Editing Transistor (section)

===Semiconductor material===
{|class="wikitable" style="float:right; margin:10px;"
|+ Semiconductor material characteristics
|-
! Semiconductor <br/>material
! Junction forward <br/>voltage @ 25&nbsp;°C, V 
! Electron mobility <br/>@ 25&nbsp;°C, m<sup>2</sup>/(V·s)
! Hole mobility <br/>@ 25&nbsp;°C, m<sup>2</sup>/(V·s)
! {{abbr|Max.|Maximum}} junction <br/>{{abbr|temp.|temperature}}, °C
|-
! Ge
|0.27||0.39||0.19||70 to 100
|-
! Si
|0.71||0.14|| 0.05||150 to 200
|-
! GaAs
|1.03||0.85||0.05||150 to 200
|-
! Al–Si junction
|0.3||—||—||150 to 200
|}

The first BJTs were made from [[germanium]] (Ge). [[Silicon]] (Si) types currently predominate but certain advanced microwave and high-performance versions now employ the ''compound semiconductor'' material [[gallium arsenide]] (GaAs) and the ''semiconductor alloy'' [[silicon–germanium]] (SiGe). Single-element semiconductor material (Ge and Si) is described as ''elemental''.

Rough parameters for the most common semiconductor materials used to make transistors are given in the adjacent table. These parameters will vary with an increase in temperature, electric field, impurity level, strain, and sundry other factors.

The ''junction forward voltage'' is the voltage applied to the emitter-base junction of a BJT to make the base conduct a specified current. The current increases exponentially as the junction forward voltage is increased. The values given in the table are typical for a current of 1&nbsp;mA (the same values apply to semiconductor diodes). The lower the junction forward voltage the better, as this means that less power is required to drive the transistor. The junction forward voltage for a given current decreases with an increase in temperature. For a typical silicon junction, the change is −2.1&nbsp;mV/°C.<ref name=Sedra>{{cite book |author1=Sedra, A.S. |author2=Smith, K.C. |name-list-style=amp |title=Microelectronic circuits |url=https://archive.org/details/microelectronicc00sedr_571 |url-access=limited |year=2004 |page=[https://archive.org/details/microelectronicc00sedr_571/page/n426 397] and Figure 5.17 |publisher=Oxford University Press |edition=Fifth |location=New York |isbn=978-0-19-514251-8}}</ref> In some circuits special compensating elements ([[sensistor]]s) must be used to compensate for such changes.

The density of mobile carriers in the channel of a MOSFET is a function of the electric field forming the channel and of various other phenomena such as the impurity level in the channel. Some impurities, called dopants, are introduced deliberately in making a MOSFET, to control the MOSFET electrical behavior.

The ''[[electron mobility]]'' and ''[[hole mobility]]'' columns show the average speed that electrons and holes diffuse through the semiconductor material with an [[electric field]] of 1&nbsp;volt per meter applied across the material. In general, the higher the electron mobility the faster the transistor can operate. The table indicates that Ge is a better material than Si in this respect. However, Ge has four major shortcomings compared to silicon and gallium arsenide:
# Its maximum temperature is limited.
# It has relatively high [[Reverse leakage current|leakage current]].
# It cannot withstand high voltages.
# It is less suitable for fabricating integrated circuits.
Because the electron mobility is higher than the hole mobility for all semiconductor materials, a given bipolar [[n–p–n transistor]] tends to be swifter than an equivalent [[p–n–p transistor]]. GaAs has the highest electron mobility of the three semiconductors. It is for this reason that GaAs is used in high-frequency applications. A relatively recent{{When|date=May 2018}} FET development, the ''[[high-electron-mobility transistor]]'' (HEMT), has a [[heterojunction|heterostructure]] (junction between different semiconductor materials) of aluminium gallium arsenide (AlGaAs)-gallium arsenide (GaAs) which has twice the electron mobility of a GaAs-metal barrier junction. Because of their high speed and low noise, HEMTs are used in satellite receivers working at frequencies around 12&nbsp;GHz. HEMTs based on [[gallium nitride]] and [[aluminum gallium nitride]] (AlGaN/GaN HEMTs) provide still higher electron mobility and are being developed for various applications.

Maximum [[junction temperature]] values represent a cross-section taken from various manufacturers' datasheets. This temperature should not be exceeded or the transistor may be damaged.

''Al–Si junction'' refers to the high-speed (aluminum-silicon) metal–semiconductor barrier diode, commonly known as a [[Schottky diode]]. This is included in the table because some silicon power IGFETs have a [[parasitic structure|parasitic]] reverse Schottky diode formed between the source and drain as part of the fabrication process. This diode can be a nuisance, but sometimes it is used in the circuit.