JPS6242569A - Field effect transistor - Google Patents

Abstract

PURPOSE:To reduce a short channel effect significantly by a method wherein a doped layer is provided in a single quantum well to be utilized as a channel and the carriers are confined by the hetero junctions of the single quantum well to have two-dimensional mobility. CONSTITUTION:An I-type AlGaAs layer 2, a single quantum well SQW, an I-type AlGaAs layer 6 and an I-type GaAs layer 7 are laminated on a semi- insulating GaAs substrate 1 wherein molar (x) ratios of Al in the layers 2 and 6 are selected to be 0.2-0.3. The SQW consists of an I-type GaAs layer 3, an N-type GaAs layer 4 and an I-type GaAs layer 5. The N-type layer 4 is formed by providing a layer of Se atoms of the concentration of 10<19>cm<-3> or higher or a doped layer of the concentration of about 10<18>cm<-3> between the two I-type layers 3 and 5 and the two I-type layers 3 and 5 protect the I-type layers 2 and 6 from impurity diffusion. N<+> type source and drain layers 8 and 9 are formed to complete an FET. With this constitution, as the doping concentration of the channel is high and, moreover, electrons are confined by the hetero junctions of the SQW, a narrow channel can be obtained and a short channel effect can be reduced significantly.

Description

[Detailed Description of the Invention] [Summary] As a channel structure in a field effect transistor,
A device with a single quantum well (SQW), in which a channel formed by a doped layer within the well is called SQ
Two-dimensionality is provided by the W heterojunction.

Thereby, short channel effects are reduced, linearity of device characteristics is improved, subthreshold characteristics are improved, and fluctuations in threshold values due to temperature are reduced.

[Industrial application field]

The present invention relates to a field effect transistor, and more particularly to an element having a single quantum well (SQW) as a channel structure, and a channel formed in the well having two-dimensionality due to a heterojunction of the SQW.

[Conventional technology]

Conventionally, various attempts have been made to improve the characteristics of field effect transistors, such as increasing mutual conductance (!I1.n) and reducing short channel effects.

FIG. 3 shows a conventional GaAs MESFET. In the figure, 31 is a semi-insulating GaAs substrate, 32 is an n-GaA substrate.
s layer, 33.34 is n+ layer for source and drain contacts, 36.37 is source and drain electrode, 35
is the gate electrode. FET operation is performed by controlling the channel with the depletion layer 38 extending by applying a bias voltage to the gate electrode 35, but in this case, when the channel length is shortened, the short channel effect shown in FIG. 5 becomes a problem. As shown in FIG. 5, when the channel length becomes about 1 μm or less, the threshold value vth of the field effect transistor changes as shown in the figure. This variation becomes much smaller as the impurity concentration N of the active layer of the channel decreases. Therefore, conventionally, the active layer has been highly doped in order to reduce the short channel effect. Furthermore, when the active layer is highly doped, the depletion layer 41 becomes thinner as shown in the energy hand in FIG. becomes larger9. This means that you can improve your performance.

[Problems to be Solved by the Invention] However, conventional devices still have problems such as a decrease in device breakdown voltage or a decrease in mobility due to highly doped active layers. The present invention aims to solve these problems and provide an element with excellent characteristics.

[Means for solving problems]

In the present invention, a doped layer is formed in a single quantum well (SQW), the quantum well layer is used as a channel, and the channel is made two-dimensional by a heterojunction of the single quantum well (SQW). I try to let it stand.

The present invention will be explained using the energy band diagram of the device according to the embodiment of the present invention shown in FIG. 2. In the figure, 1-AI! Ga
As (6), 1-AI! A single quantum well (SQW) is formed between GaAs (2). A Brehner-doped or highly doped layer d is formed in the single quantum well (SQW), and the electron gas e supplied from this layer is confined by a heterojunction to give it two-dimensionality. The width of the single quantum well (SQW) is preferably within about 100 layers in order to provide two-dimensionality.

[Effect]

According to the above-mentioned structure of the invention, the doping concentration of the channel is high, and electrons are further confined by the SQW heterojunction, resulting in a narrow channel.
The short channel effect can be prevented more than T, and the short channel effect can be significantly reduced.

Furthermore, since the electronic system is confined by the relatively high barrier of the heterojunction with the lower 1-AItGaAs (2), the subthreshold characteristics are very good even in the vicinity of pinch-off. In contrast, conventional MESFET
In this case, it is a homojunction or has a low barrier, and as shown in Fig. 6, in the characteristic diagram of gate voltage Vgs and drain current 1d, it does not close as shown in b but as shown in a, and a subthreshold occurs. become.

In addition, for the same reason and because the gate capacitance is constant, the linearity of the device characteristics is good, and the channel electrons are supplied from GaAs, so there is little variation in the amount of electrons supplied due to temperature, and the The fluctuation of the value with respect to temperature is reduced.

