JPH02291134A - Bipolar transistor and operation thereof - Google Patents

Abstract

PURPOSE:To realize a high-speed operation even in a low electric-current region by a method wherein a Fermi energy from the bottom of a conduction band of an emitter layer is specified, a band gap difference at an emitter-base junction and a base-collector junction is specified and a specific buffer layer whose conductivity type is identical to that of the emitter layer is formed between the base and the emitter. CONSTITUTION:A Fermi energy EF (Eemitter) from the bottom of a conduction band of an emitter layer 19 satisfies an inequality EF (Eemitter)>¦1.3¦Emin¦ regarding Emin=(-0.175).13.6(M*/m0)/epsilons<2>, where M* represents an effective mass of an electron, m0 a mass of the electron in a vacuum and epsilons a relative permittivity of a base layer 16 at an emitter-base junction. DELTAEG, which is a value obtained by subtracting a band gap at a base-collector junction from a bang gap at the emitter-base junction, satisfies an inequality DELTAEG>2¦Emin¦. In addition, an emitter buffer layer 17 whose layer thickness is 10Angstrom or higher, whose impurity concentration is 10<16> to 10<18>cm<-3> and whose conductivity type is identical to that of the emitter layer is constituted between the base and the emitter.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to the structure and operating method of a hyperpolar transistor.

(Prior Art) Conventional high-voltage high speeds are mostly used for indoor operation, but recently, lowering the temperature has been studied in order to make the high voltage high speeds smaller and lower the voltage. However, its operation was essentially the same as that at room temperature.

(Problems that the invention attempts to solve) One of the disadvantages of conventional junction transistors is that the transconductance becomes small in the low current range, so the current cannot be lowered for high-speed operation. power consumption was increasing. An object of the present invention is to propose a hyperpolar I transistor that can operate at high speed even in a low current range by realizing negative transconductance, and a method for operating the same.

(Means for Solving the Problem) The present invention provides that the Fermi energy EF (Eemitter) from the bottom of the conduction band of the eminota layer is equal to the effective mass m* of the electron.
and the specific induction g of the base layer in the eminota-base junction.
Emin = (-0.175)・
13.6 Inequality E1 regarding (m*/mo)/ε3,
(Eemitter)>1.3|Emin|, and the value ΔEo obtained by subtracting the node\doganop at the base collector junction from the hand gap at the eminota-base junction satisfies the inequality △Eo>2|Emin| The present invention provides an npn type bibolar transistor characterized by having an eminota layer having a layer thickness of 100 or more between the base and eminota, an impurity concentration of 10 to 10 cm, and having the same conductivity type as the eminota layer. .

In addition, as a pnp type bibolar 1 transistor, the Fermi energy EF (
Emitter) is expressed by the effective mass of the hole and the relative dielectric constant εs of the base layer in the emitter-base junction.Emin=(-0.175)-13.6(m*/m
0)/εs2 inequality EF(Eemitter)
> 1. ．． 3｜Emin｜ and the emitter
The value ΔEo obtained by subtracting the hand cassop at the base-collector junction from the hand gap at the base-collector junction satisfies the inequality ΔEo>2|Emin|, and the layer thickness between the pace emitters is 100 or more - I2 and 10 to 10
A structure having an impurity concentration of cm 2 and an emitter layer having the same conductivity type as the emitter layer is provided.

In addition, as for the operation method of the npn type high boler 1 transistor, the Fermi energy E1, (Eemitter) from the bottom of the conduction band of the emisoter layer is the effective mass m* of the electron.
and the dielectric constant ε of the base layer at the emitter-base junction
Emin represented by s = (- 0.175)・13
．． 6m*/mo/ε, the inequality EF(Eemit
ter)> 1.3 |Emin|, and Eminota. The value ΔEo obtained by subtracting the band gap at the base-collector junction from the band gap at the base junction satisfies the inequality ΔEa > 2 | Emin il
It is characterized by operating at a temperature T of Eminl.

