extremely pure form of semiconductor is called intrinsic semiconductor.

Intrinsic semiconductor
An extremely pure form of semiconductor is called intrinsic semiconductor.

Why we need extrinsic semiconductor?
Because the conduction capability of intrinsic semiconductor is very small. But for useful purposes in semiconductor device we need to increase the amount of current. That’s means we need to increase the conductivity of the material Increase in conductivity means increase in carrier concentration. Carrier concentration can be altered in a semiconductor by adding impurity “The process of impurity addition to semiconductor is called doping”

Types of doping Two types n- type doping (n- negative type)
p- type doping (p- positive type) “Extrinsic semiconductor is obtained from the process of impurity adding to the intrinsic semiconductor to change the carrier concentration to change the electronic property of the semiconductor material, this process is named as doping”

n- type doping From n-type doping we obtain n-type semiconductor “When a small amount of pentavalent impurity is added to a pure semiconductor, it is known as n-type semiconductor” Impurities which produce n-type semiconductor are known as donor impurities because they donate or provide free electrons to the semiconductor crystal.

n- type doping The current conduction in an n-type semiconductor is predominantly by free electrons i.e. negative charges and is called n-type or electron type conductivity “So, majority carrier in n- type semiconductor is electron and minority carrier is hole”

p- type doping From p-type doping we obtain p-type semiconductor “When a small amount of trivalent impurity (In, Ga) is added to a pure semiconductor, it is called p-type semiconductor.” Such impurities which produce p-type semiconductor are known as acceptor impuritiesbecause the holes created can accept the electrons.

p- type doping The current conduction in an p-type semiconductor is predominantly by free electrons i.e. negative charges and is called n-type or electron type conductivity. “Majority carrier in p- type semiconductor is hole and minority carrier is electron”

Majority and minority carrier

Semiconductor devices (crystal diode, transistors) semiconductor device is nothing but a simple p-n junction or many p-n junctions

p-n junction When a pure semiconductor’s one side is doped with n-type doping and another side in doped with p-type doping a junction is formed between the n-type and p-type semiconductor is called p-n junction “p-n junction is important since the control element for semiconductor devices.” n-type semiconductor – majority carrier(electron), minority carrier (Hole) p- type semiconductor- majority carrier(hole), minority carrier (electron) The density of electrons and holes are high in n side and p side respectively

p-n junction The density of electrons and holes are high in n side and p side respectively. So electrons are diffused from n side and move towards p side and holes are diffused from p side and move towards n side. These electrons and holes recombine near the junction Since the density of electrons is decreased at n side, so positive charges appears near n side at the junction. And similarly density of holes is decreased at p side, so negative charges appears near the p side at the junction. This region of positive and negative charges at the junction is called depletion region. (depleted– emptied, no free charge carrier in this region)

p-n junction

p-n junction At equilibrium the depletion region acts as a barrier for further diffusion of electron and holes, stops the movement of electrons and holes. The positive and negative charges at this region set up electric filed. Direction of electric field is from positive charge to negative charge The potential difference in the depletion region is called, Built in potential or barrier potential. The value of built in potential, 𝑣 𝑜 in a Si p-n junction is 0.7 v And the value of built in potential, 𝑣 𝑜 in a Ge p-n junction is 0.3 v

p-n junction In electronics, the term bias refers to the use of d.c voltage to establish certain operating conditions. Two types of biasing Forward bias Reverse bias

Forward Biasing When external d.c. voltage applied to the junction is in such a direction that it cancels the potential barrier, thus permitting current flow, it is called forward biasing.

