[Basics of Analog Electronics Technology] Chapter 1 Commonly Used Semiconductor Devices (Diodes and Transistors)

According to the object's ability to conduct electricity, electrical materials can be divided into three categories: conductors, semiconductors and insulators.

Semiconductors can be defined as electrical materials whose electrical conductivity is between conductors and insulators. The resistivity of semiconductors is $10^{-3}\sim10^{-9}\Omega \cdot cm$ . Typical semiconductors include silicon Si, germanium Ge, and gallium arsenide GaAs.

The conductivity of semiconductors varies greatly under different conditions: when affected by external heat and light, its conductivity changes significantly; when certain impurity elements are mixed into pure semiconductors, their conductivity will change. The ability to conduct electricity is controllable; these special properties determine that semiconductors can be made into various devices.

1.2 Structure and conductive properties of intrinsic semiconductors

Intrinsic semiconductors are pure semiconductor single crystals without structural defects. The purity of semiconductor materials used to manufacture semiconductor devices must reach 99.9999999%, often called "nine nines". Its physical structure is covalently bonded and in the form of a single crystal. At thermodynamic temperature zero and without external excitation, intrinsic semiconductors do not conduct electricity.

1.3 Intrinsic excitation and recombination phenomena of semiconductors

When a conductor is at thermodynamic temperature OK, there are no free electrons in the conductor. When the temperature rises or is illuminated by light, the energy of valence electrons increases, and some valence electrons can break away from the shackles of the nucleus and participate in conduction, becoming free electrons. This phenomenon is called intrinsic excitation (also called thermal excitation). The free electrons and holes that appear due to thermal excitation appear in pairs at the same time and are called electron-hole pairs.
Some of the free electrons may also return to the hole, which is called recombination.
At a certain temperature, intrinsic excitation and recombination will reach dynamic equilibrium. At this time, the carrier concentration is constant and the number of free electrons and holes are equal.

1.4 Conductive mechanism of semiconductors

The directional movement of free electrons forms electron current, and the directional movement of holes can also form hole current. Therefore, there are two kinds of current-carrying particles (i.e., carriers) in semiconductors: free electrons and holes. This is the characteristic of semiconductors. Special properties. The essence of hole conduction is that the valence electrons in adjacent atoms (bound electrons in covalent bonds) fill the holes in turn to form an electric current. Since electrons are negatively charged and electrons move in the opposite direction to holes, holes are considered to be positively charged.

1.5 Impurity semiconductors

Intrinsic semiconductors doped with impurities are called impurity semiconductors. Impurity semiconductors are the basic materials of semiconductor devices.

Doping pentavalent elements (such as phosphorus) into intrinsic semiconductors (such as pure silicon crystals) and replacing the positions of silicon atoms in the crystal lattice forms N-type (electron-type) semiconductors;

Doping trivalent elements (such as boron, ammonium, indium, etc.) into intrinsic semiconductors (such as pure silicon crystals) and replacing the positions of silicon atoms in the crystal lattice forms Р-type (hole-type) semiconductors.

The electrical conductivity of impurity semiconductors is related to its doping concentration and temperature. The greater the doping concentration and the higher the temperature, the stronger its electrical conductivity. The concentration of majority carriers is approximately equal to the concentration of dopant atoms, and the concentration of minority carriers is very sensitive to temperature.

In N-type semiconductors, electrons are majority carriers and holes are minority carriers.

Number of majority carriers (free electrons) = number of positive ions + number of minority carriers (holes)

In Р-type semiconductors, holes are majority carriers and electrons are minority carriers.

The number of major carriers (holes) = the number of negative ions + the number of minor carriers (free electrons)

1.6 PN junction

(1) Formation of PN junction

There are two orderly movements of carriers in semiconductors: diffusion movement of carriers under the action of concentration difference and drift movement under the action of electric field.

Р-type and N-type semiconductor regions are formed on the same semiconductor single crystal. At the junction of these two regions, when multi-carrier diffusion and minority carrier drift reach a dynamic balance, the space charge region (also known as the depletion layer or barrier region ) is basically stabilized, and the PN junction is formed.