The operating method of a pnp bipolar transistor is to use the Fermi energy E from the top of the valence electron emitter in the emisota layer.
F (Eemitter) is expressed by the effective mass m* of the hole and the dielectric constant ε of the base layer in the emitter-base junction. Emin = (-0.175)·1 3.
6m*/mo/e, the inequality EF (Eemitt
er) > 1.3 |Emin|, and the value obtained by subtracting the band gap at the base-collector junction from the band gap at the eminota-base junction is 8Eo
is pn that satisfies the inequality 8Eo>2|Emin|
It is characterized in that a p-type bipolar transistor is operated at a temperature T such that the inequality kBT<+Emin1 is expressed by the Portzmann constant kB and Emin.

(Embodiment) FIG. 1 shows an embodiment of the present invention, in which the upper side shows a cross-sectional view of the structure of a transistor, and the lower side shows a hand diagram. First, we will explain the basic principle using the structure and hand diagram.

The materials will be described later.

1 in Figure 1. 2. 3 are the emitter, base, and collector of the junction 1 and the transistor, respectively. 4, 5, 6.
7 indicates the conduction bands of the emitter, base, base collector depletion layer, and collector, respectively; 4', 5', 6', and 7' indicate the valence bands of the regions corresponding to the columns with the same numbers. 8 is the Fermi level of holes at the base, 10. 11 are the quasi-Fermi levels of electrons at the eminota and base, respectively. Although an example of an NPN I-transistor is given here, the following also applies, with obvious modifications, to a PNP I-transistor.

Examples of specific structures are summarized in Table 1 below.Although not shown in FIG. 1, Table 1 shows a structure in which an emitter Hanofa layer is sandwiched between emitter bases. This is to prevent tunnel current from flowing since the emitter base is heavily doped with impurities. A thickness of about 300 people or more is sufficient. In addition to the N-type collector layer, the collector has a highly doped conductive layer (n
There is a type collector conductive layer (not shown in FIG. 1) for lowering the collector resistance.

Next, the principle of the octopolar transistor of the present invention will be explained. In the following, we will consider the case of extremely low temperatures, but to simplify the equation, we will consider the case when T=O. At this time, the electric current (M'b density J) injected from the emitter to the base is given by the following equation.

However, q is the electron charge, P is the electron mobility at the base,
W is the base "1", n(0) is the electron density of B at the emisotapace junction, m* is the effective mass of electrons at the base, h is Dirac's constant (h=h/2n: h is a blank constant) , ■ is the eminotabase bias voltage.

Here, ■ represents the threshold voltage for electron injection from the emitter to the base to occur. First, J=0
When , ■=■. Here, VTo is T as shown in Figure 1.
TO is the energy difference between the fermi level 8 of the hole in the base and the bottom 9 of the conduction band in the emisoter base junction. By the way, when v>vTo and injection of electrons into the base begins, since electrons are degenerate at extremely low temperatures, quantum mechanical exchange interactions occur between the electrons and the energy of the electrons decreases. The higher the density of electrons, the greater the energy drop, so when current is flowing, dotted line 12 in Figure 1. It looks like 13. As the energy at the bottom of the conduction band decreases in this way, the energy threshold (2) for electron injection decreases. This VT decrease E is expressed by the following formula, where T mo is the mass of free electrons, m* is effective mass, and ε is the dielectric constant of the semiconductor, ε. is the dielectric constant of vacuum. Using this E, ■1 can be written as follows.

■1=vTo1Ex (8)
This decrease in vT causes an increase in current.

On the other hand, such a reduction in the energy of the conduction band lowers the slope of the band gap at the base, as shown in FIG.

This effect causes a decrease in the built-in voltage Δ■8 at the base in equation (1). ΔvB at J=00 now is Δ■Bo
Then, ΔVB when J≠00 is given by the following equation.