Reverse Biasing When the external d.c. voltage applied to the junction is in such a direction that potential barrier is increased, it is called reverse biasing. “In forward biasing the junction resistance decreases and in reverse biasing resistance increases” Breakdown voltage: It is the minimum reverse voltage at which pn junction breaks down with sudden rise in reverse current. Peak inverse voltage: It is the maximum reverse voltage that can be applied to the p-n junction without damage to the junction. PIV<Breakdown voltage

Crystal diode A pn junction is known as a semi-conductor or *crystal diode. Anode (+) cathode (-)

V-I characteristics of a p-n junction diode (crystal diode)

Crystal diode Rectifier
Rectification means converting AC signal into DC signal Crystal diode converts AC signal into pulsating DC signal Pulsating DC– Filtering– Constant DC Tow types of rectifier circuits Half wave rectifier Full wave rectifier

Half wave Rectifier(Clipper circuit)
In half-wave rectification, the rectifier conducts current only during the positive half-cycles of input ac supply. The negative half-cycles of ac supply are suppressed i.e. during negative half-cycles, no current is conducted and hence no voltage appears across the load. Therefore, current always flows in one direction (i.e. dc) through the load though after every half-cycle. “That’s why diode is a unidirectional device”

Half wave Rectifier(Clipper circuit)

Disadvantages of Half wave Rectifier
The main disadvantages of a half-wave rectifier are : (i) The pulsating current in the load contains alternating component whose basic frequency is equal to the supply frequency ( 𝑓 𝑜𝑢𝑡 = 𝑓 𝑖𝑛 ). Therefore, an elaborate filtering is required to produce steady direct current. (ii) The a.c. supply delivers power only half the time. Therefore, the output is low.

Efficiency of Half wave Rectifier
The ratio of d.c. power output to the applied input a.c. power is known as rectifier efficiency. 𝜂= 𝐷𝐶 𝑜𝑢𝑡𝑝𝑢𝑡 𝑝𝑜𝑤𝑒𝑟 𝑖𝑛𝑝𝑢𝑡 𝑎𝑐 𝑝𝑜𝑤𝑒𝑟 Diode conducts and supplies power to the load only during positive half cycle of the input ac supply. Output dc power is pulsating dc. So for output dc power we need to find the average load current.

Let voltage at the secondary side of the transformer, v= 𝑣 𝑚 sin 𝜃 Resistances of the diode and the load are 𝑟 𝑓 𝑎𝑛𝑑 𝑅 𝐿 So the average current, 𝐼 𝑎𝑣 = 1 2𝜋 0 𝜋 𝑣 𝑅 𝑑𝜃 = 1 2𝜋 0 𝜋 𝑣 𝑚 sin 𝜃 𝑅 𝐿 + 𝑟 𝑓 𝑑𝜃 = 𝑣 𝑚 2𝜋( 𝑅 𝐿 + 𝑟 𝑓 ) 0 𝜋 sin 𝜃 𝑑𝜃 = 𝑣 𝑚 2𝜋( 𝑅 𝐿 + 𝑟 𝑓 ) ∗2 = 𝑣 𝑚 𝜋( 𝑅 𝐿 + 𝑟 𝑓 ) = 𝐼 𝑚 𝜋 (ℎ𝑒𝑟𝑒, 𝐼 𝑚 = 𝑣 𝑚 𝑅 𝐿 + 𝑟 𝑓 )

So output dc power, 𝑃 𝑑𝑐 = 𝐼 𝑎𝑣 2 ∗ 𝑅 𝐿 = 𝐼 𝑚 𝜋 2 ∗ 𝑅 𝐿 Input ac power, 𝑃 𝑎𝑐 = 𝐼 𝑟𝑚𝑠 2 ∗ (𝑅 𝐿 + 𝑟 𝑓 )= 𝐼 𝑚 2 2 ∗ (𝑅 𝐿 + 𝑟 𝑓 ) 𝜂= 𝑃 𝑑𝑐 𝑃 𝑎𝑐 = 𝐼 𝑚 𝜋 2 ∗ 𝑅 𝐿 𝐼 𝑚 2 2 ∗ (𝑅 𝐿 + 𝑟 𝑓 ) = 𝑅 𝐿 (𝑅 𝐿 + 𝑟 𝑓 ) = 𝑟 𝑓 𝑅 𝐿 =.406×100%=40.6% 𝑟 𝑓 ≪ 𝑅 𝐿 , 𝑠𝑜,1+ 𝑟 𝑓 𝑅 𝐿 ≈1 Max. rectifier efficiency = 40.6% This shows that in half-wave rectification, a maximum of 40.6% of a.c. power is converted into d.c. power. ℎ𝑒𝑟𝑒, 𝐼 𝑟𝑚𝑠 𝑓𝑜𝑟 ℎ𝑎𝑙𝑓 𝑐𝑦𝑐𝑙𝑒 = × 1 2𝜋 0 2𝜋 𝑣 𝑚 𝑠𝑖𝑛𝜃 𝑅 𝐿 + 𝑟 𝑓 𝑑𝜃 = 𝐼 𝑚 2

