Bipolar Junction Transistors (BJTBJT)

EBB424E

Dr. Sabar D. HutagalungSchool of Materials & Mineral Resources Engineering, Universiti Sains Malaysia

Transistors Two main categories of transistors:

bipolar junction transistors (BJTs) and field effect transistors (FETs).

Transistors have 3 terminals where the application of current (BJT) or voltage (FET) to the input terminal increases the amount of charge in the active region.

The physics of "transistor action" is quite different for the BJT and FET.

In analog circuits, transistors are used in amplifiers and linear regulated power supplies.

In digital circuits they function as electrical switches, including logic gates, random access memory (RAM), and microprocessors.

The First Transistor: Point-contact transistor

A point-contact transistor was the first type of solid state electronic transistor ever constructed.

It was made by researchers John Bardeen & Walter Houser Brattain at Bell Laboratories in December 1947.

The point-contact transistor was commercialized and sold by Western Electric and others but was rather quickly superseded by the junction transistor.

The Junction Transistor

First BJT was invented early in 1948, only weeks after the point contact transistor.

Initially known simply as the junction transistor.

It did not become practical until the early 1950s. The term “bipolar” was tagged onto the name to

distinguish the fact that both carrier types play important roles in the operation.

Field Effect Transistors (FETs) are “unipolar” transistors since their operation depends primarily on a single carrier type.

Bipolar Junction Transistors (BJT) A bipolar transistor

essentially consists of a pair of PN Junction diodes that are joined back-to-back.

There are therefore two kinds of BJT, the NPN and PNP varieties.

The three layers of the sandwich are conventionally called the Collector, Base, and Emitter.

The First BJT

Transistor Size (3/8”L X 5/32”W X 7/32”H)No Date Codes. No Packaging.

Modern Transistors

BJT Fabrication

BJT can be made either as discrete devices or in planar integrated form.

In discrete, the substrate can be used for one connection, typically the collector.

In integrated version, all 3 contacts appear on the top surface. The E-B diode is closer to the surface

than the B-C junction because it is easier make the havier doping at the top.

BJT Structure - Discrete

Early BJTs were fabricated using alloying - an complicated and unreliable process.

The structure contains two p-n diodes, one between the base and the emitter, and one between the base and the collector.

BJT Structure - Planar

In the planar process, all steps are performed from the surface of the wafer

The “Planar Structure” developed by Fairchild in the late 50s shaped the basic structure of the BJT, even up to the present day.

BJTs are usually constructed vertically Controlling depth of the emitter’s n doping sets the

base width

np

n

E B C

Advanced BJT Structures The original BJT structure survived, practically

unchanged, since the mid 60’s. As the advances in MOS development appears,

some of the fabrication technology are also applied to the BJT. Low defect epitaxy Ion implant Plasma etching (dry etch) LOCOS (local oxidation of Si) Polysilicon layers Improved lithography

Isolation Methods

The most significant advances in reducing overall device size and packing density have come from improved isolation methods.

The traditional junction isolation technique requires the p+ deep diffusion to be aligned to the n+ buried layer that is covered by a thick epitaxial layer.

The area (and hence junction capacitance) is determined by alignment tolerance, area for side diffusion, and allowance for the spread of the depletion region.

Modern isolation techniques: oxide isolation, and trench isolation.

Oxide & Trench Isolation

Oxide isolation processes were intorduced in the late 70’s. They utilize wet anisotropic etch (KOH) of the <100> Si wafer with Si3N4 as mask.

The KOH etch will erode the <111> plane. Oxide is either deposited or grown to fill the V-grooves.

The base and emitter are formed on the large mesa and the collector on the small mesa.

To further reduce the area between adjacent mesa, trench isolation can be used, making use of trench etching.

The trench is typically 2µm wide and 5µm deep. The trench walls are oxidized and the remaining volume is filled with polysilicon.

Double Poly Transistors A further extension of the self-aligned BJT structure is to use

double polysilicon (n+ for emitter, p+ for base) to reduce the area required for contacts.

Example of BJT Specification Sheet

How the BJT works Figure shows the energy

levels in an NPN transistor under no externally applying voltages.

In each of the N-type layers conduction can take place by the free movement of electrons in the conduction band.

In the P-type (filling) layer conduction can take place by the movement of the free holes in the valence band.

However, in the absence of any externally applied electric field, we find that depletion zones form at both PN-Junctions, so no charge wants to move from one layer to another.

NPN Bipolar Transistor

How the BJT works What happens when we

apply a moderate voltage between the collector and base parts.

The polarity of the applied voltage is chosen to increase the force pulling the N-type electrons and P-type holes apart.

