Zener diode,Varicap diodes (varactor diode),Tunnel diode,The transistor,Common base amplifier and Field effect transistor.

Zener diode

If a p-n junction is reverse biased, leakage current flows. Normally this current is negligible, but if the reverse voltage is increased, a breakdown voltage is reached where a large current flows. The breakdown is caused by two distinct mechanisms, both involving somewhat complex semiconductor physics:

(I) the avalanche effect. High velocity minority carriers travel through the depletion layer and dislodge valence electrons. The effect is cumulative, causing a sudden increase in current

(2) the zener effect. The diode is manufactured with a deliberate narrow depletion layer. The applied voltage can thus produce a large electric field across the layer which is sufficient to break down the covalent pairs.

These effects occur in normal diodes at voltages well in excess of the peak inverse voltage (PIV). Devices are manufactured, however, to exhibit these breakdowns at predictable low voltages. Regardless of the mechanism of breakdown, the devices are known as Zener diodes.

When breakdown occurs it is essential that current flow is limited. Usually this is done by the series resistor shown in (a). The slope of the VII curve at breakdown is very steep, and the circuit of (a) has an output impedance of a few ohms.

The current flowing through the zener produces heat, dissipation being given by:

Zener diodes are available for dissipations up to 5 W.

The characteristics of a zener diode make them a useful voltage reference for power supplies and other circuits. It should be noted, however, that the mechanisms of breakdown produce a 'noisy' voltage and care should be taken in the application of zener diodes inside amplifiers.

Varicap diodes (varactor diode)

It is mentioned earlier that the depletion layer in a reversed biased junction behaves as a capacitor. As the reverse voltage is increased, carriers are drawn away from the junction. This increases the depletion layer width and reduces the junction capacitance.

The varicap diode is specially designed to utilise this effect. A typical diode exhibits a change in capacitance from around 20 pF to 40 pF for a 5 V change in reverse bias.

Varicap diodes are widely used as tuning devices in LC circuits.

Most TV tuners now utilise varicap tuning.

Tunnel diode

The tunnel diode (sometimes called the Esaki diode, after its inventor) is a p-n junction in which the doping is very heavy. This results in a narrow depletion layer, and breakdown occurs without any external bias at all.

The forward characteristics of the diode are shown below. There are three distinct regions. In region I, breakdown is occurring and forward current is increasing. In region 2, the device comes out of

breakdown and exhibits negative resistance (falling I for increasing V). In region 3, the device is completely out of breakdown and behaves like a normal diode.

The useful portion of the characteristic is region 2, the negative resistance making the device useful as an oscillator or storage device.

The transistor

The junction transistor is a three-layer device consisting basically of two p-n junctions back to back. It may be pnp or npn in construction, as shown below. The three regions are called the base, collector and emitter.

For a transistor to operate, the base region must be made very thin and the doping of the emitter must be much heavier than that of the base region. The following diagram shows a pnp transistor biased correctly for operation (an npn transistor could be substituted if the voltages are reversed in polarity).

The emitter base junction is forward biased, and holes diffuse into the base region. These holes would normally exit via the base, but because of the narrowness of the base region they also come under the influence of the negatively charged collector. The holes pass into the collector assisted by the collector base potential, and collector current Ic flows.

A few holes do recombine with electrons in the base region to form a small base current lb. The ratio I,JI, is approximately constant, at about 0.98, and is sometimes referred to as a. Since Kirchoff's laws apply we can say that:

le=lc+lh

and it follows that the base current is about 0.02/e. Correctly, a is referred to as hFB.

Common base amplifier

Below is a practical circuit with a signal source Vin and a load RL. Resistor R 1 sets le and lh to suitable levels. This arrangement is known as the common base connection and is, in fact, the least used amplifier circuit.

The input impedance is low (around 30 !1) due to the forward biased base emitter junction. If RL is high (say 3 k!l) the voltage gain is:

The power gain is a2 (RdRin).