Full wave Rectifier The following two circuits are commonly used for full-wave rectification: Centre-tap full-wave rectifier Full-wave bridge rectifier

Full wave bridge Rectifier
Working: During positive half cycle diode D1 and D3 are forward biased. D2 and D4 are reverse biased So current flows through D1, 𝑅 𝐿 , D3 and return back to the secondary winding of the transformer output voltage is obtained across the load, 𝑅 𝐿

Working: During negative half cycle diode D2 and D4 are forward biased. D1 and D3 are reverse biased So current flows through D2, 𝑅 𝐿 , D4 and return back to the secondary winding of the transformer and output voltage is obtained across the load, 𝑅 𝐿 . So we obtain power at load for both cycle of the input ac

Working: During negative half cycle diode D2 and D4 are forward biased. D1 and D3 are reverse biased So current flows through D2, 𝑅 𝐿 , D4 and return back to the secondary winding of the transformer and output voltage is obtained across the load, 𝑅 𝐿 . So we obtain power at load for both cycle of Input ac supply

Load Current path for positive and negative cycle of the input ac
Here, 𝑓 𝑜𝑢𝑡 =2 𝑓 𝑖𝑛

Peak Inverse Voltage The peak inverse voltage is the maximum voltage that can be applied across the diode during reverse biased condition. It must be less than the breakdown voltage of the diode. Here, the PIV of the diodes is 𝑉 𝑚 volt.

disadvantages Full wave bridge Rectifier
It requires four diodes As during each half-cycle of a.c. input two diodes that conduct are in series, therefore, voltage drop in the internal resistance of the rectifying unit will be twice as great as in the center tap circuit. This is objectionable when secondary voltage is small.

Efficiency of Full wave Rectifier
𝜂= 𝐷𝐶 𝑜𝑢𝑡𝑝𝑢𝑡 𝑝𝑜𝑤𝑒𝑟 𝑖𝑛𝑝𝑢𝑡 𝑎𝑐 𝑝𝑜𝑤𝑒𝑟 Diodes conduct and supplies power to the loads for full cycle of the input ac supply. Output dc power is pulsating dc. So for output dc power we need to find the average load current.

Efficiency of Full wave Rectifier
Let voltage at the secondary side of the transformer, v= 𝑣 𝑚 sin 𝜃 Resistances of the diode and the load are 𝑟 𝑓 𝑎𝑛𝑑 𝑅 𝐿 So the average current, 𝐼 𝑎𝑣 = 1 𝜋 0 𝜋 𝑣 𝑅 𝑑𝜃 = 1 𝜋 0 𝜋 𝑣 𝑚 sin 𝜃 𝑅 𝐿 + 𝑟 𝑓 𝑑𝜃 = 𝑣 𝑚 𝜋( 𝑅 𝐿 + 𝑟 𝑓 ) 0 𝜋 sin 𝜃 𝑑𝜃 = 𝑣 𝑚 𝜋( 𝑅 𝐿 + 𝑟 𝑓 ) ∗2 = 2𝑣 𝑚 𝜋( 𝑅 𝐿 + 𝑟 𝑓 ) = 2 𝐼 𝑚 𝜋 (ℎ𝑒𝑟𝑒, 𝐼 𝑚 = 𝑣 𝑚 𝑅 𝐿 + 𝑟 𝑓 )

