PHYS 3050 – Electronics I

Chapter 4. Semiconductor Diodes and Transistors

Dr. Jinjun Shan, Associate Professor of Space Engineering

Department of Earth and Space Science and EngineeringRoom 255, Petrie Science and Engineering Building

Tel: 416-736 2100 ext. 33854Email: jjshan@yorku.ca

Homepage: http://www.yorku.ca/jjshan

Earth, Moon, Mars, and Beyond

Semiconductor Diodes and Transistors 2

Introduction

In order to understand the extremely fast diode switching speeds or the offset voltage, we have to understand and view the diode as a pn junction.

This in turn requires an elementary understanding of electron and hole motion in semiconducting materials.

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Intrinsic Semiconductors

Semiconductor Diodes and Transistors 4

Intrinsic Semiconductors

It is interesting to observe that conduction is by electrons and by positively charged carries, called holes, which are created when a bond is broken and an electron is freed.

This is commonly referred to as production of a hole-electron pair.

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Intrinsic Semiconductors

This leaves behind a vacancy or a hole.

Another electron from some adjacent broken bond can jump into the hole and fill the vacancy, leaving a hole somewhere else.

This is commonly referred to as elimination of a hole-electron pair byrecombination.

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Intrinsic Semiconductors

Silicon conduction?

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Example:

Determine the conductivity for intrinsic silicon (Si) at room temperature (300 K).

Intrinsic Semiconductors

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Intrinsic Semiconductors

Conduction by holes and electrons in silicon

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Extrinsic Semiconductors

The ability to vary the conductivity of semiconducting material over a large range leads directly to many useful devices, including the diode and transistor.

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Extrinsic Semiconductors

There is a better way.

The conductivity of a semiconductor can be substantially increased by addingsome (typically 1 in 10 million) impurity atoms (called dopants) to the pure crystal structure.

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Semiconductor

Semiconductors materials such as silicon (Si), germanium (Ge), have electrical properties between those of a “conductor” and an “insulator”.

They are not good conductors nor good insulators (hence their name “semi”-conductors). They have very few “fee electrons” because their atoms are closely grouped together in a crystalline pattern called a “crystal lattice”.

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Semiconductor

However, their ability to conduct electricity can be greatly improved by adding certain “impurities” to this crystalline structure thereby, producing more free electrons than holes or vice versa.

By controlling the amount of impurities added to the semiconductor material it is possible to control its conductivity.

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Semiconductor

These impurities are called donors or acceptors depending on whether they produce electrons or holes respectively.

This process of adding impurity atoms to semiconductor atoms (the order of 1 impurity atom per 10 million (or more) atoms of the semiconductor) is called Doping.

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Semiconductor – Silicon

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Semiconductor – Germanium

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Semiconductor – Pure Silicon

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Semiconductor

As there are very few free electrons available to move around the silicon crystal, crystals of pure silicon are therefore good insulators, or very high value resistors.

A crystal of pure silica is generally said to be an intrinsic crystal (it has no impurities) and therefore has no free electrons.

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N-type Semiconductor

In order for silicon crystal to conduct electricity, we need to introduce an impurity atom such as Arsenic, Antimony or Phosphorus into the crystalline structure making it extrinsic.

These atoms have five outer electrons in their outermost orbital to share with neighbouring atoms and are commonly called “Pentavalent” impurities.

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N-type Semiconductor

This allows four out of the five orbital electrons to bond with its neighbouring silicon atoms leaving one “free electron” to become mobile when an electrical voltage is applied (electron flow).

As each impurity atom “donates” one electron, pentavalent atoms are generally known as “donors”.

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N-type Semiconductor

Antimony Atom and Doping

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N-type Semiconductor

A semiconductor material is classed as N-type when its donor density is greater than its acceptor density, in other words, it has more electrons than holes thereby creating a negative pole.

