Heterojunction Bipolar Transistor

The first SiGe device to go into production was the HBT in 1999. The performance of a Si bipolar (BJT) can be enhanced by the addition of SiGe to the base region of the transistor. Two schemes exist to enhance performance, a rectangular Ge base profile (HBT) and a linearly graded base. The linear base provides a built in electric field which accelerates electrons across the base region. The band structure for the HBT and the linear grade transistors are shown on the right.

The demonstrated cutoff frequencies (fT - the frequency where the gain for the transistor equals 1) for HBTs has now reached 360 GHz and this is predicted to increase as technology progresses. There is a downside to this, however, in that the breakdown voltage between the collector and the emitter also decreases as fT is increased.

Freddy R Vallenilla R

16.791.006

EES SEC 2

The Schottky barrier diode

The Schottky diode or Schottky Barrier diode is an electronics component that is widely used for radio frequency (RF) applications as a mixer or detector diode. 

The Schottky diode is also used in power applications as a rectifier, again because of its low forward voltage drop leading to lower levels of power loss compared to ordinary PN junction diodes. Although normally called the Schottky diode these days, named after Schottky, it is also sometimes referred to as the surface barrier diode, hot carrier diode or even hot electron diode.

Despite the fact that Schottky barrier diodes have many applications in today's high tech electronics scene, it is actually one of the oldest semiconductor devices in existence. As a metal-semiconductor devices, its applications can be traced back to before 1900 where crystal detectors, cat's whisker detectors and the like were all effectively Schottky barrier diodes.

Structure

The Schottky barrier diode can be manufactured in a variety of forms. The most simple is the point contact diode where a metal wire is pressed against a clean semiconductor surface. This was how the early Cat's Whisker detectors were made, and they were found to be very unreliable, requiring frequent repositioning of the wire to ensure satisfactory operation. In fact the diode that is formed may either be a Schottky barrier diode or a standard PN junction dependent upon the way in which the wire and semiconductor meet and the resulting forming process.

Point contact Schottky diode

Although point contact diodes were manufactured many years later, these diodes were also unreliable and they were subsequently replaced by a technique in which metal was vacuum deposited.

Deposited metal Schottky barrier diode

One of the problems with the simple deposited metal diode is that breakdown effects are noticed around the edge of the metalised area. This arises from the high electric fields that are present around the edge of the plate. Leakage effects are also noticed. To overcome these problems a guard ring of P+ semiconductor fabricated using a diffusion process is used along with an oxide layer around the edge. In some instances metallic silicides may be used in place of the metal.

Deposited metal and oxide film Schottky diode

There are a number of points of interest from the fabrication process. The most critical element in the manufacturing process is to ensure a clean surface for an intimate contact of the metal with the semiconductor surface, and this is achieved chemically. The metal is normally deposited in a vacuum either by the use of evaporation or sputtering techniques. However in some instances chemical deposition is gaining some favour, and actual plating has been used although it is not generally controllable to the degree required.

When silicides are to be used instead of a pure metal contact, this is normally achieved by depositing the metal and then heat treating to give the silicide. This process has the advantage that the reaction uses the surface silicon, and the actual junction propagates below the surface, where the silicon will not have been exposed to any contaminants. A further advantage of the whole Schottky structure is that it can be fabricated using relatively low temperature techniques, and does not generally need the high temperature steps needed in impurity diffusion.

Characteristics

The Schottky diode is what is called a majority carrier device. This gives it tremendous advantages in terms of speed because it does not rely on holes or electrons recombining when they enter the opposite type of region as in the case of a conventional diode. By making the devices small the normal RC type time constants can be reduced, making these diodes an order of magnitude faster than the conventional PN diodes. This factor is the prime reason why they are so popular in radio frequency applications.

The diode also has a much higher current density than an ordinary PN junction. This means that forward voltage drops are lower making the diode ideal for use in power rectification applications.

Its main drawback is found in the level of its reverse current which is relatively high. For many uses this may not be a problem, but it is a factor which is worth watching when using it in more exacting applications.

The overall I-V characteristic is shown below. It can be seen that the Schottky diode has the typical forward semiconductor diode characteristic, but with a much lower turn on voltage. At high current levels it levels off and is limited by the series resistance or the maximum level of current injection. In the reverse direction breakdown occurs above a certain level. The mechanism is similar to the impact ionisation breakdown in a PN junction.