Efficiency of full wave Rectifier
So output dc power, 𝑃 𝑑𝑐 = 𝐼 𝑎𝑣 2 ∗ 𝑅 𝐿 = 2𝐼 𝑚 𝜋 2 ∗ 𝑅 𝐿 Input ac power, 𝑃 𝑎𝑐 = 𝐼 𝑟𝑚𝑠 2 ∗ (𝑅 𝐿 + 𝑟 𝑓 )= 𝐼 𝑚 2 2 ∗ (𝑅 𝐿 + 𝑟 𝑓 ) 𝜂= 𝑃 𝑑𝑐 𝑃 𝑎𝑐 = 2𝐼 𝑚 𝜋 2 ∗ 𝑅 𝐿 𝐼 𝑚 2 2 ∗ (𝑅 𝐿 + 𝑟 𝑓 ) = 𝑅 𝐿 (𝑅 𝐿 + 𝑟 𝑓 ) = 𝑟 𝑓 𝑅 𝐿 =.812×100%=81.2% 𝑟 𝑓 ≪ 𝑅 𝐿 , 𝑠𝑜,1+ 𝑟 𝑓 𝑅 𝐿 ≈1 Max. rectifier efficiency =81.2% This is double the efficiency due to half-wave rectifier. Therefore, a full-wave rectifier is twice as effective as a half-wave rectifier. ℎ𝑒𝑟𝑒, 𝐼 𝑟𝑚𝑠 𝑓𝑜𝑟 𝑓𝑢𝑙𝑙 𝑐𝑦𝑐𝑙𝑒 = 𝜋 0 2𝜋 𝑣 𝑚 𝑠𝑖𝑛𝜃 𝑅 𝐿 + 𝑟 𝑓 𝑑𝜃 = 𝐼 𝑚 2

Transistor Transfer+ resistor “It transfers a current from a low resistance to high resistance”

Transistor A transistor consists of two p-n junctions formed by sandwiching either p-type or n- type semiconductor between a pair of opposite types. Accordingly ; there are two types of transistors, namely; n-p-n transistor p-n-p transistor A transistor has two p-n junctions. And three terminals, named Emitter, base and collector. And the middle section is a very thin layer.

Transistor A transistor has two p-n junctions. Three terminals. Among them BE is always forward biased and BC junction is reversed biased. So, One junction is forward biased and the other is reverse biased. The forward biased junction has a low resistance path whereas a reverse biased junction has a high resistance path. The weak signal is introduced in the low resistance circuit and output is taken from the high resistance circuit. Therefore, a transistor transfers a signal from a low resistance to high resistance.

Transistor Emitter: The section on one side that supplies charge carriers (electrons or holes) is called the emitter. The emitter is always forward biased w.r.t. base so that it can supply a large number of majority carriers. Collector: The section on the other side that collects the charges is called the collector. The collector is always reverse biased. Base: The middle section which forms two p-n junctions between the emitter and collector is called the base. The BE junction is forward biased, allowing low resistance for the emitter circuit. The BC junction is reverse biased and provides high resistance in the collector circuit.

Transistor Base is thinner than emitter
Collector is wider than both emitter and base Emitter is highly doped, base is lightly doped and collector is moderately doped Base is thinner and lightly dope to reduce the recombination of the carrier that is injected from emitter to base junction. Emitter is highly doped so that it can inject a large amount of majority carrier Collector is wider than both emitter and base. During transistor operation, much heat is produced at the collector junction. The collector is made larger to dissipate the heat

Transistor To operate transistor, it has two ports.
Input and output ports. Each port has two terminals So for two port we need 4 terminals But in transistor, we have only three terminals So one terminal is considered in both port (one terminal is common in both input and output) So there can be three configuration. Common Base configuration(input port BE, output port BC) Common emitter configuration (input port BE, output port CE) Common collector configuration (input port CE, output port CB)

Working principle of n-p-n Transistor(CB)
𝐼 𝐸 (100%)= 𝐼 𝐶 (95%)+ 𝐼 𝐵 (5%)

Symbols of n-p-n and p-n-p transistors

Why it is called bipolar junction transistors?
The current in this transistors, flows due to the flow of both electrons and holes. That’s why it is called bipolar junction transistors. Transistor is called current controlled device. 𝐼 𝐸 (100%)= 𝐼 𝐶 (95%)+ 𝐼 𝐵 (5%) Output current is controlled by controlling the input current, that’s why it is called current controlled device.