ΔV=ΔV +E (9)B
We considered only the exchange interaction between the electrons injected above BO x , but
The effects of exchange interactions between electrons and holes, interactions between electrons and impurities, and holes and holes at the base all have no or small dependence on the injected electron current. Therefore, all these effects can be included in vTo and vBo.

Now, let's give a numerical example to the above results.

p-1000cm/V-see, ε5/ε. -10
, W, = 150OA, m*/mO = 0.01
, 0.05, 0.1, 0.2, 0.4, 0.
5, 0.6, 0.7, 0.8, 0.9. FIG. 2 shows the IV characteristics when the voltage is changed to 1. The numerical example shown in Table 1 corresponds to the case where m*/mo=0.06. In FIG. 2, it can be seen that a current is observed below the threshold voltage vT, and a negative transconductance is observed.

Now, let us define the conditions under which the negative resistance described in the present invention occurs. First, let's talk about Emisotar's conditions. In the emitter, the electrons must be degenerate at low temperatures. As for this condition, it is desirable that the wave functions of the donors overlap each other to form an impurity hand, and that the energy of the impurity band overlaps with the conduction band. This condition is determined by the fact that one donor exists on average in the spherical surface determined by the Bohr radius of the donor (6), and the competition between the decrease in electron energy due to exchange interaction and the increase in the kinetic energy of the electrons.

becomes. However, ml. , *, εso are each larger than the presence of emisoter electrons, and if εSE'ε0 becomes smaller, ND
requires even higher concentrations. Also, the eminotar's hand camp must be a so-called Ganop eminotar with a wider bandgap than the base. Roughly speaking, this condition is sufficient if the difference in hand width is 0.1 eV or more.

Next, let's discuss the basic conditions for negative resistance to appear. Negative resistance appears when minority carrier electrons are injected into the base, and the quasi-Fermi level of the electrons is at the bottom of the conduction band (the first
This is because the energy is lower than the energy shown in 9) in the figure. The change in the pseudo-Fermi level of electrons due to the increase in electron density is (
3) It is given by the intersection of Eq. The total electron energy E=Ek×Ex(12) has a negative minimum value 1△E as a function of n(0), because Ek is positive and increases monotonically as a function of n(0).
Ex is negative and monotonically decreasing, and in the region where n(0) is small, E is dominant and negative and decreasing, and in the region where n(0) is large, E,
is dominant and becomes a positive increasing function. The magnitude of this 18E is approximately lower than the forward bias ■To.

As mentioned above, if the absolute temperature is 0 degrees, negative resistance will always appear. However, in reality, this can only be achieved at a finite temperature, so it is necessary to limit the operating temperature. That is, in the temperature range of kBT≦8E (13), negative resistance still appears. However, kB is the Borckmann constant, and T is the absolute temperature of the lattice.

Next, we need to consider the conditions for the base hand gap slope. Lowering the bottom of the conduction band in the pace emisoter junction by E in equation (3) is equivalent to weakening the electric field due to the bandgap gradient of the base. In order to prevent such an effect from causing a current to flow, the difference ΔvB between the value at the eminota side junction of the base band ganop and the value at the collector side junction must be the minimum of the electron energy E (11 (formula)). Must be sufficiently large for the absolute value ΔE qΔ■B > ΔE As for the size range, qΔ■3≧28E should be sufficient 0 More details < Negative resistance of J-V characteristic Let us clearly define the mechanism with reference to Figure 2. NPN now}
The transistor will be explained. The designated Fermi level measured from the bottom of the conduction band at the emitter-base junction when no electrons are injected from the emitter to the base is the kinetic energy, and the decrease in the bottom of the conduction band when electrons are injected using the equation (3). However, the unit of energy is R* unit. Here, the y-th term is the kinetic energy term, and the second and third terms are contributions from equation (3). r is a decreasing function of the electron density n(0) injected by equation (5). Equation (9) is a function of n(0), and in the region where n(0) is small, rs is thick <EF
When is close to 0, r becomes even smaller, the first term becomes increasingly dominant, and EF becomes an increasing function after passing through a minimum, and eventually becomes positive. FIG. 3 shows the change in EF with respect to n(0).