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P-type Semiconductor

If we go the other way, and introduce a “Trivalent” (3-electron) impurity into the crystalline structure, such as Aluminium, Boron or Indium, which have only three valence electrons available in their outermost orbital, the fourth closed bond cannot be formed.

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P-type Semiconductor

Therefore, a complete connection is not possible, giving the semiconductor material an abundance of positively charged carriers known as holes in the structure of the crystal where electrons are effectively missing.

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P-type Semiconductor

As there is now a hole in the silicon crystal, a neighbouring electron is attracted to it and will try to move into the hole to fill it.

However, the electron filling the hole leaves another hole behind it as it moves.

This in turn attracts another electron which in turn creates another hole behind it, and so forth giving the appearance that the holes are moving as a positive charge through the crystal structure.

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P-type Semiconductor

This movement of holes results in a shortage of electrons in the silicon turning the entire doped crystal into a positive pole.

As each impurity atom generates a hole, trivalent impurities are generally known as “Acceptors” as they are continually “accepting” extra or free electrons.

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P-type Semiconductor

Boron (symbol B) is commonly used as a trivalent additive as it has only five electrons arranged in three shells with the outermost orbital having only three electrons.

The doping of Boron atoms causes conduction to consist mainly of positive charge carriers resulting in a P-type material with the positive holes being called “Majority Carriers” while the free electrons are called “Minority Carriers”.

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P-type Semiconductor

Boron Atom and Doping

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P-type Semiconductor

Then a semiconductor basics material is classed as P-type when its acceptor density is greater than its donor density.

Therefore, a P-type semiconductor has more holes than electrons.

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Doping

Antimony (Sb) and Boron (B) are two of the most commonly used doping agents as they are more feely available compared to other types of materials.

However, the periodic table groups together a number of other chemical elements all with either three, or five electrons in their outermost orbital shell making them suitable as a doping material.

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Doping

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Conduction in Doped Semiconductors

The product of electrons and holes in doped silicon is equal to electron squared (or holes squared) in pure silicon.

Increasing the majority carriers by increasing the doping level will decrease the minority carriers proportionally.

We conclude that in doped silicon, conduction is primarily by the impurity carriers.

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Conduction in Doped Semiconductors

Conductivity for n-type semiconductors

Conductivity for p-type semiconductors

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Conduction in Doped Semiconductors

Example:

(a) Find the conductance of arsenic- and indium-doped silicon if the doping level is 1022 atoms/m3.

(b) Find the resistance of a cube of the above material if the cube measures 1 mm on a side.

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pn-junction

How about if we join (or fuse) N-type and P-type semiconductor materials together?

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pn-junction, diode-junction, the diode

The uncircled quantities (holes and electrons) are the free-charge carriers which, when in motion, constitute an electric current.

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pn-junction, diode-junction, the diode

Near the junction, the charge distribution is unstable and can exist only for a very brief time during manufacture of the junction.

The free charges on opposite sides of the junction will immediately combine.

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pn-junction, diode-junction, the diode

As a result, the charge density of the P-type along the junction is filled with negatively charged acceptor ions ( NA ), and the charge density of the N-type along the junction becomes positive.

This charge transfer of electrons and holes across the PN junction is known as diffusion.

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pn-junction, diode-junction, the diode

Since no free charge carriers can rest in a position where there is a potential barrier, the regions on either sides of the junction now become completely depleted of any more free carriers.

This area around the pn-junction is now called the Depletion Layer.

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pn-junction, diode-junction, the diode

Semiconductor Diodes and Transistors 40

pn-junction, diode-junction, the diode

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pn-junction, diode-junction, the diode

We now have four currents in the junction.

Majority current by holes and electrons

Minority current by holes and electrons

Fortunately, in most practical situations we can ignore drift current as being negligible.

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pn-junction, diode-junction, the diode

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pn-junction, diode-junction, the diode

We observe that the V-field increases when moving from the p-region to the n-region.