This widens the depletion zone between the collector and base and so no current will flow.

In effect we have reverse-biassed the Base-Collector diode junction.

Apply a Collector-Base voltage

Charge Flow What happens when we apply a

relatively small Emitter-Base voltage whose polarity is designed to forward-bias the Emitter-Base junction.

This 'pushes' electrons from the Emitter into the Base region and sets up a current flow across the Emitter-Base boundary.

Once the electrons have managed to get into the Base region they can respond to the attractive force from the positively-biassed Collector region.

As a result the electrons which get into the Base move swiftly towards the Collector and cross into the Collector region.

Hence a Emitter-Collector current magnitude is set by the chosen Emitter-Base voltage applied.

Hence an external current flowing in the circuit.

Apply an Emitter-Base voltage

Charge Flow Some of free electrons crossing the Base encounter a hole and 'drop into it'.

As a result, the Base region loses one of its positive charges (holes).

The Base potential would become more negative (because of the removal of the holes) until it was negative enough to repel any more electrons from crossing the Emitter-Base junction.

The current flow would then stop.Some electron fall into a hole

Charge Flow To prevent this happening we use the applied E-B voltage to remove the captured electrons from the base and maintain the number of holes.

The effect, some of the electrons which enter the transistor via the Emitter emerging again from the Base rather than the Collector.

For most practical BJT only about 1% of the free electrons which try to cross Base region get caught in this way.

Hence a Base current, IB, which is typically around one hundred times smaller than the Emitter current, IE.

Some electron fall into a hole

Terminals & Operations

Three terminals: Base (B): very thin and lightly doped central region (little

recombination). Emitter (E) and collector (C) are two outer regions

sandwiching B. Normal operation (linear or active region):

B-E junction forward biased; B-C junction reverse biased. The emitter emits (injects) majority charge into base region

and because the base very thin, most will ultimately reach the collector.

The emitter is highly doped while the collector is lightly doped.

The collector is usually at higher voltage than the emitter.

Terminals & Operations

Operation Mode

Operation Mode

Active: Most importance mode, e.g. for amplifier operation. The region where current curves are practically flat.

Saturation: Barrier potential of the junctions cancel each other out

causing a virtual short. Ideal transistor behaves like a closed switch.

Cutoff: Current reduced to zero Ideal transistor behaves like an open switch.

Operation Mode

BJT in Active Mode

Operation Forward bias of EBJ injects electrons from emitter into base

(small number of holes injected from base into emitter) Most electrons shoot through the base into the collector across

the reverse bias junction (think about band diagram) Some electrons recombine with majority carrier in (P-type) base

region

Circuit Symbols

Circuit Configuration

Band Diagrams (In equilibrium)

No current flow Back-to-back PN diodes

Ec

Ev

Ef

Emitter Base Collector

N P N

Band Diagrams (Active Mode)

EBJ forward biased Barrier reduced and so electrons diffuse into the base Electrons get swept across the base into the collector

CBJ reverse biased Electrons roll down the hill (high E-field)

Ec

Ev

Ef

Emitter Base Collector

N P N

Minority Carrier Concentration Profiles

Current dominated by electrons from emitter to base (by design) b/c of the forward bias and minority carrier concentration gradient (diffusion) through the base some recombination causes bowing of electron concentration (in the base) base is designed to be fairly short (minimize recombination) emitter is heavily (sometimes degenerately) doped and base is lightly doped

Drift currents are usually small and neglected

Diffusion Current Through the Base

Diffusion of electrons through the base is set by concentration profile at the EBJ

Diffusion current of electrons through the base is (assuming an ideal straight line case):

Due to recombination in the base, the current at the EBJ and current at the CBJ are not equal and differ by a base current

Collector Current Electrons that diffuse across the base to the CBJ junction are swept across

the CBJ depletion region to the collector b/c of the higher potential applied to the collector.

Note that iC is independent of vCB (potential bias across CBJ) ideally Saturation current is

inversely proportional to W and directly proportional to AE

Want short base and large emitter area for high currents dependent on temperature due to ni

2 term

Collector Current Electrons that diffuse across the base to the CBJ junction are swept across

the CBJ depletion region to the collector b/c of the higher potential applied to the collector.

Note that iC is independent of vCB (potential bias across CBJ) ideally Saturation current is

inversely proportional to W and directly proportional to AE

Want short base and large emitter area for high currents dependent on temperature due to ni

2 term

Collector Current Electrons that diffuse across the base to the CBJ junction are swept across

the CBJ depletion region to the collector b/c of the higher potential applied to the collector.

