Methods Of Biasing

Base Bias

 

The simplest form of biasing is to use a single resistor.  A single source provides the biasing requirements of both PN junctions.  The base of the transistor is returned to  via and the value of  determines the level of

This form of biasing is not as accurate or stable as the aforementioned potential divider bias. 


Feedback base bias

A better more stable form of biasing arrangement is shown below, is to connect the bias resistor  between the collector and base terminals of the transistor.

Now any increase in  increases the potential difference across , lowers the quiescent collector potential set by the biasing arrangement, decreasing  as before.  However, this time the fall is fed back to the base via , decreasing the forward bias on the base , decreasing  and ultimately decreasing , thus correcting the original rise owing to thermal runway.

This NEGATIVE FEEDBACK also occurs when an ac signal is applied to the base and causes a severe reduction in amplification.  As the example above shows, the value of the biasing resistor is dependent upon the gain of the transistor and this circuit does NOT stabilise the quiescent conditions against changes in transistor parameters (production spread).

Photo Diodes

The photodiode is another light-sensitive device which utilises a PN junction.  It is constructed in a manner similar to the photovoltaic cell, but it is used in basically the same way as the photoconductive cell.  In other words it is used essentially as a light-variable resistor.

The photodiode is a semiconductor device (usually made from silicon) and may be constructed in basically two ways.  One type of photodiode utilises a simple PN junction as shown below.  A P-type region is diffused into an N-type substrate as shown.  This diffusion takes place through a round window that is etched into a silicon dioxide layer that is formed on top of the N-type substrate.  Then a metal ring or window is formed over the silicon dioxide layer (through an evaporation process) as shown.  This window makes electrical contact with the P-type region and serves as an electrode to which an external lead can be attached.  however, the window also accurately controls the area that will receive or respond to light.  A metal base is then formed on the bottom N-type layer.  This metal layer serves as a second electrode to which another lead is attached

  

When used as a photovoltaic cell, the device is said to be operating in the photovoltaic mode and it will generate an output voltage (across its electrodes) that varies with the intensity of the light striking it’s P-type layer.  However, the photodiode is most commonly subjected to a reverse bias voltage as shown in Fig 1A.  In other words its P-type region is made negative with respect to it’s N-type region. Under these conditions a wide depletion region forms around the PN junction.  When photons enter this region to create electron-hole pairs, the separated electrons and holes are pulled in opposite directions because of the influence of the charges that exist on each side of the junction and the applied reverse bias.  The electrons are drawn toward the positive side of the bias source (the N-type region) and the holes are attracted toward the negative side of the bias voltage (the P-type region).  The separated electrons and holes therefore support a small current flow in the reverse direction through the photodiode.  As the light intensity increases, more photons produce more electron-hole pairs which further increase the conductivity of the photodiode resulting in a proportionally higher current.  When a photodiode is used in this manner it is said to be operating in the photoconductive or photocurrent mode.

PN junction and PIN photodiodes are often mounted on an insulative platform or substrate and sealed within a metal case as shown in Fig 2.  A glass window is provided at the top of the case, as shown, to allow light to enter and strike the photodiode.  The two leads extend through the insulative base at the bottom of the case and are internally bonded (with fine wires) to the photodiode’s electrodes.

Multi Segment LED

A further development of the standard LED package is the seven segment numerical indicator and the sixteen segment alpha-numeric indicator. In these devices, the PN junctions are elongated into a rectangular format and the light is emitted in a bar shape. The letter or number which a multi-segment display is required to produce is formed from a combination of illuminated segments.
Below shows the layout of the constituent light emitting diodes which are used in seven segment displays and  shows the layout for sixteen segment indicators. A sample of the letters or figures which may also be produced by some of the possible combinations of illuminated diodes is also illustrated.

Because these displays are composed of linear segments (that is, there are no curls or twists which can be produced), some anomalies could exist between similarly formed letters or numbers. Any combination which may introduce a misinterpretation is usually not specified in the equipment manual which covers the interpretation of the display.
As an example, the number 1 and the letter I could easily be read one for the other, and the distinction will be shown in the display dictionary.


The diode junctions which form the segments of the display require both an anode and a cathode connection. For a sixteen segment display this would result in thirty two connectors. Fortunately, most situations allow for a common connection to all the cathodes (or, alternatively, all the anodes). The displays are then referred to as common anode connected or common cathode connected and the number of leads reduces to eight or seventeen for the two types of display.