(2) Unidirectional conductivity of PN junction

When the potential of the Р region is higher than the potential of the N region, it is called forward voltage (or forward bias). At this time, the PN junction is turned on, showing low resistance, and mA-level current flows, which is equivalent to a switch. Closed;
when the potential of the N region is higher than the potential of the Р region, it is called reverse voltage (or reverse bias). At this time, the PN junction is cut off, showing high resistance, and a uA-level current flows, which is equivalent to The switch is open.
The PN junction is the basic structural unit of semiconductors, and its basic characteristic is unidirectional conductivity: that is, when the polarity of the applied voltage is different, the PN junction shows completely different conductive properties.
When a forward voltage is applied to the PN junction, it exhibits low resistance and has a large forward diffusion current; when a reverse voltage is applied to the PN junction, it exhibits high resistance and has a small reverse drift current. This is the specific manifestation of the unidirectional conductivity of the PN junction.

(3) Current equation of PN junction

$i=I_s(e^{u/U_T}-1)$

In the formula: $I_S$ is the reverse saturation current; $U_T$ is the temperature voltage equivalent, when T=300K, $U_T$ ≈26mV.

(4) Volt-ampere characteristics of PN junction

When u>0 and u>>UT, $i=I_se^{u/U_T}$ , the volt-ampere characteristics show a nonlinear exponential law;
when u<0 and |u|>>uT, , $i\approx-I_s\approx0$ the current is basically independent of u; this also shows that the PN junction has a single To conductive properties.

Reverse breakdown: When the reverse voltage of the PN junction increases to a certain value, the reverse current increases sharply as the voltage value increases. There are two types of reverse breakdown of PN junction: Zener breakdown and avalanche breakdown.

Zener breakdown : In the case of high doping, because the depletion layer width is very narrow, when the reverse voltage increases to a certain critical value, the electrons in the valence band will cross the forbidden band and recombine with holes to form electrons. -pairs of holes, causing a rapid increase in current.

Avalanche breakdown : In the case of low doping, the depletion layer width is wide, and Zener breakdown will not occur at low reverse voltage. When the reverse voltage is close to the breakdown voltage UB, these carriers with higher energy meet neutral atoms in the space charge region and undergo collision ionization to generate new electron-hole pairs. These newly generated electrons and holes will regain energy under the action of the electric field, collide with other neutral atoms to ionize them, and generate more electron-hole pairs. This chain reaction continues, causing the number of carriers in the space charge region to increase sharply, just like an avalanche, causing the reverse current to increase sharply and causing breakdown.

No matter what kind of breakdown occurs, if the current is not limited, it may cause permanent damage to the PN junction.

(5) PN junction temperature characteristics

When the temperature increases, the reverse current of the PN junction increases and the forward conduction voltage decreases. This is also the main reason for the poor thermal stability of semiconductor devices.

(6) PN junction capacitance effect

The PN junction has a certain capacitance effect, which is determined by two factors: barrier capacitance and diffusion capacitance, both of which are nonlinear capacitances. The junction capacitance of the PN junction is the sum of the two, that is

$C_j=C_b+C_d$

The barrier capacitance is the capacitance equivalent to the change in the width of the depletion layer $C_b$ . The barrier capacitance is related to factors such as the area of the PN junction, the width of the space charge region, and the applied voltage.
Diffusion capacitance is the capacitance equivalent to the accumulation and release of charge in the diffusion area $C_d$ . Diffusion capacitance is related to factors such as PN junction forward current and temperature.

PN junction capacitance consists of barrier capacitance and diffusion capacitance. When the PN junction is forward biased, the diffusion capacitance is dominant; when the PN junction is reverse biased, the barrier capacitance is dominant. Only when the signal frequency is higher, the effect of junction capacitance is considered.

2 semiconductor diodes

2.1 Several common structures of semiconductor diodes and their applications

Adding leads and packaging to the PN junction becomes a diode.