Transistor Circuit as an Amplifier
A transistor raises the strength of a weak signal and thus acts as an amplifier. The weak signal is applied between emitter-base junction and output is taken across the load 𝑅 𝐶 connected in the collector circuit. In order to achieve faithful amplification, the input circuit should always remain forward biased. As the input circuit has low resistance, therefore, a small change in signal voltage causes an appreciable change in emitter current. This causes almost the same change in collector current due to transistor action. The collector current flowing through a high load resistance RC produces a large voltage across it. Thus, a weak signal applied in the input circuit appears in the amplified form in the collector circuit. It is in this way that a transistor acts as an amplifier.

Transistor Circuit as an Amplifier
The reason is as follows. The collector-base junction is reversed biased and has a very high resistance of the order of mega ohms. Thus collector-base voltage has little effect on the collector current. This means that a large resistance 𝑅 𝐶 can be inserted in series with collector without disturbing the collector current relation to the emitter current viz. 𝐼 𝐶 = 𝛼 𝐼 𝐸 + 𝐼 𝐶𝐵𝑂 Therefore, collector current variations caused by a small base-emitter voltage fluctuations result in voltage changes in RC that are quite high—often hundreds of times larger than the emitter-base voltage.

Transistor circuit as an amplifier

Emitter Current amplification factor(α) in common base configuration
Common base means base is common to both input and output circuit So input current is 𝐼 𝐸 and output current, 𝐼 𝐶 .

Common base means base is common to both input and output circuit So input current is 𝐼 𝐸 and output current, 𝐼 𝐶 . The ratio of change in collector current to the change in emitter current at constant collector-base voltage 𝑽 𝑪𝑩 is known as current amplification factor. Then amplification factor 𝛼= ∆ 𝐼 𝐶 ∆ 𝐼 𝐸 ( 𝐼 𝐸 = 𝐼 𝐵 + 𝐼 𝐶 ) So 𝛼 is less than unity. Usually 0.9 to 0.99 Total collector current consists of : i)Part of emitter current which reaches the collector terminal i.e. ***α 𝐼 𝐸 ii) The leakage current 𝐼 𝑙𝑒𝑘𝑎𝑔𝑒 This current is due to the movement of minority carriers across base-collector junction on account of it being reverse biased. This is generally much smaller than α 𝐼 𝐸

So total collector current, 𝐼 𝐶 =𝛼 𝐼 𝐸 + 𝐼 𝑙𝑒𝑎𝑘𝑎𝑔𝑒 if 𝐼 𝐸 = 0 (i.e., emitter circuit is open), a small leakage current still flows in the collector circuit. This 𝐼 𝑙𝑒𝑎𝑘𝑎𝑔𝑒 is abbreviated as 𝐼 𝐶𝐵𝑂 , meaning collector-base current with emitter open. 𝐼 𝐶 =𝛼 𝐼 𝐸 + 𝐼 𝐶𝐵𝑂 𝐼 𝐸 = 𝐼 𝐵 + 𝐼 𝐶 𝐼 𝐶 =𝛼 (𝐼 𝐵 + 𝐼 𝐶 )+ 𝐼 𝐶𝐵𝑂 𝐼 𝐶 1−𝛼 =𝛼 𝐼 𝐵 + 𝐼 𝐶𝐵𝑂 𝐼 𝐶 = 𝛼 1−𝛼 𝐼 𝐵 + 𝐼 𝐶𝐵𝑂 1−𝛼

Input and output characteristics of common base configuration

Base Current amplification factor(𝜷) in common emitter configuration
Common emitter means emitter is common to both input and output circuit So input current is 𝐼 𝐵 and output current, 𝐼 𝐶 .