Light Emitting Diodes

Light emitting diodes are available in a wide variety of shapes and sizes, and they can be manufactured to fulfil a specific purpose or for general use. In size, they range from a miniature decimal point for use in multi-number configuration to a three-inch high single character display.


The A diagram shows the PN junction encapsulated in clear plastic with the light from the device focused by a lens.  The lens is usually made of self-colored plastic to match, or modify slightly, the colour being emitted by the junction.  The lens can be made to focus the light to a small pinpoint or to spread it over a wide area.

More detailed drawing of the PN wafer of the light emitting diode is shown in B, from this diagram, it can be seen that one face of the silicon is left clear of any obstructions or connecting wire which may impede the path of the light rays.  For a single indicator diode, the junction uses a square section of silicon.  During manufacture it is considerably easier to slice the silicon die into squares than it is to cut and trim complicated shapes; the desired outward appearance of the LED can be more easily obtained by shaping the lens of the device.  Thus, from a square section of silicon, plastic lens caps of square, rectangular, circular or star shape can achieve the shapes required for a particular indicator.

Hot Carrier Diode (HCD)

The hot carrier diode (HCD) is formed by placing an N-type semiconductor material (usually silicon) in contact with a metal such as gold, silver or aluminium to form a metal-to-semiconductor junction. This diode operates in a manner similar to ordinary PN junction diodes but there are several important differences. The barrier voltage developed within the device is approximately one half as great as the barrier voltage within an ordinary silicon diode. This means that the forward voltage drop across the diode is approximately 0.3 volts instead of 0.6 or 0.7 volts. Also, the HCD operates with majority carriers (electrons); virtually no minority carriers are involved. This means that the reverse or leakage current through the device is extremely small.

 The term hot carrier diode is used because the electrons move from the N-type semiconductor material cathode across the junction to the metallic anode (the forward-biased direction of current flow) in a manner similar to the movement of electrons through a vacuum tube diode.  In other words the electrons possess a high level of kinetic energy just like the electrons leaving the heated cathode of a vacuum tube.

The barrier voltage produced within the HCD is often referred to as the Schottky-barrier because the German scientist Schottky discovered the operating principle of the device in 1938.  For this reason the HCD is also commonly referred to as a Schottky-barrier diode or simply a Schottky-diode.

The HCD is able to change operating states (turn on and off) much faster than ordinary PN junction diodes, and it is used extensively to process high frequency AC signals.  This device finds extensive use in microwave electronic mixers (circuits which combine AC signals), detectors (circuits which use rectification as a means of extracting information from AC signals) , and high speed digital logic circuits

Varactor Diodes

When any PN junction diode is reverse-biased, majority carriers are swept away from the diode’s junction and a relatively wide depletion area is formed.  When the diode is subjected to varying reverse bias voltage, the width or thickness of this depletion region will also vary.  When the reverse bias voltage increases in value, the depletion region becomes wider.  When the reverse voltage decreases, the depletion region becomes narrower.  The depletion region acts like an insulator since it provides an area through which no conduction can take place.  It also effectively separates the N and P sections of the diode in the same way that a dielectric separates the two plates of a capacitor.  In fact, the entire PN junction diode is basically a small electronic capacitor that changes its capacitance as its depletion region changes in size.

Ordinary PN junction diodes possess only a small amount of internal junction capacitance and in most cases this capacitance is too small to be effectively used.  However, special diodes are constructed so that they have an appreciable amount of internal capacitance and are used in much the same way that ordinary capacitors are used in electronic circuits.  These special diodes are commonly referred to as VARACTOR DIODES or simply VARACTORS, or VARICAP on CAPACITANCE DIODES.

Rectifier Diode Characteristics And Parameters

The forward characteristics for silicon and germanium diodes are shown below. For forward voltages below about 0.6 v for silicon and 0.2 v for germanium the current is very small.  In the reverse bias condition the reverse saturation current (leakage current) is of the order of several nA to several mA.  Leakage current is very temperature-dependent and it is found that the reverse saturation current approximately doubles for every 10°C rise in temperature.

At the reverse breakdown voltage (or peak inverse voltage, PIV) the diode will break down, and unless the current is limited by a resistor the device will be damaged.  Diodes are not used on this part of the characteristic except in the case of zener diodes.