Diodes are divided into three categories according to their structure: point contact type, surface contact type and planar type. Point contact diodes have small PN junction area and small junction capacitance, and are often used in high-frequency circuits such as detection and frequency conversion. Surface contact diodes have a large PN junction area and large junction capacitance, and are used in power frequency high current rectification circuits. The PN junction area of planar diodes can be large or small. Those with large PN junction area are mainly used for power rectification; those with small junction area can be used as switching tubes in digital pulse circuits.

2.2 Volt-ampere characteristics of diode

(1) The difference between the volt-ampere characteristics of diodes and PN junctions

The volt-ampere characteristic curve of a semiconductor diode is the forward volt-ampere characteristic curve in the first quadrant, and the reverse volt-ampere characteristic curve is in the third quadrant.

Positive characteristics: When V>0, it is in the positive characteristics area. The forward area is divided into two sections: ① When 0<V<Uon, the forward current is zero, and Uon is called the dead zone voltage or turn-on voltage. ②When V>Uon, forward current begins to appear and grows exponentially.

Reverse characteristics: When V<0, it is in the reverse characteristics area. The reverse area is also divided into two areas:
① When VBR<V<0, the reverse current is very small and basically does not change with the change of the reverse voltage. The reverse current at this time is also called the reverse saturation current Is.
②When V≤VBR, the reverse current increases sharply, and VBR is called the reverse breakdown voltage. From the perspective of the breakdown mechanism, if |V| ≥ 7V, the silicon diode is mainly avalanche breakdown; if VR ≤ 4V, it is mainly Zener breakdown. When between 4V and 7V, both breakdowns are present. It is possible to obtain a zero temperature coefficient point.

The difference between the volt-ampere characteristics of the diode and the volt-ampere characteristics of the PN junction: the basic characteristics of the diode are the characteristics of the PN junction. Different from the ideal PN junction, the diode has a turn-on voltage Uon in the forward characteristic. Generally, the Uon of silicon diodes is about 0.5V, and the Uon of germanium diodes is about 0.1V; the reverse saturation current of the diode is larger than that of the PN junction.

(2) Effect of temperature on diode volt-ampere characteristics

Temperature has a great impact on the performance of the diode. When the temperature rises, the reverse current will increase exponentially. For every 8°C increase in silicon diode temperature, the reverse current will approximately double; for every 12°C increase in germanium diode temperature, the reverse current will approximately double. The reverse current is approximately doubled.
As the temperature increases, the forward characteristic curve of the diode will shift to the left and the reverse characteristic curve will shift downward. The forward voltage drop of the diode will be reduced. For every increase of 1℃, the forward voltage drop Up decreases by approximately 2mV, that is, it has a negative temperature coefficient.

2.3 Main parameters of diodes

(1) Maximum rectified current $I_F$ : the maximum forward average current allowed to pass through the diode during long-term operation. Under specified heat dissipation conditions, if the average forward current of the diode exceeds this value, it will burn out due to excessive junction temperature.

(2) Maximum reverse working voltage $U_{R}$ : the maximum reverse voltage allowed to be applied when the diode is working. If this value is exceeded, the diode may be damaged by reverse breakdown. Generally half $U_{(BR)}$ the value .

(3) Reverse current $I_R$ : The reverse current when the diode has not broken down. Sensitive to temperature. The smaller the value, the better the unidirectional conductivity of the diode.

(4) Maximum operating frequency $f_M$ : the upper limit cutoff frequency of the diode's normal operation. If it exceeds this value, its unidirectional conductivity will be affected due to the effect of junction capacitance.

2.4 Equivalent circuit (or equivalent model) of diode

(1) Ideal model: that is, when forward biased, the tube voltage drop is 0 and the on-resistance is 0; when reverse biased, the current is 0 and the resistance is $\infty$ . It is called an ideal diode, which is equivalent to an ideal switch and is suitable for approximate analysis when the signal voltage is much larger than the diode voltage drop.
(2) Simplified circuit model: It is a model established based on the approximate volt-ampere characteristic curve of the diode. It uses two straight lines to approximate the volt-ampere characteristic, that is, the voltage drop is a constant during forward conduction; the reverse current is 0 when turned off $U_{on}$ .
(3) Small signal circuit model: that is, within the range of small changes, the diode is approximately regarded as a linear device and is equivalent to a dynamic resistance r. This model is limited to calculating the response to small voltage or current changes superimposed on the DC operating point Q.