Applications

The Schottky barrier diodes are widely used in the electronics industry finding many uses as diode rectifier. Its unique properties enable it to be used in a number of applications where other diodes would not be able to provide the same level of performance. In particular it is used in areas including:

• RF mixer and detector diode
• Power rectifier
• Power OR circuits
• Solar cell applications
• Clamp diode - especially with its use in LS TTL

The use in each of these applications is slightly different, sometimes focussing on different properties of the diode. Accordingly they will be addressed separately.

Schottky diode as an RF mixer and detector diode

The Schottky diode has come into its own for radio frequency applications because of its high switching speed and high frequency capability. In view of this Schottky barrier diodes are used in many high performance diode ring mixers. In addition to this their low turn on voltage and high frequency capability and low capacitance make them ideal as RF detectors.

Schottky diode as a power rectifier diode

Schottky barrier diodes are also used in high power applications, as rectifiers. Their high current density and low forward voltage drop mean that less power is wasted than if ordinary PN junction diodes were used. This increase in efficiency means that less heat has to be dissipated, and smaller heat sinks may be able to be incorporated in the design.

Schottky diode in power OR circuits

Schottky diodes can be used in applications where a load is driven by two separate power supplies. One example may be a mains power supply and a battery supply. In these instances it is necessary that the power from one supply does not enter the other. This can be achieved using diodes. However it is important that any voltage drop across the diodes is minimised to ensure maximum efficiency. As in many other applications, the Schottky diode is ideal for this in view of its low forward voltage drop.

Schottky diodes tend to have a high reverse leakage current. This can lead to problems with any sensing circuits that may be in use. Leakage paths into high impedance circuits can give rise to false readings. This must therefore be accommodated in the circuit design.

Schottky diode in solar cell applications

Solar cells are typically connected to rechargeable batteries, often lead acid batteries because power may be required 24 hours a day and the Sun is not always available. Solar cells do not like the reverse charge applied and therefore a diode is required in series with the solar cells. Any voltage drop will result in a reduction in efficiency and therefore a low voltage drop diode is needed. As in other applications, the low voltage drop of the Schottky diode is particularly useful, and as a result Schottky diodes are normally used in this application.

Schottky diode as a clamp diode

Schottky barrier diodes may also be used as a clamp diode in a transistor circuit to speed the operation when used as a switch. They were used in this role in the 74LS (low power Schottky) and 74S (Schottky) families of logic circuits. Schottky barrier diodes are inserted between the collector and base of the driver transistor to act as a clamp. To produce a low or logic "0" output the transistor is driven hard on, and in this situation the base collector junction in the diode is forward biased. When the Schottky diode is present this takes most of the current and allows the turn off time of the transistor to be greatly reduced, thereby improving the speed of the circuit.

An NPN transistor with Schottky diode clamp

Schottky diode summary

Schottky barrier diodes are used in many areas of electronics because of the properties thay offer. As a result Schottky diodes are used as discrete components for RF and power applications as well as being incorporated within devices as protection devices or for charge removal in devices from photodiodes to MESFETs. Not only do Schottky barrier diodes find widespread use in many applications in its own right, but it an essential part of many other components as well.

Freddy R Vallenilla R

16.791.006

EES SEC 2

The PNP Transistor

The PNP Transistor is the exact opposite to the NPN Transistor device we looked at in the previous tutorial. Basically, in this type of transistor construction the two diodes are reversed with respect to the NPN type, with the arrow, which also defines the Emitter terminal this time pointing inwards in the transistor symbol. Also, all the polarities are reversed which means that PNP Transistors "sink" current as opposed to the NPN transistor which "sources" current. Then, PNP Transistors use a small output base current and a negative base voltage to control a much larger emitter-collector current. The construction of a PNP transistor consists of two P-type semiconductor materials either side of the N-type material as shown below.

A PNP Transistor Configuration

Note: Conventional current flow.

The PNP Transistor has very similar characteristics to their NPN bipolar cousins, except that the polarities (or biasing) of the current and voltage directions are reversed for any one of the possible three configurations looked at in the first tutorial, Common Base, Common Emitter and Common Collector. Generally, PNP Transistors require a negative (-ve) voltage at their Collector terminal with the flow of current through the emitter-collector terminals being Holes as opposed to Electrons for the NPN types. Because the movement of holes across the depletion layer tends to be slower than for electrons, PNP transistors are generally more slower than their equivalent NPN counterparts when operating.