Base Current amplification factor(𝛽) in common emitter configuration
The ratio of change in collector current ( ∆𝐼 𝐶 ) to the change in base current ( ∆𝐼 𝐵 ) is known as emitter current amplification factor. 𝛽= ∆𝐼 𝐶 ∆𝐼 𝐵 Again, α= ∆𝐼 𝐶 ∆𝐼 𝐸 𝛽= ∆𝐼 𝐶 ∆𝐼 𝐸 × ∆𝐼 𝐸 ∆𝐼 𝐵 =𝛼× ∆ 𝐼 𝐶 +∆ 𝐼 𝐵 ∆ 𝐼 𝐵 =𝛼 ∆𝐼 𝐶 ∆𝐼 𝐵 +1 =𝛼(𝛽+1) 𝛽=𝛼𝛽+𝛼 𝛽 1−𝛼 =𝛼 𝛽= 𝛼 1−𝛼

𝛽= 𝛼 1−𝛼 If 𝛼 tends to 1 then 𝛽 tends to infinity. So current amplification factor is very high in common emitter configuration. That’s why it is used in about 90 to 95 percent of all transistor applications.

In common emitter circuit, 𝐼 𝐵 is the input current and 𝐼 𝐶 is the output current. We know that, 𝐼 𝐸 = 𝐼 𝐵 + 𝐼 𝐶 And 𝐼 𝐶 =𝛼 𝐼 𝐸 + 𝐼 𝐶𝐵𝑂 𝐼 𝐶 =𝛼( 𝐼 𝐵 + 𝐼 𝐶 )+ 𝐼 𝐶𝐵𝑂 𝐼 𝐶 =𝛼 𝐼 𝐵 + 𝛼𝐼 𝐶 + 𝐼 𝐶𝐵𝑂 (1− 𝛼)𝐼 𝐶 =𝛼 𝐼 𝐵 + 𝐼 𝐶𝐵𝑂 𝐼 𝐶 = 𝛼 1−𝛼 𝐼 𝐵 + 𝐼 𝐶𝐵𝑂 1−𝛼 𝐼 𝐶 =𝛽 𝐼 𝐵 + 𝐼 𝐶𝐵𝑂 1−𝛼 𝐼 𝐶 =𝛽 𝐼 𝐵 + 𝐼 𝐶𝐸𝑂 Where, 𝐼 𝐶𝐸𝑂 is the collector emitter current when base is open

base Current amplification factor(𝜸) in common collector configuration
The ratio of change in emitter current ( ∆𝐼 𝐸 ) to the change in base current ( ∆𝐼 𝐵 ) is known as emitter current amplification factor. 𝛾= ∆𝐼 𝐸 ∆𝐼 𝐵 Again, α= ∆𝐼 𝐶 ∆𝐼 𝐸 𝑎𝑛𝑑 𝛽= ∆𝐼 𝐶 ∆𝐼 𝐵 𝛾= ∆𝐼 𝐸 ∆𝐼 𝐶 × ∆𝐼 𝐶 ∆𝐼 𝐵 = 1 𝛼 ×𝛽 = 𝛼 1−𝛼 × 1 𝛼 𝛾= 1 1−𝛼

Comparison of transistor configuration

Transistor switching action
Switch has two state. On state and off state. To operate transistor as a switch transistor is needed to operate in cut-off region (switch off) and saturation region (switch on).

In cut-off region, both BE and BC junctions must be reversed biased. In saturation region, both of junctions must be forward biased.

In cut-off region, both BE and BC junctions must be reversed biased. In saturation region, both of junctions must be forward biased. For switch off, 𝑉 𝐵𝐸 ≤0 𝑎𝑛𝑑 𝑉 𝐵𝐶 ≤0 And for on, 𝑉 𝐵𝐸 >0 𝑎𝑛𝑑 𝑉 𝐵𝐶 >0 During cut off 𝐼 𝐵 =0 and output current 𝐼 𝐶 = 𝛽𝐼 𝐵 so output current is also 0. That means no current flows through collector emitter terminal. So CE junction is opened and 𝑉 𝐶𝐸 = 𝑉 𝐶𝐶 During saturation both BE and BC junctions are forward biased. So CE junction is also forward biased and ideally output voltage, 𝑉 𝐶𝐸 =0𝑉and output current becomes maximum and 𝐼 𝐶 = 𝐼 𝐶𝑠𝑎𝑡 . So in cut-off and saturation region transistor operate as switch off and switch on ,respectively.