2.5 Zener diode

1. Volt-ampere characteristics of voltage regulator tube

The Zener diode is a special surface contact semiconductor diode that achieves voltage stabilization through reverse breakdown characteristics. The volt-ampere characteristics of the voltage regulator tube are similar to those of ordinary diodes, and their forward characteristics are exponential curves; when the value of the external back pressure increases to a certain level, breakdown occurs. The breakdown curve is very steep, almost parallel to the vertical axis. When When the current is within a certain range, the voltage regulator tube exhibits good voltage stabilization characteristics.

2. Main parameters of voltage regulator tube

(1) Stable voltage $U_Z$ : the reverse breakdown voltage of the voltage regulator tube under the specified current.
(2) Stable current $I_Z$ : $I_Z$ It is the reference current when the voltage regulator tube works in a stable voltage state. If the current is lower than this value, the voltage cannot be stabilized, so it is often $I_Z$ written as $I_{Zmin}$ .
(3) Rated power consumption $P_{ZM}$ : $U_Z$ equal to the product of the stable voltage of the voltage regulator tube and the maximum stable current $I_{ZM}$ (or recorded as $I_{Zmax}$ ). If this value is exceeded, the tube will burn out due to the junction temperature rising too high.
(4) Dynamic resistance $r_z$ : When the voltage regulator tube works in the voltage stabilization zone, the ratio of the terminal voltage change to its current change, that is

$r_z=\Delta U_Z/\Delta I_Z$

The concept is the same as the dynamic resistance of a general diode, except that the dynamic resistance of a Zener diode is obtained from its reverse characteristics. $r_z$ The smaller it is, the steeper the breakdown characteristics of the voltage regulator tube are and the better the voltage stabilization characteristics are.
(5) Temperature coefficient α: represents the change in voltage regulation value for every 1°C change in temperature. Changes in temperature will cause $U_Z$ changes. In the voltage regulator tube, when | $U_Z$ |>7V, Uz has a positive temperature coefficient, and the reverse breakdown is avalanche breakdown; when $U_Z$ | $U_Z$ Breakthrough is Zener breakdown; when 4V<| $U_Z$ |<7V, the voltage regulator tube can obtain a temperature coefficient close to zero. Such a Zener diode can be used as a standard Zener diode.

3. Voltage regulator equivalent circuit

The equivalent circuit of the voltage regulator tube consists of two parallel branches:

(1) When forward voltage and reverse voltage are applied without breakdown, the characteristics are the same as those of ordinary silicon tubes;

(2) After applying reverse voltage and breakdown, it is equivalent to the series connection of the ideal diode, voltage source Uz and dynamic resistor rz.

4. Voltage regulator tube voltage stabilizing circuit

The Zener diode should be reverse-connected during operation. Since the reverse current of the voltage regulator tube is less than the $I_{Zmin}$ voltage, it will not stabilize the voltage. $I_{Zmax}$ If it is greater than the reverse current, it will be damaged due to exceeding the rated power consumption, so a resistor must be connected in series to limit the current.

The resistor has two functions: one is to limit the current to protect the voltage regulator tube; the other is to take out the error signal through the change in voltage drop on the resistor to adjust the operating current of the voltage regulator tube when the input voltage or load current changes. , thus playing a voltage stabilizing role.

2.6 Special diodes

Like ordinary diodes, special diodes also have unidirectional conductivity. The breakdown characteristics of the PN junction can be used to make a zener diode, the luminescent material can be used to make a light-emitting diode, and the photosensitive characteristics of the PN junction can be used to make a photodiode.