In all practical transistors the reverse biased collector base junction has a leakage current independent of any emitter current. This is denoted by /co' so the total collector current is represented by:

Common emitter amplifier

In the common base circuit the emitter is used as the input. A more usual arrangement (known as the common emitter amplifier) uses the base as the input. It has been shown earlier that if Ie changes by, say, 1 rnA, /b changes by 20 J.1A for an a of 0.98. It follows that if /b is changed by 20 J.IA, the emitter current changes by 1 rnA, because the basic relationship between the three currents /b, lc and /e holds regardless of the controlling element.

The current gain is given by lcflb. this is often denoted by /3. By simple analysis it can be seen that:

 

The characteristics of a typical common emitter amplifier are shown below. The output impedance is around 50 kQ, the input impedance around I kQ. Again there is voltage and power gain.

Leakage current presents a problem with common emitter amplifiers. The leakage current in the common emitter amplifier is denoted by Iceo• and has a typical value of 150 IJA. Effectively this is the collector base leakage current being treated as a base current i.e.

Leakage current is highly temperature-dependent and care needs to be taken in design of common emitter transistor amplifiers.

Practical details of transistor amplifiers are given elsewhere.

Field effect transistor

The conventional transistor described here has the disadvantage of being a low impedance device, and complex designs are needed to produce circuits with high input impedances.

The transistor is basically a current-operated device, but the conductivity of a semiconductor material can also be controlled by means of an electric field. A device using this principle is known as a field effect transistor (FET).

The simplest FET is shown diagrammatically below, in (a). The FET is a three-terminal device comprising a slice of n-type silicon (called the channel) with a p-n junction diffused into it.

The drain is biased positive to the source, so current flows from drain to source. If the gate is taken negative, however, a depletion layer is formed at the p-n junction. This depletion layer causes a decrease in the channel conductance and a decrease in the current Ict·

The conductance of the channel is thus controlled by the voltage on the gate; the more negative the gate, the less current flows. At all times the gate/channel junction is back-biased and negligible gate current flows.

If the gate voltage is taken further negative, the current Id ceases altogether. The gate voltage at which this occurs is called the pinch­ off voltage, and is typically around 5 V.

In many respects a FET resembles a thermionic valve, because itis a voltage-controlled device. Typical curves for a FET are shown in

(b).

The FET has a very simple equivalent circuit, as shown below. It consists of an input resistance and capacitance, a current generator and an output resistor. Typical values are:

It can be seen that, for all practical circuits, the input impedance is determined by Cin' and for reasonable values of RL the voltage gain is given by g L·

The above FET is known as ann-channel JUGFET. It operates in the depletion mode, so called because the gate voltage depletes the conduction of the channel. By using p-type material, a p-channel JUGFET is produced. This operates in a similar manner, except that all polarities are reversed.

An alternative method of producing a FET is the so-called insulated gate FET (or IGFET), also known as the metal oxide semiconductor FET (or MOSFET). In this type of FET the gate is insulated from the channel.

The construction of an IGFET is shown. Two n-type regions are diffused into a p-type substrate. On top of this, an insulating layer of silicon dioxide is grown. An aluminium gate is then evaporated on to the oxide layer.

The device has four terminals. With the gate and base shorted, the only current flowing between drain and source is a negligible leakage current. If the gate potential is taken positive with respect to the base, electrons are attracted to the surface of the base, enhancing the conductivity between the two n-type regions. Drain current can now flow, the current being controlled by the gate/base voltage. This circuit operates by the gate voltage being used to enhance the conductivity between source and drain, and is hence known as an n­ channel enhancement mode IGFET.

A depletion mode IGFET can be constructed as shown. A thin layer of n-type connects source and drain regions. The gate is taken negative with respect to the base, in a similar manner to the JUGFET, to control the source/drain current. This type of device is known as ann-channel depletion mode IGFET.

As might be expected, enhancement and depletion mode IGFETs can be made with p- and n-channel material. Circuit symbols for all six basic FET types are shown.