To cause the Base current to flow in a PNP transistor the Base needs to be more negative than the Emitter (current must leave the base) by approx 0.7 volts for a silicon device or 0.3 volts for a germanium device with the formulas used to calculate the Base resistor, Base current or Collector current are the same as those used for an equivalent NPN transistor and is given as.

Generally, the PNP transistor can replace NPN transistors in electronic circuits, the only difference is the polarities of the voltages, and the directions of the current flow. PNP Transistors can also be used as switching devices and an example of a PNP transistor switch is shown below.

A PNP Transistor Circuit

The Output Characteristics Curves for a PNP transistor look very similar to those for an equivalent NPN transistor except that they are rotated by 180^o to take account of the reverse polarity voltages and currents, (the currents flowing out of the Base and Collector in a PNP transistor are negative).

Transistor Matching

You may think what is the point of having a PNP Transistor, when there are plenty of NPN Transistors available?. Well, having two different types of transistors PNP & NPN, can be an advantage when designing amplifier circuits such as Class B Amplifiers that use "Complementary" or "Matched Pair" transistors or for reversible H-Bridge motor control circuits. A pair of corresponding NPN and PNP transistors with near identical characteristics to each other are called Complementary Transistors for example, a TIP3055 (NPN), TIP2955 (PNP) are good examples of complementary or matched pair silicon power transistors. They have a DC current gain, Beta, (Ic / Ib) matched to within 10% and high Collector current of about 15A making them suitable for general motor control or robotic applications.

Identifying the PNP Transistor

We saw in the first tutorial of this Transistors section, that transistors are basically made up of two Diodes connected together back-to-back. We can use this analogy to determine whether a transistor is of the type PNP or NPN by testing its Resistance between the three different leads, Emitter, Base and Collector. By testing each pair of transistor leads in both directions will result in six tests in total with the expected resistance values in Ohm's given below.

• 1. Emitter-Base Terminals - The Emitter to Base should act like a normal diode and conduct one way only.
•  
• 2. Collector-Base Terminals - The Collector-Base junction should act like a normal diode and conduct one way only.
•  
• 3. Emitter-Collector Terminals - The Emitter-Collector should not conduct in either direction.

Transistor Resistance Values for the PNP transistor and NPN transistor types

Between Transistor Terminals		PNP	NPN
Collector	Emitter	RHIGH	RHIGH
Collector	Base	RLOW	RHIGH
Emitter	Collector	RHIGH	RHIGH
Emitter	Base	RLOW	RHIGH
Base	Collector	RHIGH	RLOW
Base	Emitter	RHIGH	RLOW

Freddy R Vallenilla R

16.791.006

EES SEC 2

The NPN Transistor

In the previous tutorial we saw that the standard Bipolar Transistor or BJT, comes in two basic forms. AnNPN (Negative-Positive-Negative) type and a PNP (Positive-Negative-Positive) type, with the most commonly used transistor type being the NPN Transistor. We also learnt that the transistor junctions can be biased in one of three different ways - Common Base, Common Emitter and Common Collector. In this tutorial we will look more closely at the "Common Emitter" configuration using NPN Transistors and an example of its current flow characteristics is given below.

An NPN Transistor Configuration

Note: Conventional current flow.

We know that the transistor is a "CURRENT" operated device and that a large current (Ic) flows freely through the device between the collector and the emitter terminals. However, this only happens when a small biasing current (Ib) is flowing into the base terminal of the transistor thus allowing the base to act as a sort of current control input. The ratio of these two currents (Ic/Ib) is called the DC Current Gain of the device and is given the symbol of hfe or nowadays Beta, (β). Beta has no units as it is a ratio. Also, the current gain from the emitter to the collector terminal, Ic/Ie, is called Alpha, (α), and is a function of the transistor itself. As the emitter current Ie is the product of a very small base current to a very large collector current the value of this parameter α is very close to unity, and for a typical low-power signal transistor this value ranges from about 0.950 to 0.999.

α and β Relationships

By combining the two parameters α and β we can produce two mathematical expressions that gives the relationship between the different currents flowing in the transistor.

