Types of Bipolar Junction Transistors

1. NPN is one of the two types of bipolar transistors, in which the letters «N» and «P» refer to the majority charge carriers inside the different regions of the transistor. Most bipolar transistors used today are NPN, because electron mobility is higher than hole mobility in semiconductors.

NPN transistors consist of a layer of P-doped semiconductor (the «base») between two N-doped layers. NPN transistors are commonly operated with the emitter at ground and the collector connected to a positive voltage through an electric load. A small current entering the base in common-emitter mode is amplified in the collector output.

The arrow in the NPN transistor symbol is on the emitter leg and points in the direction of the conventional current flow when the device is in forward active mode.

2. The other type of BJTs is PNP with the letters «P» and «N» referring to the majority charge carriers inside the different regions of the transistor. Few transistors used today are PNP, since the NPN type gives better performance in most circumstances.

PNP transistors consist of a layer of N-doped (often doped with boron) semiconductor between two layers of P-doped (often with arsenic) material. PNP transistors are commonly operated with the collector at ground and the emitter connected to a positive voltage through an electric load. A small current entering the base prevents current from flowing between the collector and emitter.

3. The heterojunction bipolar transistor (HBT) is an improvement of the BJT that can handle signals of very high frequencies up to several hundred GHz. It is common nowadays in ultrafast circuits, mostly RF systems.

Heterojunction transistors have different semiconductors for the elements of the transistor. Usually the emitter is composed of a larger bandgap material than the base. This helps reduce minority carrier injection from the base when the emitter-base junction is under forward bias and increases emitter injection efficiency. The improved injection of carriers into the base allows the base to have a higher doping level, resulting in lower resistance to access the base electrode. With a regular transistor, also referred to as homojunction, the efficiency of carrier injection from the emitter to the base is primarily determined by the doping ratio between the emitter and base. Because the base must be lightly doped to allow the high injection efficiency its resistance is relatively high. With a heterojunction the base can be highly doped allowing a much lower base resistance and consequently higher frequency operation.

4. Two commonly used HBT’s are silicon-germanium and aluminum-gallium arsenide. Silicon-germanium is widely used because it is compatible with standard silicon digital processes, allowing integration of very high speed circuitry with complex lower speed digital circuitry.

Notes:

junction transistor	плоскостной транзистор
to dope	добавить примесь
majority/minority current carriers	основные/неосновные носители тока
performance	(рабочие) характеристики
homo/heterojunction	гомо/гетероструктурный переход
forward/reverse bias	прямое/обратное смещение

5. Укажите, какие из данных утверждений соответствуют содержанию текста.

1. The arrow on the emitter leg in the NPN transistor symbol indicates the direction of the conventional current flow in reverse operating mode.

2. Nowadays, NPN transistors are more popular.

3. In homojunction transistors the base is usually doped lightly.

6. Выберите правильные ответы на вопросы.

1. What voltage is usually applied to the collector of NPN transistor in active mode?

(a) positive;

(b) negative.

2. What circuits are HBTs used today?

(a) in very high frequency circuits;

(b) in very low frequency circuits.

7. Прочитайте предложения. Выберите правильный вариант перевода.

1. NPN-transistors are commonly operated with the emitter at ground and the collector connected to a positive voltage through an electric load.

(a) NPN-транзисторы обычно работают при эмиттере подключенном к земле, а на коллектор через электрическую нагрузку подается положительное напряжение.

(b) NPN-транзисторы обычно работают с эмиттером на земле и коллектором, подсоединенном к положительному напряжению через электрическую нагрузку.

2. A small current entering the base prevents current from flowing between the collector and emitter.

(a) Небольшой ток, поступающий на базу, отсекает ток между коллектором и эмиттером.

(b) Небольшой ток, поступающий на базу, предотвращает ток от протекания между коллектором и эмиттером.

3. NPN transistors consist of a layer of P-doped semiconductor (the base) and two N-doped layers commonly referred to as collector and emitter.

(a) NPN транзисторы состоят из полупроводникового слоя р-типа (базы) и двух слоев n-типа, обычно называемых эмиттером и коллектором.

(b) NPN транзисторы состоят из полупроводникового слоя р-типа (базы) и двух слоев n-типа, обычно относящихся к эмиттеру и коллектору.