Note that iC is independent of vCB (potential bias across CBJ) ideally Saturation current is

inversely proportional to W and directly proportional to AE

Want short base and large emitter area for high currents dependent on temperature due to ni

2 term

Base Current Base current iB composed of two components:

holes injected from the base region into the emitter region

holes supplied due to recombination in the base with diffusing electrons and depends on minority carrier lifetime b in the base

And the Q in the base is

So, current is

Total base current is

Beta

Can relate iB and iC by the following equation

and is

Beta is constant for a particular transistor On the order of 100-200 in modern devices (but can be higher) Called the common-emitter current gain

For high current gain, want small W, low NA, high ND

Emitter Current

Emitter current is the sum of iC and iB

is called the common-base current gain

I-V Characteristics

Collector current vs. vCB shows the BJT looks like a current source (ideally) Plot only shows values where BCJ is reverse biased and so BJT

in active region

However, real BJTs have non-ideal effects

VCE

IC

VBE1

VBE2

VBE3

VBE3 > VBE2 > VBE1

VBE

IC

VCE

I-V Characteristics

Base-emitter junction looks like a forward biased diode

Collector-emitter is a family ofcurves which are a function ofbase current.

I-V Characteristics

Example:

Calculate the values of β and α from the transistor shown in the previous graphs.

Early Effect

Early Effect Current in active region depends (slightly) on vCE

VA is a parameter for the BJT (50 to 100) and called the Early voltage Due to a decrease in effective base width W as reverse bias increases Account for Early effect with additional term in collector current equation Nonzero slope means the output resistance is NOT infinite, but…

IC is collector current at the boundary of active region

VCE

VBE1

VBE2

VBE3Active region

Saturation region

-VA

Early Effect What causes the Early Effect?

Increasing VCB causes depletion region of CBJ to grow and so the effective base width decreases (base-width modulation)

Shorter effective base width higher dn/dx

EBJ CBJ

dn/dxVCB > VCB

Wbase

Common-emitter

It is called the common-emitter configuration because (ignoring the power supply battery) both the signal source and the load share the emitter lead as a common connection point.

Common-collector

It is called the common-collector configuration because both the signal source and the load share the collector lead as a common connection point. Also called an emitter follower since its output is taken from the emitter resistor, is useful as an impedance matching device since its input impedance is much higher than its output impedance.

Common-base

This configuration is more complex than the other two, and is less common due to its strange operating characteristics.

Used for high frequency applications because the base separates the input and output, minimizing oscillations at high frequency. It has a high voltage gain, relatively low input impedance and high output impedance compared to the common collector.

Collector Resistance, rC

Emitter Resistance, rE

Base Resistance, rB

Mainly effects small-signal and transient responses.

Difficult to measure since it depends on bias condition and is influenced by rE.

In the Ebers-Moll model (SPICE’s default model for BJTs), rB is assumed to be constant.

Breakdown Voltages

The basic limitation of the max. voltage in a transistor is the same as that in a pn junction diode.

However, the voltage breakdown depends not only on the nature of the junction involved but also on the external circuit arrangement.

In Common Base configuration, the maximum voltage between the collector and base with the emitter open, BVCBO is determined by the avalanche breakdown voltage of the CBJ.

In Common Emitter configuration, the maximum voltage between the collect and emitter with the base open, BVCEO can be much smaller than BVCBO.

Breakdown Voltages

Breakdown Voltages

Breakdown Voltages

BJT Analysis

Here is a common emitter BJT amplifier:

What are the steps?

Input & Output

We would want to know the collector current (iC), collector-emitter voltage (VCE), and the voltage across RC.

To get this we need to fine the base current (iB) and the base-emitter voltage (VBE).

Input Equation

To start, let’s write Kirchoff’s voltage law (KVL) around the base circuit.

Output Equation

Likewise, we can write KVL around the collector circuit.

Use Superposition: DC & AC sources Note that both equations are written so as to calculate the

transistor parameters (i.e., base current, base-emitter voltage, collector current, and the collector-emitter voltage) for both the DC signal and the AC signal sources.

Use superposition, calculate the parameters for each separately, and add up the results: First, the DC analysis to calculate the DC Q-point

Short Circuit any AC voltage sources Open Circuit any AC current sources

Next, the AC analysis to calculate gains of the amplifier. Depends on how we perform AC analysis

Graphical Method Equivalent circuit method for small AC signals

BJT - DC Analysis

Using KVL for the input and output circuits and the transistor characteristics, the following steps apply:1. Draw the load lines on the transistor characteristics2. For the input characteristics determine the Q point for

the input circuit from the intersection of the load line and the characteristic curve (Note that some transistor do not need an input characteristic curve.)

3. From the output characteristics, find the intersection of the load line and characteristic curve determined from the Q point found in step 2, determine the Q point for the output circuit.