3 transistor

3.1 Structure and types of transistors

The bipolar transistor BJIT is a device formed by joining two PN junctions together through a certain process. It is the core component of the amplifier circuit. It can control the conversion of energy and amplify the output without distortion of any small changes in the input. The object of amplification is the amount of change.
There are four common shapes of BJT, which are used in different situations such as low power, medium power or high power, high frequency or low frequency.

The P located in the middle is the base region, which is very thin and has a low impurity concentration;

The N region located in the upper layer has a very high doping concentration;

The N area located on the lower level is the current collecting area, which is very large.

3.2 BJT has internal and external conditions for amplification

(1) The internal conditions of BJT are: BJT has three areas (emitter area, collector area and base area), two PN junctions (emitter junction and collector junction), and three electrodes (emitter, collector and base) ) composition; and the impurity concentration in the emitter region is much greater than the impurity concentration in the base region, and the thickness of the base region is very small.

(2) The external conditions for BJT to work in the amplified state are: the emitter junction is forward biased and the collector junction is reverse biased.

3.3 Current amplification effect and current distribution relationship of BJT

The transistor has an amplification effect, which means that a small base current can control a large collector current.

When the emitter junction is forward biased and the collector junction is reverse biased, only a small part of the non-equilibrium minority carriers injected from the emitter region into the base region recombine with the majority carriers in the base region to form a base current; while most A drift current is formed under the action of the external electric field of the collector junction $I_C$ , which reflects the control effect $I_B$ of the $I_C$ current collector. At this time, it can be regarded as a current source controlled by $I_C$ current . $I_B$

Three important current distribution relationships

$I_E=I_B+I_C$

$IC=\beta I_B+I_{CEO}\approx\beta I_B$

$IC=\alpha I_C+I_{CBO}\approx\alpha I_E$

2.4 Input characteristics and output characteristics of a transistor
The input characteristics and output characteristics of a transistor indicate the relationship between the current and voltage between each electrode. Now take the common emitter circuit as an example.
(1) Common emission input characteristics: is-8uBE)l VcE=constant. The input characteristic curve is divided into three zones: dead zone, nonlinear zone and linear zone. The one with vcE=0V is equivalent to the forward characteristic curve of the emitter junction. When vcE≥IV, the characteristic curve will move slightly to the right. But when vcz increases again, the curve shifts to the right very little. The rightward shift of the curve is caused by the internal feedback of the transistor. The insignificant rightward shift indicates that the internal feedback is very small.
(2) Common emission output characteristics: ic=fucE)l is=-constant, which is a family of characteristic curves with is as a parameter. For one of the curves, when vcE=0V, ic=0; when vcr increases slightly, ic is mainly determined by vE; when vc increases to make the collector junction reverse bias voltage larger, the characteristic curve enters the axis with vcE Basically parallel areas (this is consistent with the reason why the input characteristic curve shifts to the right as vcr increases). Therefore, the output characteristic curve can be divided into three areas: saturation area, cut-off area and amplification area.
(3) Conditions and characteristics of transistors operating outside three different working areas

3.4 Main parameters of transistors

(1) DC parameters

①Common-emitter DC current amplification factor $\bar{\beta}=\frac{I_C-I_{CEO}}{I_B}$

At that time $I_C \gg I_{CEO}$ , $\bar{\beta}=\frac{I_C}{I_B}$ . β is basically unchanged in the amplification region.

② Common base DC amplification coefficient $\bar{\alpha}=\frac{I_C-I_{CBO}}{I_E}\approx\frac{I_C}{I_E}$ . (When $I_{CEO}$ ignored, $\bar{\alpha}\approx\frac{I_C}{I_E}$ )

Obviously there is the following relationship $\alpha$ between $\beta$

$\bar{\alpha }=\frac{I_C}{I_E}=\frac{\bar{\beta }I_B}{(1+\beta )I_B}=\frac{\bar{\beta }}{(1+\bar{\beta })}$
③Inter-electrode reverse current

$I_{CBO}$ is the reverse saturation current of the collector junction when the emitter is open. $I_{CEO}$ is the penetration current between the collector and emitter when the base is open, and $I_{CEO}=(1+\bar{\beta })I_{CBO}$ . The smaller the reverse current of tubes of the same model, the more stable the performance.