On the other hand, in conventional HEMTs (high electron mobility transistors), the electron supply layer is AlGaAs with a deep donor level and a DX center, so the amount of electron supply changes easily depending on the temperature, and the dark value changes with temperature. big.

〔Example〕

FIG. 1 shows the main parts of a device according to an embodiment of the present invention. In the figure, each undoped 1-A7! on a semi-insulating (Sr)C, aAs substrate 1! GaAs layer 2. Single quantum well (SQW), 1-AlGaAs6.1-GaAs7
Each layer is formed. 1-A of AIGaAs layers 2 and 6
jl! The molar ratio X is 0°2 to 1. In this example, it is set to 0.2 to 0.3.

Single quantum well (SQW) layer is 1-GaAs3゜n-Ga
It is formed from As4 and 1-GaAs5. The four layers of single quantum well (SQW) doping layers are planar doped or highly doped.

Examples of each of the above layers are shown below.

2.6:1-AlGaAs layer, undoped, several hundred thick (thickness where carriers cannot tunnel) 3.5:1
-GaAs layer undoped, film thickness several tens of 4:n-GaAs
Layer planar dope (atomic planar dope: t
- An atomic layer of St or Se is interposed between the GaAs layers.

), the doping concentration is 10I9cIII-3 or more, the film thickness is several tens of layers, and the doping concentration is I Q ” in the case of high doping.
It should be about C11-3. Doping layer n-GaAS
The undoped 1-GaAs layer 3.5 is provided on both sides of 4 because the dopant is diffused into i-Ga, A
This is to prevent diffusion into the I A 3 layer 2.6. Incidentally, the thickness of the SQW consisting of the 3rd and 4.5th layers is 100 Å or less in order to ensure two-dimensionality. 7: 1-G
In addition to several hundreds of aAs layers, undoped, and over several hundred films, 8.9 in FIG.
12 is a gate electrode (AN).

〔Effect of the invention〕

As is clear from the above description, the following effects can be obtained according to the present invention.

■According to the above-mentioned structure of the invention, not only the doping concentration of the channel is high, but also the electrons are confined by the SQW heterojunction, resulting in a narrow channel, which is different from the conventional improved ME S
The short channel effect can be prevented more than the FET, and the short channel effect can be significantly reduced.

■Since the electronic system is confined by the relatively high barrier of the lower heterojunction, the subthreshold characteristics are very good even near pinch-off.

(2) For the same reason and because the gate capacitance is constant, the linearity of the device characteristics becomes good and it is possible to achieve equalization.

■GaAs doping layer with relatively shallow donor level
etc., and since the channel electrons are supplied from here, there is little variation in the amount of electron supply due to temperature, and the variation of the dark value with respect to temperature is reduced.

On the other hand, in a conventional HEMT (high electron mobility transistor), the electron supply layer is made of AlGaAs with a deep donor level, so the amount of electron supply changes easily depending on the temperature, and the dark value changes greatly depending on the temperature.

■Since the channel has two-dimensionality and impurity doping is limited to a part of the single quantum well (SQW) that constitutes the channel, carrier mobility is lower than that of conventional M
Better than ESFET etc.

(2) Since the gate electrode can be formed on top of a high resistance layer such as a 1-GaAs layer, there is no deterioration in breakdown voltage.

[Brief explanation of drawings]

Fig. 1 is a sectional view of the main part of the embodiment of the present invention, Fig. 2 is an energy band diagram of the embodiment of the present invention, and Fig. 3 is a conventional MES.
A cross-sectional view showing the outline of the FET, Figure 4 is a conventional ME S
The energy band diagram of the FET, FIG. 5 is an explanatory diagram of the short channel effect, and FIG. 6 is a diagram showing the subthreshold. Main code 1... Semi-insulating (Sl) GaAs substrate 2 ・-・1-
AlGaAs layer 3...-1-Qa, 6. s layer 4-...n-GaAs layer 5...1-GaA3 layer 6...1-GaAlAs layer 7 .--1-GaAs layer 8.9...n+ region], 0.11... Source, drain electrode 12...gate electrode

Claims (1)

[Claims] 1. First and second semiconductor layers on a semiconductor substrate, and a single quantum well (SQ) having a narrower band gap between the two semiconductor layers.
W), a doping layer is formed in the third semiconductor layer, and a doping layer is formed in the third semiconductor layer, and the single quantum well (SQW)
) is used as a channel. 2. The semiconductor substrate is made of semi-insulating GaAs, the first and second semiconductor layers are made of AlGaAs, the third semiconductor layer is made of GaAs, and the doped layer is a planar layer.
Doped or highly doped layer with undoped G on both sides
The field effect transistor according to claim 1, characterized in that an aAs layer is interposed therebetween.