Base-Emitter Circuit Q point

The Load Line intersects the Base-emitter characteristics at VBEQ = 0.6 V and IBQ = 20 µA

Collector-Emitter Circuit Q point

Now that we have the Q-point for the base circuit, let’s proceed to the collector circuit.

The Load Line intersects the Collector-emitter characteristic, iB = 20 µA at VCEQ = 5.9 V and ICQ = 2.5mA, then β = 2.5m/20 µ = 125

BJT DC Analysis - Summary Calculating the Q-point for BJT is the first step in

analyzing the circuit To summarize:

We ignored the AC (variable) source Short circuit the voltage sources Open Circuit the current sources

We applied KVL to the base-emitter circuit and using load line analysis on the base-emitter characteristics, we obtained the base current Q-point

We then applied KVL to the collector-emitter circuit and using load line analysis on the collector-emitter characteristics, we obtained the collector current and voltage Q-point

This process is also called DC Analysis We now proceed to perform AC Analysis

BJT - AC Analysis

How do we handle the variable source Vin(t) ?

When the variations of Vin(t) are large we will use the base-emitter and collector-emitter characteristics using a similar graphical technique as we did for obtaining the Q-point.

When the variations of Vin(t) are small we will shortly use a linear approach using the BJT small signal equivalent circuit.

BJT - AC Analysis

Let’s assume that Vin(t) = 0.2 sin(ωt). Then the voltage sources at the base vary from a

maximum of 1.6 + 0.2 = 1.8 V to a minimum of 1.6 -0.2 = 1.4 V

We can then draw two “load lines” corresponding the maximum and minimum values of the input sources

The current intercepts then become for the: Maximum value: 1.8 / 50k = 36 µA Minimum value: 1.4 / 50k = 28 µA

AC Analysis Base-Emitter Circuit

From this graph, we find:At Maximum Input Voltage: VBE = 0.63 V, iB = 24 µAAt Minimum Input Voltage: VBE = 0.59 V, iB = 15 µARecall: At Q-point: VBE = 0.6 V, iB = 20 µA

Note the asymmetry around the Q-point of the Max and Min Values for the base current and voltage which is due to the non-linearity of the base-emitter characteristics

∆iΒmax = 24-20 = 4 µA; ∆iBmin = 20-15 = 5 µA

AC Analysis Base-Emitter Circuit

AC Characteristics-Collector Circuit

Using these max and min values for the base current on the collect circuit load line, we find:At Max Input Voltage: VCE = 5 V, iC = 2.7mAAt Min Input Voltage: VCE = 7 V, iC = 1.9mARecall: At Q-point: VCE = 5.9 V, iB = 2.5ma

AC Characteristics-Collector Circuit

BJT AC Analysis - Amplifier Gains

From the values calculated from the base and collector circuits we can calculate the amplifier gains:

BJT AC Analysis - Summary

Once we complete DC analysis, we analyze the circuit from an AC point of view.

AC analysis can be performed via a graphical processes Find the maximum and minimum values of the input

parameters (e.g., base current for a BJT) Use the transistor characteristics to calculate the output

parameters (e.g., collector current for a BJT). Calculate the gains for the amplifier

The pnp Transistor

Basically, the pnp transistor is similar to the npn except the parameters have the opposite sign. The collector and base currents flows out of the

transistor; while the emitter current flows into the transistor

The base-emitter and collector-emitter voltages are negative

Otherwise the analysis is identical to the npn transistor.

Current flow in a pnp transistor biased to operate in the active mode.

The PNP Transistor

The pnp Transistor

Two junctions Collector-Base and Emitter-Base

Biasing vBE Forward Biased

vCB Reverse Biased

pn

(0)pn

(x )

pn o

W E B W B CW B

np o

np

(0)

E

ICIE

I B

xB

V C BV E B

np(x )

E

(b)

C

p + n p

(a)

E m itter B ase C ollector

H o l ed r i f t

R e c o b i n a t i o n

E l e c t r o n s

E lec tronD i ff u s io n

L e akag e c urre nt

IB

(d)

ICIE

BE C

H ole d i ffus ion

E

ICIE

I B

V C BV EB

O u tp u tc irc u it

In p u tc irc u it

pnpE C

B(c)

(a) A schematic illustration of pnp BJT with 3 differently doped regions. (b) The pnp bipolar operated under normal and active conditions. (c) The CB configuration with input and output circuits identified. (d) The illustration of various current component under normal and active conditions.

Current flow in an pnp transistor biased to operate in the active mode.

The pnp Transistor

Two large-signal models for the pnp transistor operating in the active mode.

The pnp Transistor