Field Effect transistors (FET)
Why FET? Because, BJT has low input impedance and considerable noise level. But FET has very much higher input impedance and noise level is smaller than BJT. BJT is Bipolar junction transistor, here current is due to flow of both electrons and holes. But in FET current flows only either flow of electron or hole. That’s why it is unipolar transistor . In BJT output current is controlled by input current, that’s why it is called current controlled device. In FET output current is controlled by input voltage (electric field), that’s why it is called voltage controlled device.

Types of Field Effect transistors (FET)
Two basic types of FET. Junction field effect transistors (JFET) Metal oxide semiconductor field effect transistors (MOSFET)

Junction Field Effect transistors (JFET)
A junction field effect transistor is a three terminal semiconductor device in which current conduction is by one type of carrier i.e., electrons or holes. Two types, a) n- channel JFET b) p-channel JFET.

Gate must be reversed with respect to source. The input circuit (i.e. gate to source) of a JFET is reverse biased. This means that the device has high input impedance. The drain is so biased w.r.t. source that drain current flows from the drain to source. (iii) In all JFETs, source current 𝐼 𝑠 is equal to the drain current 𝐼 𝑠 i.e. 𝐼 𝑠 = 𝐼 𝐷

Operation of n-channel Junction Field Effect transistors (JFET)
𝑉 𝑔𝑠 =0V, And 𝑉 𝑑𝑠 >0𝑉

Pinch-off condition

𝑉 𝑔𝑠 <0𝑉 and 𝑉 𝑑𝑠 >0𝑉

The two p-n junctions at the sides form two depletion layers. The current conduction by charge carriers is through the channel between the two depletion layers and out of the drain. The width and hence resistance of this channel can be controlled by changing the input voltage 𝑉 𝐺𝑆 The greater the reverse voltage 𝑉 𝐺𝑆 , the wider will be the depletion layers and narrower will be the conducting channel. The narrower channel means greater resistance and hence source to drain current decreases. Reverse will happen should 𝑉 𝐺𝑆 decrease. Thus JFET operates on the principle that width and hence resistance of the conducting channel can be varied by changing the reverse voltage 𝑉 𝐺𝑆 .In other words, the magnitude of drain current can be changed by altering 𝑉 𝐺𝑆

Symbols of n-channel and p-channel JFET

Difference between JFET and BJT
See article 19.6 Vk Mehta Salient features of JFET, 19.9

Shorted gate drain current ( 𝑰 𝑫𝒔𝒔 ) and Pinch-off voltage( 𝑽 𝒑 )
It is the drain current with source short-circuited to gate (i.e. 𝑉 𝐺𝑆 = 0) and drain voltage ( 𝑉 𝐷𝑆 = 𝑉 𝑃 ) equal to pinch off voltage. It is sometimes called zero-bias current. Pinch off voltage: It is the minimum drain-source voltage at which the drain current essentially becomes constant.

Metal oxide semiconductor Field Effect transistors (MOSFET)
In JFET gate is always reversed biased w.r.t source. So we can only decrease the channel width (this is depletion mode). For n channel with negative gate voltage and for p channel with positive gate voltage. But there is another type of FET where we can enhance the width of the channel This is called MOSFET. A field effect transistor (FET) that can be operated in the enhancement-mode is called a MOSFET Two types Depletion type MOSFET (D-MOS) (can be operated in both depletion and enhancement mode) Enhancement type MOSFET (E-MOS) (only in enhancement mode)

n-channel depletion type MOSFET

Operation of n-channel D-MOSFET (Depletion mode)
i) 𝑉 𝑔𝑠 =0𝑉, 𝑉 𝑑𝑠 >0𝑉 ii) 𝑉 𝑔𝑠 <0𝑉, 𝑉 𝑑𝑠 >0𝑉

Operation of n-channel D-MOSFET (Enhancement mode)

for your kind attention
Thank you for your kind attention

Lecturer, Dept. of EEE, KUET

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