The values of Beta vary from about 20 for high current power transistors to well over 1000 for high frequency low power type bipolar transistors. The equation for Beta can also be re-arranged to make Ic as the subject, and with zero base current (Ib = 0) the resultant collector current Ic will also be zero, (β x 0). Also when the base current is high the corresponding collector current will also be high resulting in the base current controlling the collector current. One of the most important properties of the Bipolar Junction Transistor is that a small base current can control a much larger collector current. Consider the following example.

Example No1.

An NPN Transistor has a DC current gain, (Beta) value of 200. Calculate the base current Ib required to switch a resistive load of 4mA.

Therefore, β = 200, Ic = 4mA and Ib = 20µA.

One other point to remember about NPN Transistors. The collector voltage, (Vc) must be greater than the emitter voltage, (Ve) to allow current to flow through the device between the collector-emitter junction. Also, there is a voltage drop between the base and the emitter terminal of about 0.7v for silicon devices as the input characteristics of an NPN Transistor are of a forward biased diode. Then the base voltage, (Vbe) of an NPN Transistor must be greater than this 0.7 V otherwise the transistor will not conduct with the base current given as.

Where:   Ib is the base current, Vb is the base bias voltage, Vbe is the base-emitter volt drop (0.7v) and Rbis the base input resistor.

Example No2.

An NPN Transistor has a DC base bias voltage, Vb of 10v and an input base resistor, Rb of 100kΩ. What will be the value of the base current into the transistor.

Therefore, Ib = 93µA.

The Common Emitter Configuration.

As well as being used as a switch to turn load currents "ON" or "OFF" by controlling the Base signal to the transistor, NPN Transistors can also be used to produce a circuit which will also amplify any small AC signal applied to its Base terminal. If a suitable DC "biasing" voltage is firstly applied to the transistors Base terminal thus allowing it to always operate within its linear active region, an inverting amplifier circuit called a Common Emitter Amplifier is produced.

One such Common Emitter Amplifier configuration is called a Class A Amplifier. A Class A Amplifier operation is one where the transistors Base terminal is biased in such a way that the transistor is always operating halfway between its cut-off and saturation points, thereby allowing the transistor amplifier to accurately reproduce the positive and negative halves of the AC input signal superimposed upon the DC Biasing voltage. Without this "Bias Voltage" only the positive half of the input waveform would be amplified. This type of amplifier has many applications but is commonly used in audio circuits such as pre-amplifier and power amplifier stages.

With reference to the common emitter configuration shown below, a family of curves known commonly as the Output Characteristics Curves, relates the output collector current, (Ic) to the collector voltage, (Vce) when different values of base current, (Ib) are applied to the transistor for transistors with the same βvalue. A DC "Load Line" can also be drawn onto the output characteristics curves to show all the possible operating points when different values of base current are applied. It is necessary to set the initial value ofVce correctly to allow the output voltage to vary both up and down when amplifying AC input signals and this is called setting the operating point or Quiescent Point, Q-point for short and this is shown below.

The Common Emitter Amplifier Circuit

Output Characteristics Curves for a Typical Bipolar Transistor

The most important factor to notice is the effect of Vce upon the collector current Ic when Vce is greater than about 1.0 volts. You can see that Ic is largely unaffected by changes in Vce above this value and instead it is almost entirely controlled by the base current, Ib. When this happens we can say then that the output circuit represents that of a "Constant Current Source". It can also be seen from the common emitter circuit above that the emitter current Ie is the sum of the collector current, Ic and the base current, Ib, added together so we can also say that " Ie = Ic + Ib " for the common emitter configuration.

By using the output characteristics curves in our example above and also Ohm´s Law, the current flowing through the load resistor, (RL), is equal to the collector current, Ic entering the transistor which inturn corresponds to the supply voltage, (Vcc) minus the voltage drop between the collector and the emitter terminals, (Vce) and is given as:

Also, a Load Line can be drawn directly onto the graph of curves above from the point of "Saturation" whenVce = 0 to the point of "Cut-off" when Ic = 0 giving us the "Operating" or Q-point of the transistor. These two points are calculated as:

Then, the collector or output characteristics curves for Common Emitter NPN Transistors can be used to predict the Collector current, Ic, when given Vce and the Base current, Ib. A Load Line can also be constructed onto the curves to determine a suitable Operating or Q-point which can be set by adjustment of the base current.

Freddy R Vallenilla R

16.791.006

EES SEC 2