The fact that EF becomes negative with respect to the bottom of the conduction band at the emitter-base junction when no injection is present is the reason why negative resistance appears in the J-V characteristic shown in FIG. Equation (9) is r = 5
．． 27, minimum value EF = Em, n = - 0.175
It becomes R, *.

The temperature T-0 (Kelvin) has been considered above, but at a finite temperature T, blurring due to Fermi level temperatures must be considered. In this case, negative resistance is less likely to appear, but the temperature range in which negative resistance appears is kBT<4 |Emin| (kB is Borckmann's constant)
(15) is fine.

The difference Eo between the band gap at the emitter-base junction and the hand gap at the collector-base junction due to the base bandgap gradient decreases due to the lowering of the conduction band due to the implantation, and the accelerating electric field due to the band gap gradient decreases. However, the following formula ΔEa>2 |Emin|
If (16) is satisfied, the effect is small.

FIG. 4 shows an example of the structure of an NPN transistor. The transistor has an N-type collector InGaAsl4. 15 and P
base 16, and N emitter 17. It is formed from 18.

The P base 16 has a composition of Ga and AI from the emitter side to the collector side. s. A1o 1. AS to I
It changes continuously up to nGaAs, and the band gap is from 0.85 eV (eminotabase junction) to 0.75 eV.
(base collector junction). The emitter layer 18 has an n-type concentration of InAIAs 2 × 1019 cm, and the emitter path layer 17 has a composition of Ga and A1 of InAIAs from the boundary with the base 18 toward the junction with the base.
From InGao8. A1o1. It is changed continuously up to As, and is 3×10 cm in n type. This changes the bandgap of the emitter from ~1.45 eV at 18 to 0.85 eV at the junction with the base. Also, the doping of the donor acceptor and the thickness of each layer are shown in the drawings. In this structure, the effective quality of electrons injected from the emisotar to the base is 0.05m. According to FIG. 2, negative resistance can be realized.

In fact, when operating a transistor with the structure shown in Figure 4 at various temperatures between 30K and 0.1K, the voltage applied across the eminotabase is very small, on the order of a few mV with respect to the threshold voltage V. Even with regard to voltage, a current of the order of 10 A/cm was observed. Note that a low temperature of 30K to 0.1K was obtained by reducing the pressure of neon, hydrogen, or helium (a mixed solution of He3 and He4). InAIAs-InG
ao8. AIo1. The As layer 17 is an emitter buffer layer for preventing tunnel current from flowing between pace emitters.

Next, as a second embodiment, consider a PNP transistor having a semiconductor structure similar to that shown in FIG. 4, in which the emitter and collector are doped with P-type impurities, and the base is doped with N-type impurities.

The band structure is the same as in Figure 3 if the direction of hole energy is directed upward. At this time, holes are injected from the emitter into the base. Although the exact value of the effective mass of holes in the base is unknown, InAs, GaAs
, the effective masses of heavy holes in AIAs are each 0.41 m.

, 0.45mo, 0.22m. Therefore, it is approximately 0.
3-0.4. It would be the value of mo. According to Figure 2, np
A more significant negative resistance is seen than in the nh transistor.

NPN is generally a narrow bandgap IILV group semiconductor.
The same thing can be said when comparing PNP and PNP. Therefore PNP
If the transistor is at a low temperature given by equation (11) and has a baseband gassop slope that satisfies equation (12), the negative resistance will be more pronounced than that of the NPN type due to the effect of lowering carrier energy according to equation (3). seen in

(Effects of the Invention) Since the collector current becomes a negative resistance as a high-ass function between the emitter and base, and low-temperature operation is possible, a high-speed bibolar circuit can be realized in a low current range.

