60CHAPTER 2. SOLID-STATE DEVICE THEORYpolarity. This reduces the series resistance of the diode. The N− region is lightly doped asindicated by the (-). Light doping produces a diode with a higher reverse breakdown voltage,important for high voltage power rectiﬁer diodes. Lower voltage diodes, even low voltage powerrectiﬁers, would have lower forward losses with heavier doping. The heaviest level of dopingproduce zener diodes designed for a low reverse breakdown voltage. However, heavy dopingincreases the reverse leakage current. The P+ region at the anode contact is heavily dopedP-type semiconductor, a good contact strategy. Glass encapsulated small signal junction diodesare capable of 10’s to 100’s of mA of current. Plastic or ceramic encapsulated power rectiﬁerdiodes handle to 1000’s of amperes of current.• REVIEW:• Point contact diodes have superb high frequency characteristics, usable well into the mi-crowave frequencies.• Junction diodes range in size from small signal diodes to power rectiﬁers capable of 1000’sof amperes.• The level of doping near the junction determines the reverse breakdown voltage. Lightdoping produces a high voltage diode. Heavy doping produces a lower breakdown voltage,and increases reverse leakage current. Zener diodes have a lower breakdown voltagebecause of heavy doping.2.8Bipolar junction transistorsThe bipolar junction transistor(BJT) was named because its operation involves conduction bytwo carriers: electrons and holes in the same crystal. The ﬁrst bipolar transistor was inventedat Bell Labs by William Shockley, Walter Brattain, and John Bardeen so late in 1947 that itwas not published until 1948. Thus, many texts differ as to the date of invention. Brattainfabricated a germanium point contact transistor, bearing some resemblance to a point contactdiode. Within a month, Shockley had a more practical junction transistor, which we describe infollowing paragraphs. They were awarded the Nobel Prize in Physics in 1956 for the transistor.The bipolar junction transistor shown in Figure 70,2.33(a) is an NPN three layer semiconduc-tor sandwich with an emitterand collectorat the ends, and a basein between. It is as if a thirdlayer were added to a two layer diode. If this were the only requirement, we would have nomore than a pair of back-to-back diodes. In fact, it is far easier to build a pair of back-to-backdiodes. The key to the fabrication of a bipolar junction transistor is to make the middle layer,the base, as thin as possible without shorting the outside layers, the emitter and collector. Wecannot over emphasize the importance of the thin base region.The device in Figure 70,2.33(a) has a pair of junctions, emitter to base and base to collector,and two depletion regions.It is customary to reverse bias the base-collector junction of a bipolar junction transistoras shown in (Figure 70,2.33(b). Note that this increases the width of the depletion region. Thereverse bias voltage could be a few volts to tens of volts for most transistors. There is no currentﬂow, except leakage current, in the collector circuit.