We have now obtained the potential jump V0 across the junction

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pn-junction, diode-junction, the diode

It should now be clear that this voltage is due to the internal electric field in the depletion zone.

The region near the junction is called a depletion region or a depletion zone since it is depleted of all free carriers.

In that sense, it is a nonconducting region – a thin insulating sheet between p and n halves.

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Forward Bias

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Forward Bias

Battery of voltage V across the pn-junction with the positive of battery on the p-side and the negative to the n-side.

The battery will inject holes into the p-region and electrons into the n-region.

This is referred to as forward-biasing a pn-junction.

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Reverse Bias

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Reverse Bias

Battery connection: plus goes to n-side and minus goes to p-side.

Electrons and holes to be repelled further from the junction, greatly increasing the depletion zone.

A reverse bias increases the contact potential to V0+V at the junction, thus increasing the barrier height for majority carriers.

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Reverse Bias

Under reverse bias, the diode is not an open circuit, but the small drift current, usually referred as reverse saturation current I0, is the only current present under reverse bias, gives the diode a finite but large resistance.

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Rectifier Equation

To derive a quantitative relationship for current in a pn-junction.

The equation is better known as the diode equation.

For reverse bias, a very small drift current, the reverse saturation current I0, flows across the junction, as the majority diffusion current is blocked by the reverse bias.

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Rectifier Equation

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Example:

If the reverse saturation current I0 for a silicon diode at room temperature is 10-12A.

(a) Find the current at biasing voltages of V = -0.1, 0.1, and 0.5 V.

(b) Should the temperature of the diode rise by 30oC, find the new currents for the same biasing voltages.

Rectifier Equation

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pn-Junction and The Transistor

Bipolar Junction Transistor (BJT)

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pn-Junction and The Transistor

Bipolar Junction Transistor (BJT) Bipolar because holes and electrons are

involved in its operation.

For most part, we ignore the contribution of the small minority current.

The input region is referred to as the emitter.

The center region is referred to as the base.

The output region is the collector.

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pn-Junction and The Transistor

Bipolar Junction Transistor (BJT) With this type of transistor, we have two

junctions with the input junction always forward-biased and the output junction always reverse-biased.

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pn-Junction and The Transistor

Differences in Diodes and Transistors

Diode

Transistor

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BJT

Base, Emitter and Collector Labeling

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BJT

Common base, Common collector or Common emitter (Emitter follower)? Common base?

Common emitter?

Common collector?

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BJT

Example: Common base, collector or emitter (emitter follower)?

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BJT

Example: Common base, collector or emitter (emitter follower)?

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pn-Junction and The Transistor

Grounded-Base Transistors (Common base)

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pn-Junction and The Transistor

Grounded-Base Transistors (Common base)

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pn-Junction and The Transistor

Grounded-Base Transistors (Common base)

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pn-Junction and The Transistor

Grounded-Emitter Transistors (Common emitter)

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pn-Junction and The Transistor

Grounded-Emitter Transistors (Common emitter)

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pn-Junction and The Transistor

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pn-Junction and The Transistor

Example:

Using the collector characteristics of the grounded-emitter transistor show in Fig. 4.7b in textbook, determine the current gain β for this transistor.

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pn-Junction and The Transistor

Example:

If β for a BJT is given as 150, find the emitter current Ie if the collector current Ic is given as 4 mA.

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pn-Junction and The Transistor

Field Effect Transistor (FET) They are simpler in concept but were

invented after the bipolar transistors.

Unlike BJT, which is a current amplifier.

The FET is basically a voltage amplifier,

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pn-Junction and The Transistor

Field Effect Transistor (FET) The ends of the N-type rod are referred to

as drain and source, and the ring as a gate.

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pn-Junction and The Transistor

Field Effect Transistor (FET)

Source of electrons

Drain of electrons

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pn-Junction and The Transistor

Field Effect Transistor (FET)

The direction of the arrow is the direction of current flow of a forward-biased junction.