The figure shows the relationship between collector current density and emitter-base voltage of a transistor, FIG. 3 shows the relationship between Fermi energy and electron density, and FIG. 4 shows the cross-sectional structure of an npn transistor.

In the figure, 1...emitter, 2...base, 3...
...Collector, 4...Emisota's Contribution Emperor's Bottom, 5...
・Base conduction band bottom, 6...Collector emisota depletion layer, 7...Collector conduction band bottom, 4', 5''6+,
T...The upper end of the valence band in the region with the same number, 8...
Fermi level of hole, 9...bottom of conduction band at emitter-base junction, ]-0.11...pseudo-Fermi level of electron at emitter-base, 12. 13... Bottom of conduction band in emitter and base when considering electron exchange interaction, 14... Collector conductive layer (In.GaAs),
15--Collector layer (GaAs), 16--Base layer (InGa Al1?As), 17--Emitter layer (InGa Al-xAs), 18--
- Eminota layer (InAIAs).

Claims (1)

[Claims] 1) The Fermi energy E_F (Eemitter) from the bottom of the conduction band of the emitter layer is the effective mass of the electron m^*
Emin is expressed by the mass of electrons in vacuum m_0 and the relative dielectric constant ε_s of the base layer in the emitter-base junction.
=(-0.175)・13.6(m^*/m_0)/ε
Inequality E_F (Eemitter) with respect to _s^2>
1.3 |Emin| and the value ΔE_G obtained by subtracting the band gap at the base-collector junction from the band gap at the emitter-base junction is
The inequality ΔE_G>2|Emin| is satisfied, the layer thickness between base and emitter is 100 Å or more, and 10^1^6 to 10
An npn type bipolar transistor having an impurity concentration of ^1^8 cm^-^3 and having an emitter buffer layer of the same conductivity type as the emitter layer. 2) The Fermi energy E_F (Eemitter) from the upper end of the valence band of the emitter layer is expressed by the effective mass of the hole m^* and the dielectric constant ε_s of the base layer at the emitter-base junction.Emin=(-0.175 )・
Inequality E regarding 13.6m^*/m_0/ε_s^2
_F(Eemitter)>1.3|Emin|, and the value ΔE_G obtained by subtracting the band gap at the base collector junction from the band gap at the emitter-base junction satisfies the inequality ΔE_G>2|Emin|, and the base A pnp type layer having a layer thickness of 100 Å or more between the emitters, an impurity concentration of 10^1^6 to 10^1^8 cm^-^3, and an emitter buffer layer of the same conductivity type as the emitter layer. bipolar transistor. 3) The Fermi energy E_F (Eemitter) from the bottom of the conduction band of the emitter layer is the effective mass of the electron m^*
and the dielectric constant ε of the base layer at the emitter-base junction
Emin represented by _s=(-0.175)・13.
6m^*/m_0/ε_s^2 inequality E_F(
npn type in which the value ΔE_G obtained by subtracting the band gap at the base-collector junction from the band gap at the emitter-base junction satisfies the inequality ΔE_G>2|Emin| The bipolar transistor is defined by the Boltzmann constant k_B and the inequality k_B expressed by Emin.
A method of operating a bipolar transistor characterized by operating it at a temperature T such that BT<1/2 |Emin|. 4) The Fermi energy E__F (Eemitter) from the upper end of the valence band of the emitter layer is expressed by the effective mass of the hole m^* and the dielectric constant ε_s of the base layer at the emitter-base junction.Emin=(-0.175 )
・For 13.6m^*/m_0/ε_s^2, the inequality E_F(Eemitter)>1.3|Emin| is satisfied, and the value ΔE_G obtained by subtracting the bandgap at the base-collector junction from the bandgap at the emitter-base junction is , ΔE_G>2|Emin|
A method of operating a bipolar transistor characterized by operating it at a temperature T such that _BT<1/2|Emin|.