(2) Communication parameters

AC parameters describe the performance of a transistor for dynamic signals.

①Common-emitter AC current amplification factor $\beta$

$\beta =\frac{\Delta i_C}{\Delta i_B}|_{U_{CE}=Const}$

The β value remains basically unchanged in the amplification area. When selecting a tube, β should be moderate. If it is too small, the amplification capability will be weak, and if it is too large, the temperature stability will be poor.

② Common base AC amplification coefficient $\alpha$

$\alpha=\frac{\Delta i_C}{\Delta i_E}|_{U_{CB}=Const}$

In approximate analysis, it can be considered $\beta \approx \bar{\beta },\alpha \approx \bar{\alpha }\approx 1$

③Characteristic frequency $f_T$

The beta value of the transistor is not only related to the operating current, but also related to the operating frequency. Due to the influence of junction capacitance, when the signal frequency increases, the β of the transistor will decrease.

The frequency corresponding to when the value of the common emission current amplification coefficient drops to 1 is called the characteristic frequency.

(3) Limit parameters and safe working area of triode

①Maximum collector current $I_{CM}$

When the collector current increases, β will decrease. When the β value drops to 70 to 30% of the β value in the linear amplification zone, the corresponding collector current is called the maximum collector current $I_{CM}$ . As for how much the β value drops, different manufacturers have different regulations for different models of transistors. It can be seen that when lc>Icm, it does not mean that the transistor will be damaged.

②Maximum collector power dissipation $P_{CM}$

For a certain type of transistor, $P_{CM}=i_Cu_{CE}=Const$ . When the junction temperature of the silicon tube is greater than 150°C and the junction temperature of the germanium tube is greater than 70°C, the characteristics of the tube will obviously deteriorate or even burn out.

③Inter-electrode reverse breakdown voltage $U_{(BR)}$

When a certain stage of the transistor is open circuit, the maximum reverse voltage allowed between the other two electrodes is the inter-electrode reverse breakdown voltage. If the value exceeds this value, the tube will undergo breakdown.

There are three types of inter-electrode reverse breakdown voltage.

$U_{(BR)CBO}$ It is the reverse breakdown voltage between the collector and the base when the emitter is open circuit. This is the highest reverse voltage allowed to be applied to the collector.

$U_{(BR)CEO}$ It is the reverse breakdown voltage between the collector and the emitter when the base is open circuit. At this time, the collector is subjected to the reverse voltage.

$U_{(BR)EBO}$ It is the reverse calculated voltage between the emitter and the base when the collector is open circuit. This is the highest reverse voltage allowed to be applied to the emitter.

Since the median value of each breakdown voltage $U_{(BR)CEO}$ is the smallest, it should be greater than the working power supply of the amplifier circuit when selecting it $V_{CC}$ .

④Safe working area of triode

Four areas can be determined on the output characteristic curve by $P_{CM}$ , , $I_{CM}$ and reverse breakdown voltage : over-loss area, over-current area, breakdown area and safe operating area. $V_{(BR)CEO}$ When using, the triode should be ensured to work in a safe area.

3.5 Effect of temperature on transistor characteristics and parameters

(1) $I_{CBO}$ Effect of temperature on reverse saturation current

Temperature has a very serious influence on the current formed by the equilibrium minority carriers generated by intrinsic excitation $I_{CBO}$ and so on. $I_{CEO}$

(2) Effect of temperature on input characteristics

As the temperature increases, the positive characteristic shifts to the left. When the temperature changes by 1°C, $U_{BE}$ it drops approximately 2~2.5mV, $U_{BE}$ with a negative temperature coefficient

(3) Effect of temperature on output characteristics

When the temperature rises, because $I_{CEO}$ the sum $\beta$ increases and the input characteristics shift to the left, the collector current I increases and the output characteristics shift upward.

In summary, when the temperature increases, $I_{CEO}$ and $\beta$ increases, the input characteristics shift to the left, ultimately causing the collector current to increase.