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pn-Junction and The Transistor

Field Effect Transistor (FET) How much reverse voltage we should

apply before the gate and source cut the current in the channel?

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pn-Junction and The Transistor

Field Effect Transistor (FET)

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pn-Junction and The Transistor

Field Effect Transistor (FET)

Ohmic Region (Vds < Vov)

This is the region in which the FET acts as a variable resistor and obeys ohm’s law.

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pn-Junction and The Transistor

Field Effect Transistor (FET)

Saturation Region or Constant Current Region (Vov < Vds)

In this region, the curves become flat and horizontal. The transistor acts as a voltage-controlled current source.

Compared to BJT, which is a current-controlled current source.

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pn-Junction and The Transistor

Field Effect Transistor (FET)

BJT: current gain.

FET: transconductance gm.

gm = ∆Id/∆Vgs

The effectiveness of current control by the gate voltage is given by gm:

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pn-Junction and The Transistor

FET - Transfer Characteristics

Alternative to drain characteristics for describing the electrical properties of a FET.

drain current vs. gate voltage

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pn-Junction and The Transistor

FET - Transfer Characteristics

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pn-Junction and The Transistor

Other Types of FETS

Metal-oxide-semiconductor FET (MOSFET)

depletion-mode (DEMOSFET)

enhancement-mode

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The Transistor As Amplifier

An amplifier is a device which consists of interconnected transistors, resistors, inductors and capacitors.

Now we are ready to integrate the active and passive elements into an amplifying device.

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The Transistor As Amplifier

Elements of an Amplifier

Active element

Resistor

DC power supply

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The Transistor As Amplifier

Basic Design Considerations

How about if we replace the active element by a npn transistor?

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The Transistor As Amplifier

Basic Design Considerations

What is the correct voltage of the biasing battery?

A good design criterion is to choose a voltage for VEE that sets the output voltage at one-half of the battery voltage VB when Vs = 0.

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The Transistor As Amplifier

Basic Design Considerations

Common-base transistors, which have a low input impedance and good voltage gain but no current gain, are used as special-purpose amplifiers.

They are applicable when the driving source has an inherently low impedance and maximum power transfer is desired.

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The Transistor As Amplifier

The BJT as Amplifier

The common-emitter configuration is the most widely used for amplifiers as it combine high-gain with a moderately high input impedance.

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The Transistor As Amplifier

The BJT as Amplifier To choose a good transistor operation,

we need to choose battery voltage, load resistor, load line and Q-point.

Good transistor operation can be defined as optimal use of the operating region.

How to use this area optimally??

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The Transistor As Amplifier

DC Self-Bias Design and Thermal Runaway Protection

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The Transistor As Amplifier

Fixed-Current Bias If stabilization and drift of the Q-point

are not of primary importance, a much simpler biasing circuit that injects the correct amount of base current into the transistor for a desired Q-point may suffice.

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The Transistor As Amplifier

The FET as Amplifier

The design of a FET amplifier is similar to that for the BJT.

After picking a transistor with the desired characteristics, the DC design is carried out next.

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The Transistor As Amplifier

The FET as Amplifier

This involves choosing a suitable battery or DC power supply voltage, choosing a suitable load resistance which will determine the load line, and finally designing a biasing circuit to give a suitable Q-point.

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The Transistor As Amplifier

The FET as Amplifier

To avoid complicated algebra due to nonlinearity, we can use transfer characteristics in addition to the output characteristics to set the Q-point graphically.

A more practical technique is simply an approximate “cut-and-try”, which makes the design of the operating point for FET not complicated.

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The Transistor As Amplifier

Graphical Method

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The Transistor As Amplifier

Approximate Method

What is a good design?

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The Transistor As Amplifier

Example: Design the DC bias circuit that would fix the Q-point at Vgs = -0.6 V, when VDD = 10 V and RL+Rs = 2.5 KΩ.